As the less distracted among you have already figured out, I have permanently moved my blogging activities to www.scientificblogging.com. The reasons for the move are explained here.

Since I know that this site continues to be visited -because the 1450 posts it contains draw traffic regardless of the inactivity- I am providing here monthly updates of the pieces I write in my new blog here. Below is a list of posts published last month at the new site.

The Large Hadron Collider is Back Together – announcing the replacement of the last LHC magnets

Hera’s Intriguing Top Candidates – a discussion of a recent search for FCNC single top production in ep collisions

Source Code for the Greedy Bump Bias – a do-it-yourself guide to study the bias of bump fitting

Bump Hunting II: the Greedy Bump Bias – the second part of the post about bump hunting, and a discussion of a nagging bias in bump fitting

Rita Levi Montalcini: 100 Years and Still Going Strong – a tribute to Rita Levi Montalcini, Nobel prize for medicine

The Subtle Art of Bump Hunting – Part I – a discussion of some subtleties in the search for new particle signals

Save Children Burnt by Caustic Soda! – an invitation to donate to Emergency!

Gates Foundation to Chat with Bloggers About World Malaria Day – announcing a teleconference with bloggers

Dark Matter: a Critical Assessment of Recent Cosmic Ray Signals – a summary of Marco Cirelli’s illuminating talk at NeuTel 2009

A Fascinating New Higgs Boson Search by the DZERO Experiment – a discussion on a search for tth events recently published by the Tevatron experiment

A Banner Worth a Thousand Words - a comment on my new banner

Confirmed for WCSJ 2009 – my first post on the new site

It rarely happens that four days pass without a new post on this site, but it is never because of the lack of things to report on: the world of experimental particle physics is wonderfully active and always entertaining. Usually hiatuses are due to a bout of laziness on my part. In this case, I can blame Vodafone, the provider of the wireless internet service I use when I am on vacation. From Padola (the place in the eastern italian Alps where I spent the last few days) the service is horrible, and I sometimes lack the patience to find the moment of outburst when bytes flow freely.

Things I would have wanted to blog on during these days include:

• The document describing the DZERO search of a CDF-like anomalous muon signal is finally public, about two weeks after the talk which announced the results at Moriond. Having had in my hands a unauthorized draft, I have a chance of comparing the polished with the unpolished version… Should be fun, but unfortunately unbloggable, since I owe some respect to my colleagues in DZERO. Still, the many issues I raised after the Moriond seminar should be discussed in light of an official document.
• DZERO also produced a very interesting search for production. This is the associated production of a Higgs boson and a pair of top quarks, a process whose rate is made significant by the large coupling of top quarks and Higgs bosons, by virtue of the large top quark mass. By searching for a top-antitop signature and the associated Higgs boson decay to a pair of b-quark jets, one can investigate the existence of Higgs bosons in the mass range where the decay is most frequent -i.e., the region where all indirect evidence puts it. However, tth production is invisible at the Tevatron, and very hard at the LHC, so the DZERO search is really just a check that there is nothing sticking out which we have missed by just forgetting to look there. In any case, the signature is extremely rich and interesting to study (I had a PhD doing this for CMS a couple of years ago), thus my interest.
• I am still sitting on my notes for Day 4 of the NEUTEL2009 conference in Venice, which included a few interesting talks on gravitational waves, CMB anisotropies, the PAMELA results, and a talk by Marco Cirelli on dark matter searches. With some effort, I should be able to organize these notes in a post in a few days.
• And new beautiful physics results are coming out of CDF. I cannot anticipate much, but I assure you there will be much to read about in the forthcoming weeks!

I read with amusement (and some effort) a spanish account by Francis (th)E mule of Michael Dittmar’s controversial seminar of last March 19th. I paste the link here for several reasons: since I believe it might be of interest to some of you, to have a place to store it, and because I am not insensitive to flattery:

“Entre el público se encontraba Tomasso Dorigo [...] (r)esponsable del mejor blog sobre física de partículas elementales del mundo”

Muchas gracias, Francis -but please note: my name spells with two m’s and one s!

A recent discussion in this blog between well-known theorists and phenomenologists, centered on the real meaning of the experimental measurements of top quark and W boson masses, Higgs boson cross-section limits, and other SM observables, convinces me that some clarification is needed.

The work has been done for us: there are groups that do exactly that, i.e. updating their global fits to express the internal consistency of all those measurements, and the implications for the search of the Higgs boson. So let me go through the most important graphs below, after mentioning that most of the material comes from the LEP electroweak working group web site.

First of all, what goes in the soup ? Many things, but most notably, the LEP I/SLD measurements at the Z pole, the top quark mass measurements by CDF and DZERO, and the W mass measurements by CDF, DZERO, and LEP II. Let us give a look at the mass measurements, which have recently been updated.

For the top mass, the situation is the one pictured in the graph shown below. As you can clearly see, the CDF and DZERO measurements have reached a combined precision of 0.75% on this quantity.

The world average is now at . I am amazed to see that the first estimate of the top mass, made by a handful of events published by CDF in 1994 (a set which did not even provide a conclusive “observation-level” significance at the time) was so dead-on: the measurement back then was ! (for comparison, the DZERO measurement of 1995, in their “observation” paper, was ).

As far as global fits are concerned, there is one additional point to make for the top quark: knowing the top mass any better than this has become, by now, useless. You can see it by comparing the constraints on coming from the indirect measurements and W mass measurements (shown by the blue bars at the bottom of the graph above) with the direct measurements at the Tevatron (shown with the green band). The green band is already too narrow: the width of the blue error bars compared to the narrow green band tells us that the SM does not care much where exactly the top mass is, by now.

Then, let us look at the W mass determinations. Note, the graph below shows the situation BEFORE the latest DZERO result;, obtained with 1/fb of data, and which finds ; its inclusion would not change much of the discussion below, but it is important to stress it.

Here the situation is different: a better measurement would still increase the precision of our comparisons with indirect information from electroweak measurements at the Z. This is apparent by observing that the blue bars have width still smaller than the world average of direct measurements (again in green). Narrow the green band, and you can still collect interesting information on its consistency with the blue points.

Finally, let us look at the global fit: the electroweak working group at LEP displays in the by now famous “blue band plot”, shown below for March 2009 conferences. It shows the constraints on the Higgs boson mass coming from all experimental inputs combined, assuming that the Standard Model holds.

I will not discuss this graph in details, since I have done it repeatedly in the past. I will just mention that the yellow regions have been excluded by direct searches of the Higgs boson at LEP II (on the left, the wide yellow area) and the Tevatron ( the narrow strip on the right). From the plot you should just gather that a light Higgs mass is preferred (the central value being 90 GeV, with +36 and -27 GeV one-sigma error bars). Also, a 95% confidence-level exclusion of masses above 163 GeV is implied by the variation of the global fit with Higgs mass.

I have started to be a bit bored by this plot, because it does not do the best job for me. For one thing, the LEP II limit and the Tevatron limit on the Higgs mass are treated as if they were equivalent in their strength, something which could not be possibly farther from the truth. The truth is, the LEP II limit is a very strong one -the probability that the Higgs has a mass below 112 GeV, say, is one in a billion or so-, while the limit obtained recently by the Tevatron is just an “indication”, because the excluded region (160 to 170 GeV) is not excluded strongly: there still is a one-in-twenty chance or so that the real Higgs boson mass indeed lies there.

Another thing I do not particularly like in the graph is that it attempts to pack too much information: variations of , inclusion of low-Q^2 data, etcetera. A much better graph to look at is the one produced by the GFitter group instead. It is shown below.

In this plot, the direct search results are introduced with their actual measured probability of exclusion as a function of Higgs mass, and not just in a digital manner, yes/no, as the yellow regions in the blue band plot. And in fact, you can see that the LEP II limit is a brick wall, while the Tevatron exclusion acts like a smooth increase in the global of the fit.

From the black curve in the graph you can get a lot of information. For instance, the most likely values, those that globally have a 1-sigma probability of being one day proven correct, are masses contained in the interval 114-132 GeV. At two-sigma, the Higgs mass is instead within the interval 114-152 GeV, and at three sigma, it extends into the Tevatron-excluded band a little, 114-163 GeV, with a second region allowed between 181 and 224 GeV.

In conclusion, I would like you to take away the following few points:

• Future indirect constraints on the Higgs boson mass will only come from increased precision measurements of the W boson mass, while the top quark has exhausted its discrimination power;
• Global SM fits show an overall very good consistency: there does not seem to be much tension between fits and experimental constraints;
• The Higgs boson is most likely in the 114-132 GeV range (1-sigma bounds from global fits).

Yesterday Sven Heinemeyer kindly provided me with an updated version of a plot which best describes the experimental constraints on the Higgs boson mass, coming from electroweak observables measured at LEP and SLD, and from the most recent measurements of W boson and top quark masses. It is shown on the right (click to get the full-sized version).

The graph is a quite busy one, but I will try below to explain everything one bit at a time, hoping I keep things simple enough that a non-physicist can understand it.

The axes show suitable ranges of values of the top quark mass (varying on the horizontal axis) and of the W boson masses (on the vertical axis). The value of these quantities is functionally dependent (because of quantum effects connected to the propagation of the particles and their interaction with the Higgs field) on the Higgs boson mass.

The dependence, however, is really “soft”: if you were to double the Higgs mass by a factor of two from its true value, the effect on top and W masses would be only of the order of 1% or less. Because of that, only recently have the determinations of top quark and W boson masses started to provide meaningful inputs for a guess of the mass of the Higgs.

Top mass and W mass measurements are plotted in the graphs in the form of ellipses encompassing the most likely values: their size is such that the true masses should lie within their boundaries, 68% of the time. The red ellipse shows CDF results, and the blue one shows DZERO results.

There is a third measurement of the W mass shown in the plot: it is displayed as a horizontal band limited by two black lines, and it comes from the LEP II measurements. The band also encompasses the 68% most likely W masses, as ellipses do.

In addition to W and top masses, other experimental results constrain the mass of top, W, and Higgs boson. The most stringent of these results are those coming from the LEP experiment at CERN, from detailed analysis of electroweak interactions studied in the production of Z bosons. A wide band crossing the graph from left to right, with a small tilt, encompasses the most likely region for top and W masses.

So far we have described measurements. Then, there are two different physical models one should consider in order to link those measurements to the Higgs mass. The first one is the Standard Model: it dictates precisely the inter-dependence of all the parameters mentioned above. Because of the precise SM predictions, for any choice of the Higgs boson mass one can draw a curve in the top mass versus W mass plane. However, in the graph a full band is hatched instead. This correspond to allowing the Higgs boson mass to vary from a minimum of 114 GeV to 400 GeV. 114 GeV is the lower limit on the Higgs boson mass found by the LEP II experiments in their direct searches, using electron-positron collisions; while 400 GeV is just a reference value.

The boundaries of the red region show the functional dependence of Higgs mass on top and W masses: an increase of top mass, for fixed W mass, results in an increase of the Higgs mass, as is clear by starting from the 114 GeV upper boundary of the red region, since one then would move into the region, to higher Higgs masses. On the contrary, for a fixed top mass, an increase in W boson mass results in a decrease of the Higgs mass predicted by the Standard Model. Also note that the red region includes a narrow band which has been left white: it is the region corresponding to Higgs masses varying between 160 and 170 GeV, the masses that direct searches at the Tevatron have excluded at 95% confidence level.

The second area, hatched in green, is not showing a single model predictions, but rather a range of values allowed by varying arbitrarily many of the parameters describing the supersymmetric extension of the SM called “MSSM”, its “minimal” extension. Even in the minimal extension there are about a hundred additional parameters introduced in the theory, and the values of a few of those modify the interconnection between top mass and W mass in a way that makes direct functional dependencies in the graph impossible to draw. Still, the hatched green region shows a “possible range of values” of the top quark and W boson masses. The arrow pointing down only describes what is expected for W and top masses if the mass of supersymmetric particles is increased from values barely above present exclusion limits to very high values.

So, to summarize, what to get from the plot ? I think the graph describes many things in one single package, and it is not easy to get the right message from it alone. Here is a short commentary, in bits.

• All experimental results are consistent with each other (but here, I should add, a result from NuTeV which finds indirectly the W mass from the measured ratio of neutral current and charged current neutrino interactions is not shown);
• Results point to a small patch of the plane, consistent with a light Higgs boson if the Standard Model holds
• The lower part of the MSSM allowed region is favored, pointing to heavy supersymmetric particles if that theory holds
• Among experimental determinations, the most constraining are those of the top mass; but once the top mass is known to within a few GeV, it is the W mass the one which tells us more about the unknown mass of the Higgs boson
• One point to note when comparing measurements from LEP II and the Tevatron experiments: when one draws a 2-D ellipse of 68% contour, this compares unfavourably to a band, which encompasses the same probability in a 1-D distribution. This is clear if one compares the actual measurements: CDF (with 200/pb of data), DZERO (with five times more statistics), LEP II (average of four experiments). The ellipses look like they are half as precise as the black band, while they are actually only 30-40% worse. If the above is obscure to you, a simple graphical explanation is provided here.
• When averaged, CDF and DZERO will actually beat the LEP II precision measurement -and they are sitting on 25 times more data (CDF) or 5 times more (DZERO).

I spent this week at CERN to attend the meetings of the CMS week – an event which takes place four times a year, when collaborators of the CMS experiment, coming from all parts of the world, get together at CERN to discuss detector commissioning, analysis plans, and recent results. It was a very busy and eventful week, and only now, sitting on a train that brings me back from Geneva to Venice, can I find the time to report with the due dedication on some things you might be interested to know about.

One thing to report on is certainly the seminar I eagerly attended on Thursday morning, by Michael Dittmar (ETH-Zurich). Dittmar is a CMS collaborator, and he talked at the CERN theory division on a tickling subject:”Why I never believed in the Tevatron Higgs sensitivity claims for Run 2ab”. The title did promise a controversial discussion, but I was really startled by its level, as much as by the defamation of which I felt personally to be a target. I will explain this below.

I have also to mention that by Thursday I had already attended to a reduced version of his talk, since he had given it on the previous day in another venue. Both I and John Conway had corrected him on a few plainly wrong statements back then, but I was puzzled to see he reiterated those false statements in the longer seminar! More on that below.

Dittmar’s obnoxious seminar

Dittmar started by saying he was infuriated by the recent BBC article where “a statement from the director of a famous laboratory” claimed that the Tevatron had 50% odds of finding a Higgs boson, in a certain mass range. This prompted him to prepare a seminar to express his scepticism. However, it turned out that his scepticism was not directed solely at the optimistic statement he had read, but at every single result on Higgs searches that CDF and DZERO had produced since Run I.

In order to discuss sensitivity and significances, the speaker made a un-illuminating digression on how counting experiments can or cannot obtain observation-level significances with their data depending on the level of background of their searches and the associated systematical uncertainties. His statements were very basic and totally uncontroversial on this issue, but he failed to focus on the fact that nowadays, nobody does counting experiments any more when searching for evidence of a specific model: our confidence in advanced analysis methods involving neural networks, shape analysis, and likelihood discriminants; the tuning of Monte Carlo simulations; and the accurate analytical calculations of high-order diagrams for Standard Model processes, have all grown tremendously with years of practice and studies, and these methods and tools overcome the problems of searches for small signals immersed in large backgrounds. One can be sceptical, but one cannot ignore the facts, as the speaker seemed inclined to.

Then Dittmar said that in order to judge the value of sensitivity claims for the future, one may turn to past studies and verify their agreement with the actual results. So he turned to the Tevatron Higgs Sensitivity studies of 2000 and 2003, two endeavours to which I had participated with enthusiasm.

He produced a plot showing the small signal of decays that the Tevatron 2000 study believed the two experiments could achieve with 10 inverse femtobarns of data, expressing his doubts that the “tiny excess” could mean an evidence for Higgs production. On the side of that graph, he had for comparison placed a result of CDF on real Run I data, where a signal of WH or ZH decays to four jets had been searched in the dijet invariant mass distribution of the two b-jets.

He commented that figure by saying half-mockingly that the data could have been used to exclude the standard model process of associated production, since the contribution from Z decays to b-quark pairs was sitting at a mass where one bin had fluctuated down by two standard deviations with respect to the sum of background processes. This ridiculous claim was utterly unsupported by the plot -which had an overall very good agreement between data and MC sources- and by the fact that the bins adjacent to the downward-fluctuating one were higher than the prediction. I found this claim really disturbing, because it tried to denigrate my experiment with a futile and incorrect argument. But I was about to get more upset for his next statement.

In fact, he went on to discuss the global expectation of the Tevatron on Higgs searches, a graph (see below) produced in 2000 after a big effort from several tens of people in CDF and DZERO.

He started by saying that the graph was confusing, and that it was not clear in the documentation how it had been produced, nor that it was the combination of CDF and DZERO sensitivity. This was very amusing, since sitting from the far back John Conway, a CDF colleague, shouted: “It says it in print on top of it: combined thresholds!”, then adding, with a pacate voice “…In case you’re wondering, I made that plot.” John had in fact been the leader of the Tevatron Higgs sensitivity study, not to mention the author of many of the most interesting searches for the higgs boson in CDF since then.

Dittmar continued his surreal talk with an overbid, by claiming that the plot had been produced “by assuming a 30% improvement in the mass resolution of pairs of b-jets, when nobody had not even the least idea on how such improvement could be achieved”.

I could not have put together a more personal, direct attack to years of my own work myself! It is no mystery that I worked on dijet resonances since 1992, but of course I am a rather unknown soldier in this big game; however, I felt the need to interrupt the speaker at this point -exactly as I had done at the shorter talk the day before.

I remarked that in 1998, one year before the Tevatron sensitivity study, I had produced a PhD thesis and public documents showing the observation of a signal of decays in CDF Run I data, and had demonstrated on that very signal how the use of ingenuous algorithms could reduce by at least 30% the dijet mass resolution, making the signal more prominent. The relevant plots are below, directly from my PhD thesis: judge for yourself.

In the plots, you can see how the excess over background predictions moves to the right as more and more refined jet energy corrections are applied, starting from the result of generic jet energy corrections (top) to optimized corrections (bottom) until the signal becomes narrower and centered at the true value. The plots on the left show the data and the background prediction, those on the right show the difference, which is due to Z decays to b-quark jet pairs. Needless to say, the optimization is done on Monte Carlo Z events, and only then checked on the data.

So I said that Dittmar’s statement was utterly false: we had an idea of how to do it, we had proven we could do it, and besides, the plots showing what we had done had been indeed included in the Tevatron 2000 report. Had he overlooked them ?

Escalation!

Dittmar seemed unbothered by my remark, and he responded that that small signal had not been confirmed in Run II data. His statement constituted an even more direct attack to four more years of my research time, spent on that very topic. I kept my cool, because when your opponent offers you on a silver plate the chance to verbally sodomize him, you cannot be too angry with him.

I remarked that a signal had indeed been found in Run II, amounting to about 6000 events after all selection cuts; it confirmed the past results. Dittmar then said that “to the best of his knowledge” this had not been published, so it did not really count. I then explained it was a 2008 NIM publication, and would he please document himself before making such unsubstantiated allegations? He shrugged his shoulders, said he would look more carefully for the paper, and went back to his talk.

His points about the Tevatron sensitivity studies were laid down: for a low-mass Higgs boson, the signal is just too small and backgrounds are too large, and the sensitivity of real searches is below expectations by a large factor. To stress this point, he produced a slide containing a plot he had taken from this blog! The plot (see on the left), which is my own concoction and not Tevatron-approved material, shows the ratio between observed limit to Higgs production and the expectations of the 2000 study. He pointed at the two points for 100-140 GeV Higgs boson masses, trying to prove his claim: The Tevatron is now doing three times worse than expected. He even uttered “It is time to confess: the sensitivity study was wrong by a large factor!”.

I could not help interrupting again: I had to stress that the plot was not approved material and was just a private interpretation of Tevatron results, but I did not deny its contents. The plot was indeed showing that low-mass searches were below par, but it was also showing that high-mass ones were amazingly in agreement with expectations worked at 10 years before. Then John Conway explained the low-mass discrepancy for the benefit of the audience, as he had done one day before for no apparent benefit of the speaker.

Conway explained that the study had been done under the hypothesis that an upgrade of our silicon detector would be financed by the DoE: it was in fact meant to prove the usefulness of funding an upgrade. A larger acceptance of inner silicon tracking boosts the sensitivity to identify b-quark jets from Higgs decays by a large factor, because any acceptance increase gets squared when computing the over-efficiency. So Dittmar could not really blame the Tevatron experiments for predicting something that would not materialize in a corresponding result, given that the DoE had denied the funding to build the upgraded detector!

I then felt compelled to add that by using my plot Dittmar was proving the opposite thesis of what he wanted to demonstrate: low-mass Tevatron searches were shown to underperform because of funding issues, rather than because of a wrong estimate of sensitivity; and high-mass searches, almost unhindered by the lack of an upgraded silicon, were in excellent agreement with expectations!

The speaker said that no, the high-mass searches were not in agreement, because their results could not be believed, and moved on to discuss those by taking real-data results by the Tevatron.

He said that the is a great channel at the LHC.

“Possible at the Tevatron ? I believe that the WW continuum background is much larger at a ppbar collider than at a pp collider, so my personal conclusion is that if the Tevatron people want to waste their time on it, good luck to them.”

Now, come on. I cannot imagine how a respectable particle physicist could drive himself into making such statements in front of a distinguished audience (which, have I mentioned it, included several theorists of the highest caliber, including none less than Edward Witten). Waste their time ? I felt I was wasting my time listening to him, but my determination of reporting his talk here kept me anchored to my chair, taking notes.

So this second part of the talk was not less unpleasant than the first part: Dittmar criticized the Tevatron high-mass Higgs results in the most incorrect, and scientifically dishonest, way that I could think of. Here is just a summary:

• He picked up a distribution of one particular sub-channel from one experiment, noting that it seemed to have the most signal-rich region showing a deficit of events. He then showed the global CDF+DZERO limit, which did not show a departure between expected and observed limit on Higgs cross section, and concluded that there was something fishy in the way the limit had been evaluated. But the limit is extracted from literally several dozens of those distributions -something he failed to mention despite having been warned of that very issue in advance.

• He picked up two neural-network output distributions for a search of Higgs at 160 and 165 GeV, and declared they could not be correct since they were very different in shape! John, from the back, replied “You have never worked with neural networks, have you ?” No, he had not. Had he, he would probably have understood that different mass points, optimized differently, can provide very different NN outputs.

• He showed another Neural Network output based on 3/fb of data, which had a pair of data points lying one standard deviation above the background predictions, and the corresponding plot for a search performed with improved statistics, which had instead a underfluctuation. He said he was puzzled by the effect. Again, some intervention from the audience was necessary, explaining that the methods are constantly reoptimized, and there is no wonder that adding more data can result in a different outcome. This produced a discussion when somebody from the audience tried to speculate that searches were maybe performed by looking at the data before choosing which method to use for a limit extraction! On the contrary of course, all Tevatron searches of the Higgs are blind analyses, where the optimization is performed on expected limits, using control samples, and Monte Carlo, and the data is only looked at afterwards.
• He showed that the Tevatron 2000 report had estimated a maximum Signal/Noise ratio for the H–>WW search of 0.34, and he picked up one random plot from the many searches of that channel by CDF and DZERO, showing that the signal to noise there was never larger than 0.15 or so. Explaining to him that the S/N of searches based on neural networks and combined discriminants is not a fixed value, and that many improvements have occurred in data analysis techniques in 10 years was useless.

Dittmar concluded his talk by saying that:

“Optimistic expectations might help to get funding! This is true, but it is also true that this approach eventually destroys some remaining confidence in science of the public.”.

His last slide even contained the sentence he had previously brought himself to uttering:

“It is the time to confess and admit that the sensitivity predictions were wrong”.

Finally, he encouraged LHC experiments to looking for the Higgs where the Tevatron had excluded it -between 160 and 170 GeV- because Tevatron results cannot be believed. I was disgusted: he most definitely places a strong claim on the prize of the most obnoxious talk of the year. Unfortunately for all, it was just as much an incorrect, scientifically dishonest, and dilettantesque lamentation, plus a defamation of a community of 1300 respected physicists.

In the end, I am really wondering what really moved Dittmar to such a disastrous performance. I think I know the answer, at least in part: he has been an advocate of the signature since 1998, and he must now feel bad for that beautiful process being proven hard to see, by his “enemies”. Add to that the frustration of seeing the Tevatron producing brilliant results and excellent performances, while CMS and Atlas are sitting idly in their caverns, and you might figure out there is some human factor to take into account. But nothing, in my opinion, can justify the mix he put together: false allegations, disregard of published material, manipulation of plots, public defamation of respected colleagues. I am sorry to say it, but even though I have nothing personal against Michael Dittmar -I do not know him, and in private he might even be a pleasant person-, it will be very difficult for me to collaborate with him for the benefit of the CMS experiment in the future.

This just in – the Fermilab site has the news on the new exclusion in a range of Higgs masses. At 95% C.L., the Higgs boson cannot have a mass in the 160-170 GeV range, as shown in the graph below. The new limit is shown by the orange band.

This is the first real exclusion range on the Higgs boson mass from CDF and DZERO.  I will have more to say about this great new result during the weekend.

UPDATE: maybe the most interesting thing is not the limit shown above, but the information contained in the graph shown below. It shows how the combination of CDF and DZERO searches for the Higgs bosons end up agreeing with the background-only hypothesis (black hatched curve) or the background plus signal hypothesis (red curve), as a function of the unknown value of the Higgs boson mass. The full black line seems to favor the signal plus background hypothesis, although only marginally and at just the 1-sigma level, at around 130 GeV of mass:

However, they say that if you like sausages and if you follow laws, you should not ask how these things are made. The same goes with global limits, to some extent. In this case it is not a criticism of the limit by itself, but rather of the interpretation that one might be led to give to it. In fact, the width of the green band should put you en garde against wild speculations: It would be extremely suspicious if the black line did not venture outside of the green band somewhere, even in case the Higgs boson does not exist!

That is because the band shows the expected range of 1-sigma fluctuations -due to statistical effects, and not to systematic ones such as the real presence of a signal!- and since the black curve is extracted from the data by combining many datasets and each individual point of the line (in, say, 5-GeV intervals) has little correlation with the others, it is entirely appropriate for the curve to not be fully contained in the green area! So, the fact that the black curve overlaps with the signal plus background hypothesis at 130 GeV really -really!- means very, very little.

What does mean something is that the hatched black and red curves appear separated by about one-sigma (the width of the green band surrounding the background-only black hatched curve) over a wide range of Higgs masses. This says that the two Tevatron experiments have by now reached a sensitivity of about 1-sigma to the signal with the data they have analyzed so far. Beware: they are already sitting on about twice as much data (most analyses rely on about 2.5/fb of collisions, but the Tevatron has already delivered to the experiments over 5/fb). So they expect new results, significantly improved, by this summer.

It does seem that at last, the game of Higgs hunting is starting to get exciting again, after a hiatus of about 7 years following the tentative signal seen by the LEP II experiments!

It was with great pleasure that I found yesterday, in the public page of the DZERO analyses, a report on their new search for Higgs boson decays to photon pairs. On that quite rare decay process -along with another not trivial decay, the reaction- the LHC experiments base their hopes to see the Higgs boson if that particle has a mass close to the LEP II upper bound, i.e. not far from 115 GeV. And this is the first high-statistics search for the SM Higgs in that final state to obtain results that are competitive with the more standard searches!

My delight was increased when I saw that results of the DZERO search are based on a data sample corresponding to a whooping 4.2 inverse-femtobarns of integrated luminosity. This is the largest set of hadron-collider data ever used for an analysis. 4.2 inverse femtobarns correspond to about three-hundred trillion collisions, sorted out by DZERO. Of course, both DZERO and CDF have so far collected more than that statistics: almost five inverse femtobarns. However, it always takes some time before calibration, reconstruction, and production of the newest datasets is performed… DZERO is catching up nicely with the accumulated statistics, it appears.

The most interesting few tens of billions or so of those events have been fully reconstructed by the software algorithms, identifying charged tracks, jets, electrons, muons, and photons. Yes, photons: quanta of light, only very energetic ones: gamma rays.

When photons have an energy exceeding a GeV or so (i.e. one corresponding to a proton mass or above), they can be counted and measured individually by the electromagnetic calorimeter. One must look for very localized energy deposits which cannot be spatially correlated with a charged track: something hits the calorimeter after crossing the inner tracker, but no signal is found there, implying that the object was electrically neutral. The shape of the energy deposition then confirms that one is dealing with a single photon, and not -for instance- a neutron, or a pair of photons traveling close to each other. Let me expand on this for a moment.

Background sources of photon signals

In general, every proton-antiproton collision yield dozens, or even hundreds of energetic photons. This is not surprising, as there are multiple significant sources of GeV-energy gamma rays to consider.

1. Electrons, as well as in principle any other electrically charged particle emitted in the collision, have the right to produce photons by the process called bremsstrahlung: by passing close to the electric field generated by a heavy nucleus, the particle emits electromagnetic radiation, thus losing a part of its energy. Note that this is a process which cannot happen in vacuum, since there are no target nuclei there to supply the electric field with which the charged particle interacts (one can have bremsstrahlung also in the presence of neutral particles, in principle, since what matters is the capability of the target to absorb a part of the colliding body’s momentum; but in that case, one needs a more complicated scattering process, so let us forget about it). For particles heavier than the electron, the process is suppressed up to the very highest energy (where particle masses are irrelevant with respect to their momenta), and is only worth mentioning for muons and pions in heavy materials.
2. By far the most important process for photon creation at a collider is the decay of neutral hadrons. A high-energy collision at the Tevatron easily yields a dozen of neutral pions, and these particles decay more than 99% of the time into pairs of photons, . Of course, these photons would only have an energy equal to half the neutral pion mass -0.07 GeV- if the neutral pions were at rest; it is only through the large momentum of the parent that the photons may be energetic enough to be detected in the calorimeter.
3. A similar fate to that of neutral pions awaits other neutral hadrons heavier than the : most notably the particle called eta, in the decay . The eta has a mass four times larger than that of the neutral pion, and is less frequently produced.
4. And other hadrons may produce photons in de-excitation processes, albeit not in pairs: excited hadrons often decay radiatively into their lower-mass brothers, and the radiated photon may display a significant energy, again critically depending on the parent’s speed in the laboratory.

All in all, that’s quite a handful of photons our detectors are showered with on an event-by-event basis! How the hell can DZERO sort out then, amidst over three hundred trillion collisions, the maybe five or ten which saw the decay of a Higgs to two photons ?

And the Higgs signal amounts to…

Five to ten events. Yes, we are talking of a tiny signal here. To eyeball how many standard model Higgs boson decays to photon pairs we may expect in a sample of 4.2 inverse femtobarns, we make some approximations. First of all, we take a 115 GeV Higgs for a reference: that is the Higgs mass where the analysis should be most sensitive, if we accept that the Higgs cannot be much lighter than that: for heavier higgses, their number will decrease, because the heavier a particle is, the less frequently it is produced.

The cross-section for the direct-production process (where with X we denote our unwillingness to specify whatever else may be produced together with the Higgs) is, at the Tevatron collision energy of 1.96 TeV, of the order of one picobarn. I am here purposedly avoiding to fetch a plot of the xs vs mass to give you the exact number: it is in that ballpark, and that is enough.

The other input we need is the branching ratio of H decay to two photons. This is the fraction of disintegrations yielding the final state that DZERO has been looking for. It depends on the detailed properties of the Higgs particle, which likes to couple to particles depending on the mass of the latter. The larger a particle’s mass, the stronger its coupling to the Higgs, and the more frequent the H decay into a pair of those: the branching fraction depends on the squared mass of the particle, but since the sum of all branching ratios is one -if we say the Higgs decays, then there is a 100% chance of its decaying into something, no less and no more!- any branching fraction depends on ALL other particle masses!!!

“Wait a minute,” I would like to hear you say now, “the photon is massless! How can the Higgs couple to it?!”. Right. H does not couple directly to photons, but it can nevertheless decay into them via a virtual loop of electrically charged particles. Just as happens when your US plug won’t fit into an european AC outlet! You do not despair, and insert an adaptor: something endowed with the right holes on one side and pins on the other. Much in the same way, a virtual loop of top quarks, for instance, will do a good job: the top has a large mass -so it couples aplenty to the Higgs- and it has an electric charge, so it is capable of emitting photons. The three dominant Feynman diagrams for the decay are shown above: the first two of them involve a loop of W bosons, the third a loop of top quarks.

So, how much is the branching ratio to two photons in the end ? It is a complicated calculus, but the result is roughly one thousandth. One in a thousand low-mass Higgses will disintegrate into energetic light: two angry gamma rays, each roughly carrying the energy of a 2 milligram mosquito launched at the whooping speed of four inches per second toward your buttocks.

Now we have all the ingredients for our computation of the number of signal events we may be looking at, amidst the trillions produced. The master formula is just

where is the number of decays of the kind we want, is the production cross section for Higgs at the Tevatron, is the integrated luminosity on which we base our search, and B is the branching ratio of the decay we study.

With , , and , the result is, guess what, 4.2 events. 4.2 in three hundred trillions. A needle in the haystack is a kids’ game in comparison!

The DZERO analysis

I will not spend much of my and your time discussing the details of the DZERO analysis here, primarily because this post is already rather long, but also because the analysis is pretty straightforward to describe at an elementary level: one selects events with two photons of suitable energy, computes their combined invariant mass, and compares the expectation for Higgs decays -a roughly bell-shaped curve centered at the Higgs mass and with a width of ten GeV or so- with the expected backgrounds from all the processes capable of yielding pairs of energetic photons, plus all those yielding fake photons. [Yes, fake photons: of course the identification of gamma rays is not perfect -one may have not detected a charged track pointing at the calorimeter energy deposit, for instance.] Then, a fit of the mass distribution extracts an upper limit on the number of signal events that may be hiding there. From the upper limit on the signal size, an upper limit is obtained on the signal cross-section.

Ok, the above was a bit too quick. Let me be slightly more analytic. The data sample is collected by an online trigger requiring two isolated electromagnetic deposits in the calorimeter. Offline, the selection requires that both photon candidates have a transverse energy exceeding 25 GeV, and that they be isolated from other calorimetric activity -a requirement which removes fake photons due to hadronic jets.

Further, there must be no charged tracks pointing close to the deposit, and a neural-network classifier is used to discriminate real photons from backgrounds using the shape of the energy deposition and other photon quality variables. The NN output is shown in the figure below: real photons (described by the red histogram) cluster on the right. A cut on the data (black points) of a NN output larger than 0.1 accepts almost all signal and removes 50% of the backgrounds (the hatched blue histogram). One important detail: the shape of the NN output for real high-energy photons is modeled by Monte Carlo simulations, but is found in good agreement with that of real photons in radiative Z boson decay processes, . In those processes, the detected photon is 100% pure!

After the selection, surviving backgrounds are due to three main processes: real photon pairs produced by quark-antiquark interactions, compton-like gamma-jet events where the jet is mistaken for a photon, and Drell-Yan processes yielding two electrons, both of which are mistaken for photons. You can see the relative importance of the three sources in the graph below, which shows the diphoton invariant mass distribution for the data (black dots) compared to the sum of backgrounds. Real photons are in green, compton-like gamma-jet events are in blue, and the Drell-Yan contribution is in yellow.

The mass distribution has a very smooth exponential shape, and to search for Higgs events DZERO fits the spectrum with an exponential, obliterating a signal window where Higgs decays may contribute. The fit is then extrapolated into the signal window, and a comparison with the data found there provides the means for a measurement; different signal windows are assumed to search for different Higgs masses. Below are shown four different hypotheses for the Higgs mass, ranging from 120 to 150 GeV in 10-GeV intervals. The expected signal distribution, shown in purple, is multiplied by a factor x50 in the plots, for display purposes.

From the fits, a 95% upper limit on the Higgs boson production cross section is extracted by standard procedures. As by now commonplace, the cross-section limit is displayed by dividing it by the expected standard model Higgs cross section, to show how far one is from excluding the SM-produced Higgs at any mass value. The graph is shown below: readers of this blog may by now recognize at first sight the green 1-sigma and yellow 2-sigma bands showing the expected range of limits that the search was predicted to set. The actual limit is shown in black.

One notices that while this search is not sensitive to the Higgs boson yet, it is not so far from it any more! The LHC experiments will have a large advantage with respect to DZERO (and CDF) in this particular business, since there the Higgs production cross-section is significantly larger. Backgrounds are also larger, however, so a detailed understanding of the detectors will be required before such a search is carried out with success at the LHC. For the time being, I congratulate with my DZERO colleagues for pulling off this nice new result!

Three weeks ago the DZERO collaboration published new results of their low-mass Higgs boson search. This is about the production of Higgs bosons in association with a W boson, with the subsequent decay of the Higgs particle to a pair of b-quark jets, and the decay of the W to an electron-electron neutrino or muon- muon neutrino pair: in symbols, what I mean is , or . I wish to describe this important new analysis today, but first let me make a point about the reaction above.

In order to make this blog more accessible than it would otherwise be, I frequently write things inaccurately: precision is usually pedantic and distracting. But here I beg you to please note a detail I will not gloss over for once: to be accurate, one should write …, because what we care for is inclusive production of the boson pair. If we omit the X, strictly speaking we are implying that the two protons annihilated into the two bosons, with exactly nothing else coming out of the collision. While that reaction is possible, it is ridiculously rare -actually, the annihilation into ZH is possible, while the one into WH does not conserve electric charge and is strictly forbidden. Anyway, bringing along a symbol to remind ourselves of the fact that our projectiles are like garbage bags, which fill our detectors with debris when we throw them at one another, is cumbersome and annoying, while accurate. I hope, however, you realize that this is an important detail: Higgs bosons at a hadron collider are always accompanied by debris from the dissociating projectiles.

Two words on associated WH production and its merits

The associated production of the Higgs together with a W boson is the “golden” signature for low-mass Higgs hunters at the Tevatron collider. While producing the Higgs together with another heavy object is not effortless (you are required to produce the collision with more energetic quarks in the two colliding protons, and this makes the production less frequent), the W boson pays back with extra dividends by producing a very clean signature in its leptonic decay, and by allowing the event to be spotted easily by the online triggering system, and collected with high efficiency by the data acquisition.

If you compare the collection of WH events to the collection of directly produced Higgs bosons (, where again I prefer accuracy by specifying the X), you immediately see the advantage of the former: while their production rate is four times smaller and the leptonic W decay only occurs 20% of the times, this 0.25 x 0.2=0.05=1/20 reduction factor is a small price to pay, given the trouble one would have triggering on direct events: the decay to a pair of b quarks is the dominant one for low Higgs boson masses, but the common nature of b-jets makes it unobservable.

Higgs decays to b-quark pairs produced alone simply cannot be triggered in hadronic collisions, because they are immersed in a background which is six orders of magnitude higher in rate, namely the production of bottom-antibottom quark pairs by strong interactions. Even assuming that the online triggering system of DZERO were capable of spotting b-quark jet pairs with 100% purity (which is already a steep hypothesis), the trigger would have to accept a million background events in order to collect just one fine signal event !

Yes, life is tough for hadronic signatures at a hadron collider. Even finding the signal, which is a thousand times more frequent, is a tough business -it took CDF years to find a reasonable sample of those decays, while DZERO has not yet published anything on the matter. But the Tevatron experiments cannot ignore the fact that, if a low Higgs mass is hypothesized, the decay is the most frequent: the Higgs boson likes to decay into the heaviest pair of particles it can produce. If the total mass of a pair of W bosons or Z bosons is too heavy, the next-heaviest pair of decay products is b-quarks. This dictates the need to search for , and the trouble of triggering on such a process in turn makes the associated WH (or ZH) production the most viable signal.

The DZERO analysis

The new analysis by DZERO studies a total integrated luminosity of 2.7 inverse femtobarns. This corresponds to 150 trillion proton-antiproton collisions, but DZERO has netted almost twice as much data already by now, and it is only a matter of time before those too get included in this search: so one has to bear in mind that the statistical power of the data is soon going to increase by about 40%: the data increase corresponds to an increase in precision by the square root of two, or a factor of 1.41.

DZERO selects events which have an electron or a muon with high energy -the tag of a leptonic decay of the W boson-, missing transverse energy, and two or three hadronic jets. The presence of a energy imbalance in the plane transverse to the beam direction is a comparatively clean signature of the escape of the energetic neutrino produced together with the charged lepton by the W decay, and two jets are expected from the decay of the Higgs boson to a pair of b quarks. However, you might well ask, quid opus fuit tertium ?

No, I bet you would not ask it that way -for some reason, a reminescence of Latin sprung up in my mind. Quid opus fuit tertium – What is the matter with the third one ? The third jet is not specifically a signature of any one of the decay products of the WH pair we are after. However, if you remember what I mentioned above, we are searching for inclusive production of a WH pair: that means we accept the fact that the two projectiles also produced an additional energetic stream of hadrons in the final state. That possibility is by no means rare, and in fact it amounts to about 20% of the Higgs production events. By selecting events with two or three jets, DZERO increases its acceptance of signal events sizably.

A technique which has become commonplace in the hunt of elusive subnuclear particles is to slice and dice the data: categorizing events in disjunct classes is a powerful analysis strategy. By taking two-jet events on one side, and three-jet events on the other, DZERO can study them separately, and appreciate the different nuisances of each class. In fact, they further divide the data into subsets where one jet was tagged as a b-quark-originated one, or two of them were.

And they also keep separated the electron+jets and the muon+jets events: this also does make sense, since the experimental signatures of electrons and muons are slightly different, as are the resulting energy resolutions. In total, one has eight disjunct classes, depending on the number of jets, the number of b-tags, and the lepton species.

In order to decide whether there is a hint of Higgs bosons in any of the classes, backgrounds are studied using Monte Carlo simulations of all the Standard Model processes which could contribute to the eight selected signatures. These include the production of a W boson plus hadronic jets (“W+jets“) as well as the production of top quark pairs: both these processes produce energetic leptons in the final state; but another background is due to events which do not actually contain a lepton, and where a hadronic jet was mistook for one. The latter is called “QCD background” highlighting its origin in strong interaction processes yielding just hadronic jets: despite the rarity of a jet faking a energetic lepton, the huge rate of QCD events makes this background sizable.

Among the characteristics that can separate the WH signal from the above backgrounds, the identity of the parton originating the hadronic jets is a powerful one: b-jets are more rare than light-quark ones, but there must be two of them in a decay. DZERO uses a neural network which employs seven discriminating variables to select jets with a likely b-quark content.

The good thing with a neural-network b-tagger is that the output of the network can be dialed to decide its purity. And in fact, DZERO does exactly that. They start with a loose selection which has a rate of “false positives” of 1.5% (light-quark jets that are classified as b-tagged). If two jets have such a loose b-tag, the event is classified as a “double b-tag”; otherwise, the NN output requirement is made tighter, and “single-b-tag” events are collected by requiring that the b-tag has a better purity, with a “false positive” rate of 0.5%. These cuts have been optimized for their combined sensitivity to the Higgs signal.

Apart from b-tags, the signal displays a different kinematics than all backgrounds. Again, seven variables are used, which now describe the event kinematics: the transverse energy of the second-leading jet, the angle between jets, the dijet invariant mass, and a matrix-element discriminant, which is computed by comparing the probability density of the quadrimomenta of the objects produced in the decay in a WH event to that of backgrounds. In the figure above, the matrix element discriminant is shown for all the processes contributing to the class of W+2jet events with two b-tags. The output of the neural network shows that Higgs events fall in the right-side of the distribution, while backgrounds pile up mostly on the left, as can be seen in the figure below.

Results of the search

Since no signal is observed in the NN output distribution seen in the data, DZERO proceeds to set upper limits on the signal cross-section. For 2-jet events they use the NN output is used, while they use the dijet mass distribution for the 3-jet event classes. No justification is provided in their paper for this choice, which looks slightly odd to me, but I imagine they have done some optimization studies before taking this decision. However, I would imagine that the NN output is in principle always more discriminant than just one of the variables on which the network is constructed… Maybe somebody from DZERO could clarify this point in the comments thread, to the benefit of the other readers ?

At the end of the day, DZERO obtains limits on the cross section of the searched signal, which are still above the standard model predictions whatever the Higgs mass: therefore, they do not provide an exclusion of mass values, yet. These results, however, once combined with other results from CDF and DZERO, will one day directly imply that a SM Higgs cannot exist, if its mass is in a specified range. In the graph below you can see the limit set by this analysis on the WH production cross-section as a function of Higgs mass.

The black curve shows the 95% exclusion, while the hatched red curve shows the result that DZERO was expecting to find, based on pseudoexperiments. The comparison of the two curves is not terribly informative, but it does show that there were not surprises from the data.

The result can also be shown in the standard “LLR plot” above, which is showing, again as a function of the Higgs boson mass, the log-likelihood ratio of two hypotheses: the “background only” and the “signal+background” one. Let me explain what that is. For each mass value on the x-axis, imagine the Higgs is there. Then, with large statistics, the data would show a propension for the “signal plus background” hypothesis, and the LLR would be large and negative. If, instead, the Higgs did not exist at any mass value, the LLR would be large and positive. The two hypotheses can be run on pseudo-data of the same statistical power as the data really collected, thus producing the red and black hatched lines in the plot below. The two curves are different, but the red one does not manage to depart from the green band constructed around the black hatched one: that means that the data size and the algorithms used in the analysis do not have enough power to discriminate the two hypotheses, not even at 1-sigma level (which is the meaning of the width of the green band, while the yellow one shows two-sigma contours). The full black line shows the behavior of real data: they have a propension of confirming the background-only hypothesis at low mass, and a slight penchant for the signal+background one at about 130 GeV. But this is a really, really small fluctuation, well within the one-sigma band!

I think the LLR plot is a great way to describe the results of the search visually. It at once tells you the power of the analysis and the available data, and the outcome on the real events collected. Now, it takes twenty thick lines of text to explain it, but once you’ve grabbed its meaning…

Here I am, once again improperly and shamelessly using this public arena for my personal gain. This time, I need help from one of you who has a subscription to the American Association for the Advancement of Science.

It so happens that a couple of weeks ago I gave a phone interview to Adrian Cho about the LHC, the Tevatron, and the hunt for the Higgs boson. We discussed various scenarios, the hunt going on at the Tevatron, and other stuff. I am curious to know what Adrian made of our half-hour chat. Today, I realized that the article has been published, but I have no access to the it, since it is available at the Science Magazine site only for AAAS members.

I have to say, Adrian should have been kind enough to forward me a copy of the piece that benefitted from the interiew. I am sure he forgot to do it and once he reads this he will regret it, or maybe he thinks I am a member of the AAAS already… Adrian, you are excused. But this leaves me without the article for a while, and I am a curious person… So if you have an AAAS account and you are willing to break copyright rules, I beg you to send me a file with the article! My email is dorigo (at) pd (dot) infn (dot) it. Thank you!

And, to show you just how serious I am when I say I am shameless, here’s more embarassment: if you are a big shot of the AAAS, do you by any chance give free membership to people who do science outreach to the sole benefit of the advancement of Science  ?

UPDATE: I am always amazed by the power of internet and blogging. These days you just have to ask and you will be given! So, thanks to Peter and Senth, I got to read the article by Adrian Cho.

I must admit I am underwhelmed. Not by the article, which is incisive and to the point. Only, I should know that science journalists quote you for 1% of what you tell them, and use the rest to get informed and write a better piece. In fact, the piece starts by quoting me:

“Three years ago, nobody would have bet a lot that the Tevatron
would be competitive [with the LHC] in the Higgs search. Now I think the tables are almost turned,” says Tommaso Dorigo, a physicist from the University of Padua in Italy who works with the CDF particle detector fed by the Tevatron and the CMS particle detector fed by the LHC.

… but that is the only quote. I can console myself by noting I am in quite good company: experiment spokespersons, Fermilab director Pier Oddone, CERN spokesperson James Gillies…

Two years ago I used the combined Higgs search limits produced by the D0 experiment to evaluate how well the Tevatron was doing if compared with the predictions that had been put together by the 1999 SUSY-HIGGS working group, and later by the 2003 Higgs Sensitivity Working Group (HSWG), two endeavours to which I had participated with enthusiasm. The picture that emerged was that, although results were falling short of justifying fully the early predictions, there was still hope that those would one day be vindicated.

Indeed, I remember that when in 2003 the HSWG produced its report, we felt our results were greeted with a dose of scepticism. And we ourselves were a bit embarassed, because we knew we had been a bit optimistic in our predictions: however, that was the name of the game – looking at things on their bright side, for the sake of convincing funding agents that the Tevatron had a reason to run for a long time. I felt a strong justification for being optimistic in the incredible results on the top quark mass that the Tevatron had already started achieving: early prospects of measuring the top mass to a 1% uncertainty have in fact been surpassed by the combination of dedication of the scientists doing the analyses, and their imagination in inventing new precise methods.

We now have a chance to look back at the 1999/2003 predictions for the Higgs reach of the Tevatron with a rather solid set of hard data: the CDF combination, which I briefly discussed two days ago, is based on analyzed sets of data ranging from 2 to 3 inverse femtobarns, and the comparisons do not require a lot of extrapolations to be carried out.

If we look at the 1999/2003 predictions shown above (two basically coincident results, if one considers that the 2003 results were not accounting for systematic effects, which would worsen a bit the curves of sensitivity and bring them to match the older ones), we can read off the integrated luminosity that the Tevatron experiments needed to analyze in order to exclude, by combining their results, SM Higgs production at 95% confidence level. These numbers are as follows: for a Higgs mass of 100 GeV, 1/fb was considered sufficient; for a Higgs mass of 120 GeV, 2/fb were needed; 10/fb at 140 GeV; 4.5/fb at 160 GeV; 8/fb at 180 GeV; and 80/fb at 200 GeV. You can check them on the purple band in the graph above.

Now, let us take the actual expected limits by CDF with the analyses and the data they have based their new result upon (using expected limits rather than observed ones is correct, since the former are unaffected by statistical fluctuations). At 100 GeV, CDF has a reach in the 95%CL limit at 2.63xSM; at 120 GeV, the reach is 3.72xSM; at 140 GeV, 3.61xSM; at 160 GeV it is 1.75xSM; at 180 GeV 3.02xSM; and at 200 GeV, the reach is at 6.33xSM.

(Below, the 2009 combined CDF limits are shown by the thick red curve; the data I list above is based on the hatched curve instead, which shows the expected limit.)

How do we now compare these sets of numbers ?

Easy. As easy as 1.2.3.4 (well, not too easy, but that’s how it goes).

1. We first scale up by a factor of two the 1999/2003 luminosity numbers needed for a 95% CL exclusion, which we listed above. We thus get, for Higgs masses ranging from 100 to 200 GeV in 20-GeV steps, needed integrated luminosities of 2,4,20,9,16,160/fb.
2. Then, we take the actual luminosity used by CDF for the analyses that have been combined to yield the expected limits listed above. This is slightly tricky, since the combination includes analyses which have used 2.0/fb of data (the search), 2.1/fb (the search), 2.7/fb (the , the , and the searches), and 3.0/fb (the search). In principle, we should weight those numbers with the relative sensitivity of the various analyses, but we can approximate it by taking an “average effective luminosity” of 2.4/fb for the 100 GeV Higgs search, 2.7/fb for the 120 and 140 GeV points, and 3.0/fb for the high-mass searches. This is appropriate, since the search starts kicking in above 140 GeV.
3. We now have all the numbers we need: we divide the expected luminosity needed for one experiment by the 1999/2003 study, found at point 1 above, by the effective luminosities found at point 2, and take the square root of that number: this means finding the “reduction factor” in the sensitivity that the actual CDF data suffers with respect to the data needed to exclude the Higgs boson. We find a reduction factor of 0.91, 1.22, 2.72, 1.73, 2.31, and 7.30 for Higgs masses of 100,120,140,160,180, and 200 GeV respectively.
4. Now we are done. We can compare the “times the SM” limits of CDF with the numbers found at point 3 above. The ratio of the two says how much worse is CDF doing with respect to predictions, for each mass point. We find that CDF is doing 2.88 times worse than predictions at 100 GeV; 3.06 times worse than predictions at 120 GeV; 1.33 times worse at 140 GeV; 1.01 times worse at 160 GeV; 1.31 times worse at 180 GeV; and 0.87 times worse (i.e., 1.15 times better!) at 200 GeV.

The results of point 4 are plotted on the graph shown above, where the x-axis shows the Higgs mass, and the y axis this “shame factor”. I have given a 20% uncertainty to the figures I computed, because of the rather rough way I extracted the numbers from the 1999/2003 prediction graph. If you look at the graph, you notice that the CDF experiment has kept its (our!) promise (points bouncing around a ratio of 1.0) with its high-mass searches, while low-mass searches still are a bit below expectations in terms of reach (3x worse reach than expected). It is not a surprise: at low Higgs mass, the searches have to rely on the final state, which is very difficult to optimize (vertex b-tagging, dijet mass resolution, lepton acceptance are the three things on which CDF has been spending hundreds of man-years in the last decade). Give CDF (and DZERO) enough time, and those points will get down to 1.0 too!

A brand-new combination of Higgs boson cross-section limits has been recently produced by the CDF experiment for the 2009 winter conferences. The results are almost one month old, but I decided to wait a bit before posting them here, in order to avoid arising bad feelings in a few of my CDF colleagues, those who believe I have no right to post here published results in too timely a fashion, because they feel those results should first be presented at conferences by the real authors of the analyses.

Now I think it is due time to have the most relevant plots here, since they are all available from a public web page of CDF anyway; so here we go, with the most updated information. Mind you, these are CDF-only results: a sizable improvement in the limits will come when they get combined with the findings of DZERO. I seem to understand that the Tevatron combination group folks are dragging their feet this year, so we have better to just as well take the CDF results and comment them alone.

The first graph is the most important one of all: it describes the combination of CDF results, in the usual “95% CL limit on times the SM cross section“. It is shown below.

On the x axis is the Higgs boson mass, and on the y axis the cross-section limit. Different colors of the curves refer to different analyses, which target the various decay channels of the sought particle; hatched lines show expected limits, and full ones show instead the limits actually obtained by the analysis.

As you can see by examining the thick red curve at the bottom, CDF by itself cannot rule out the 170-GeV point, which last summer was excluded by the CDF+D0 combination. However, a sizable improvement can be observed across the board in the results. The red curve, for one thing, is considerably flatter than it used to be, a sign that the low-mass searches have started to pitch in with momentum. Another thing to note is that these results correspond to 3.0/fb of analyzed luminosity or less (2.4/fb at low mass): there is already at least twice as much data waiting to be analyzed, and results are thus expected to sizably improve their sensitivity.

The above summary brings me to mention another important point. By looking at the graph, you might run the risk of failing to appreciate the enormous effort which CDF is putting into these searches. In truth, the name of the game is not “wait more data and turn the crank”. Quite the opposite! The most important improvements in the discovery reach have been achieved in the course of the last five years by continuously improving the algorithms, the search methods, by refining tools, by finding new avenues of investigation, and new search channels neglected before. This is summarized masterfully in the two graphs shown below.

Above, you can see that for a Higgs mass of 115 GeV, the limit that CDF was able to set on its existence, in terms of cross section (well, “times the SM cross section” units to be precise: the ones shown on the y axis) has improved much more than what one would have expected by scaling down the limit with a simple square root law -the one that Poisson Statistics would dictate, for statistically-limited measurements. Quite the opposite: as time went by, the actual limits (colored points) have moved down almost vertically, a sign that the data has been used better and better! Above, if you took the extrapolation expected after the first limit was published (the one in green), you would expect that the limit today, with 2.4/fb analyzed, was at 7xSM, while it in fact is at 3xSM: this corresponds to a 2.3x improvement in the limit, which would have been granted by a 5.2 times larger analyzed dataset!!

Above, the same information is shown for the value. In this case, CDF is now expected to be able to set an exclusion alone with 9/fb of data, but we still expect to see some improvements in the data analyses, which should move the points well into the brown band. In this case, 7/fb of data might be enough.

The last two plots I wish to discuss are shown below. BEWARE: This is information that LHC scientists would really, really not like to see – so, if your life depends on the success of ATLAS or CMS, please stop reading now, take my advice.

OK. The plot below shows the probability that the Tevatron experiments, by combining their datasets and results, may observe a 2-sigma evidence for SM Higgs production, with 5/fb and 10/fb of data collected by each. If the analyses will not perform better than what they have so far, you get the full lines -red for 5/fb, blue for 10/fb. If they improve as much as it is reasonable to predict, you get the hatched lines.

What to gather from the plot ? Well: it seems that, regardless of the Higgs boson mass, the Tevatron has a sizable chance to be able to say something good, by the time CDF and D0 will have analyzed the datasets they already possess (which are in excess of 5/fb each: the delivered luminosity of the Tevatron is passing the 6/fb mark as we speak, and the typical live time of the experiments is above 80%).

Below, we see what is the chance of a 3-sigma evidence. Again, there is a sizable chance of that happening, although if no additional improvements occur in the analyses, it seems that the Tevatron will need to get lucky!

I remember that in 2005 I gave a talk in Corfù (Greece) where I ventured to speculate on the possible scenarios for Higgs searches at the Tevatron and the LHC. One of the scenarios saw the two experiments competing to find the particle with roughly equal reach, and eventually producing a combined observation. That possibility  does not seem too far-fetched any longer!

In the next few days I plan to discuss in some detail the most important analyses which contribute to the combination discussed above. Stay tuned…

Here is the second part of the list of useful physics posts I published on this site in 2008. As noted yesterday when I published the list for the first six months of 2008, this list does not include guest posts nor conference reports, which may be valuable but belong to a different place (and are linked from permanent pages above). In reverse chronological order:

December 29: a report on the first measurement of exclusive production of charmonium states in hadron-hadron collisions, by CDF.

December 19: a detailed description of the effects of parton distribution functions on the production of Z bosons at the LHC, and how these effects determine the observed mass of the produced Z bosons. On the same topic, there is a maybe simpler post from November 25th.

December 8: description of a new technique to measure the top quark mass in dileptonic decays by CDF.

November 28: a report on the measurement of extremely rare decays of B hadrons, and their implications.

November 19, November 20, November 20 again , November 21, and November 21 again: a five-post saga on the disagreement between Lubos Motl and yours truly on a detail on the multi-muon analysis by CDF, which becomes a endless diatriba since Lubos won’t listen to my attempts at making his brain work, and insists on his mistake. This leads to a back-and-forth between our blogs and a surprising happy ending when Motl finally apologizes for his mistake. Stuff for expert lubologists, but I could not help adding the above links to this summary. Beware, most of the fun is in the comments threads!

November 8, November 8 again, and November 12: a three-part discussion of the details in the surprising new measurement of anomalous multi-muon production published by CDF (whose summary is here). Warning: I intend to continue this series as I find the time, to complete the detailed description of this potentially groundbreaking study.

October 24: the analysis by which D0 extracts evidence for diboson production using the dilepton plus dijet final state, a difficult, background-ridden signature. The same search, performed by CDF, is reported in detail in a post published on October 13.

September 23: a description of an automated global search for new physics in CDF data, and its intriguing results.

September 19: the discovery of the baryon, an important find by the D0 experiment.

August 27: a report on the D0 measurement of the polarization of Upsilon mesons -states made up by a pair- and its relevance for our understanding of QCD.

August 21: a detailed discussion of the ingredients necessary to measure with the utmost precision the mass of the W boson at the Tevatron.

August 8: the new CDF measurement of the lifetime of the baryon, which had previously been in disagreement with theory.

August 7: a discussion of the new cross-section limits on Higgs boson production, and the first exclusion of the 170 GeV mass, by the two Tevatron experiments.

July 18: a search for narrow resonances decaying to muon pairs in CDF data excludes the tentative signal seen by CDF in Run I.

July 10: An important measurement by CDF on the correlated production of pairs of b-quark jets. This measurement is a cornerstone of the observation of anomalous multi-muon events that CDF published at the end of October 2008 (see above).

July 8: a report of a new technique to measure the top quark mass which is very important for the LHC, and the results obtained on CDF data. For a similar technique of relevance to LHC, also check this other CDF measurement.

To start 2009 with a tidy desk, I wish to put some order in the posts about particle physics I wrote in 2008. By collecting a few links here, I save from oblivion the most meaningful of them -or at least I make them just a bit more accessible. In due time, I will update the “physics made easy” page, but that is work for another free day.

The list below collects in reverse chronological order the posts from the first six months of 2008; tomorrow I will complete the list with the second half of the year. The list does not include guest posts nor conference reports, which may be valuable but belong to a different list (and are linked from permanent pages above).

June 17: A description of a general search performed by CDF for events featuring photons and missing transverse energy along with b-quark jets – a signature which may arise from new physics processes.

June 6: This post reports on the observation of the decay of J/Psi mesons to three photons, a rare and beautiful signature found by CLEO-c.

June 4 and June 5 offer a riddle from a simple measurement of the muon lifetime. Readers are given a description of the experimental apparatus, and they have to figure out what they should expect as the result of the experiment.

May 29: A detailed discussion of the search performed by CDF for a MSSM Higgs boson in the two-tau-lepton decay. Since this final state provided a 2.1-sigma excess in 2007, the topic deserved a careful look, which is provided in the post.

May 20: Commented slides of my talk at PPC 2008, on new results from the CDF experiment.

May 17: A description of the search for dimuon decays of the B mesons in CDF, which provides exclusion limits for a chunk of SUSY parameter space.

May 02 : A description of the search for Higgs bosons in the 4-jet final state, which is dear to me because I worked at that signature in the past.

Apr 29: This post describes the method I am working on to correct the measurement of charged track momenta by the CMS detector.

Apr 23, Apr 28, and May 6: This is a lengthy but simple, general discussion of dark matter searches with hadron colliders, based on a seminar I gave to undergraduate students in Padova. In three parts.

Apr 6 and Apr 11: a detailed two-part description of the detectors of electromagnetic and hadronic showers, and the related physics.

Apr 05: a general discussion of the detectors for LHC and the reasons they are built the way they are.

Mar 29: A discussion of the recent Tevatron results on Higgs boson searches, with some considerations on the chances for the consistence of a light Higgs boson with the available data.

Mar 25: A detailed discussion on the possibility that more than three families of elementary fermions exist, and a description of the latest search by CDF for a fourth-generation quark.

Mar 17: A discussion of the excess of events featuring leptons of the same electric charge, seen by CDF and evidenced by a global search for new physics. Can be read alone or in combination with the former post on the same subject.

Mar 10: This is a discussion of the many measurements obtained by CDF and D0 on the top-quark mass, and their combination, which involves a few subtleties.

Mar 5: This is a discussion of the CDMS dark matter search results, and the implications for Supersymmetry and its parameter space.

Feb 19: This is a divulgative description of the ways by which the proton structure can be studied in hadron collisions, studying the parton distribution functions and how these affect the scattering measurements in proton-antiproton collisions.

Feb 13: A discussion of luminosity, cross sections, and rate of collisions at the LHC, with some easy calculations of the rate of multiple hard interactions.

Jan 31: A summary of the enlightening review talk on the standard model that Guido Altarelli gave in Perugia at a meeting of the italian LHC community.

Jan 13: commented slides of the paper seminar gave by Julien Donini on the measurement of the b-jet energy scale and the cross section, the latter measured for the first time ever at a hadron machine. This is the culmination of a twelve-year effort by me and my group.

Jan 4: An account of the CDF search for Randall-Sundrum gravitons in the final state.

About a year ago I reported here on a search performed by CDF for events featuring two Z bosons, both decaying to electron-positron pairs: I had been an internal reviewer of that analysis, and I discussed it in some detail after we approved it for publication. While the standard model expectation for electroweak production of two Z bosons is of about 1.5 pb, and the process has indeed been put in evidence in CDF and D0 Run II data, the analysis was rather focused on a search for heavy mass resonances decaying to the ZZ final state: new physics, that is, either in the form
of a heavy Higgs boson, or of a graviton (in the Randall-Sundrum scenario), or other still fancier (and improbable) beasts.

CDF has now repeated that search by increasing the dataset size by a factor of three, and by including mixed final states which include muon pairs and even jet pairs. This makes the analysis intrinsically interesting to me, since I have started a similar analysis with the CMS experiment, together with a PhD student in Padova, Mia Tosi. Mia and I will be looking for Higgs bosons in the dilepton plus dijet final state, with particular emphasis on the decay, which is a signal with which we have quite some familiarity.

The new CDF search for high-mass ZZ events configures itself as a “signature-based” one: despite the reference to the Randall-Sundrum graviton, the analysis cuts are kept generic, such that a signal can be found for anything that decays to two Z bosons, and in case no signal is seen, a model-independent limit on the cross section can be set. The only limitation of the search is that the four-body mass is studied only above the minimum value of 300 GeV. Such a requirement allows to steer away from phase space regions where backgrounds dominate.

Once four objects (electrons, muons, and jets, with the specification that at most two jets are present) are selected with loose cuts, a statistical estimator is built to test the hypothesis that they originate from the decay . It is a simple function, which utilizes the expected resolution on the two two-body masses and the resulting four-body mass to estimate how much the event departs from the tentative signal interpretation. Only in the case of jet pairs, an explicit cut is set on the dijet mass to lay between 65 and 120 GeV, to avoid accepting too many random jet combinations.

While the region is the one where the signal is sought, the complementary region of the four-body mass is used as a control sample, to verify that background estimates obtained with Monte Carlo simulations are in agreement with the observed data. The nice thing about such a spectacular signature as the production of two Z bosons is that backgrounds are exclusively of electroweak nature: by having at least one decay in the final state, the signal cannot be mimicked easily by purely quantum chromodynamical processes, which plague most hadron collider searches with high rates. Besides regular pairs from standard model processes, backgrounds include WZ, WW, and Z+jets production. At high four-body mass, however, all of these are really small, and even in the 3 inverse femtobarns of proton-antiproton collisions analyzed by CDF for this search, they contribute only few events; only the dilepton+dijet signature accepts a few hundred events, because of the large cross-section of Z+2 jet production processes.

In the end, no signal is seen, and a cross-section limit is extracted as a function of the X mass. The limit is shown below, compared to the expected cross section for graviton production and decay to the ZZ final state. The comparison of upper limit (the red curve) with the theory hatched line allows to exclude gravitons with masses below 491 GeV, for a particular choice of model parameters (k is a warp factor for the extra dimensions, and is the Planck mass).

As a by-product of this analysis, a new set of excellent standard-model-like ZZ decay candidates have been selected. I am unable to show any of the new event displays here, because they have not been approved for public consumption by CDF yet… So please see the lego plot of a candidate below, extracted last year by the same authors. The two pairs of electrons make masses very close to that of the Z boson, as evidenced by the two pink numbers.

To read this graph, you have to know that the greek letter is the pseudorapidity, basically a function of the angle that particles make with the beam axis. A pseudorapidity of zero means that the particle is emitted at 90 degrees from the beam, while positive and negative values indicate the proton and antiproton directions. The other coordinate, , indicates the azimuthal angle in the transverse plane. The z axis (the height of the bars) indicates how much energy is deposited in the interval span by the bars. In bright pink are shown the four electron candidates, as measured by the CDF calorimeter, and each bar is labeled by the energy in GeV measured for each.

I am only left with the pleasant task of congratulating my colleagues Antonio Boveia, Ben Brau, and David Stuart for this new result, which greatly extends the scope of the analysis I have reviewed last year. During my review I had encouraged them to pursue the other decay modes of ZZ pairs, and so they did. Well done, folks!

The regulars here will have already noticed by now that my posting rate has fallen this week. I have been busy with three different physics analyses, trying to make some progress in each.

The first project is the calibration of the momentum scale in CMS. I have discussed the issue elsewhere a couple of times; I am slowly converging to an understanding of how to treat the Z boson lineshape -which receives contributions from a number of different sources and effects: parton distribution functions in the projectiles, electromagnetic and weak radiation effects, interaction of the final state products of Z decay with the material of the tracker. All this must be dealt with in a coherent fashion to extract the most information possible from the Z decays we will reconstruct in CMS. We have a small but focused group working at the momentum scale calibration, including worthy physicists from Torino University, plus Marco and me. This week, I have tried to determine the effect of parton distribution functions alone, to insert it in our algorithm, but something still escapes me, and I want to do things as well as I can -which sometimes take little extra effort from a mediocre result, but in this case seems to be requiring a lot more care.

The second is the search for Higgs boson decays in the final state arising when H decays to two Z bosons, and one of the Z decays to a lepton pair, while the other decays to a pair of jets. Usually this final state, which is very hard to exploit at low Higgs masses due to the large backgrounds, is used for high-mass searches only (above 200 GeV). We want to extend it to lower masses, where the Higgs is more likely to be, using the decay, which Mia and I have a lot of experience in detecting in hadronic environments. Mia will present some results of this study tomorrow at CERN, so we have been working at this heavily this week.

The third topic is the evaluation of the chances of CMS to detect a similar signature of multi-muon events that CDF has seen in its data. The CDF signal is probably just a not well understood background, but it makes sense to size up the capability of CMS to detect a similar signature with early data. This requires understanding muon sources without using real data, and it is a bit far-fetched, but it is perfectly sound as a masters’ thesis topic, one on which Franco and I in fact have a student working. I have not worked much on this topic this week, but it still has absorbed a little of CPU.

I have a thick agenda of pending things to do, which has grown longer in the last few days. One thing is to post more commentaries on the multi-muon analysis by CDF here. Another is to progress with a document I am writing. A third is to review a 40-pages long CDF paper draft for the Spokespersons Reading Group, to which I proudly belong. A fourth is to organize the upcoming meeting of the CMS-Padova software-analysis group, which will convene in ten days. A fifth is to prepare my next trip to CERN, which will be from next Monday to next Friday. I do hope that I will be able to post more in the next few days… if I survive.

The CDF authors of the study which is causing ripples in the blogosphere have published tonight a second paper, where they try to interpret the excess of events with large impact-parameter muon tracks within a phenomenological model of new physics. You can find their paper here.

In short, they try to fit the observed muon multiplicity within narrow cones, as well as their quite peculiar kinematic characteristics, with the decay of a heavy object which produces a cascade of long-lived particles, ending with a multi-muon signature.

The paper was born as part of the other document (see the story in the post below), but was extracted from it and published separately since this was the best way to proceed promptly to a publication of both. As you see by checking the arxiv entry, this second preprint has only the names of the very authors of the study on the multi-muon anomaly.

I will have more detail on the physics later…

In the previous post I mentioned a piece I wrote for Il Sole – 24 Ore, an italian daily newspaper which is less read than Repubblica and Il Corriere della Sera, but is more accurate in political and financiary analysis, and has a good reputation overall (despite being owned by Confindustria).

I wrote that piece shortly after the publication on New Scientist – soon followed by the Economist – of imprecise accounts of the issue with the small (2-sigma) excess of tau-lepton pairs  unearthed by CDF in a search for supersymmetric Higgs bosons. That story is old and I do not wish to tell it again (unless you really ask). However, I can disclose today, 18 months after the fact, a few details which I had kept for myself back then, besides the one I already disclosed in the former post (that is, that I was the author of the piece).

I was contacted by journalists from Il Sole-24 Ore for a comment on the issue, and I was kind enough to explain the matter, after asking the journalists to avoid mentioning my name in their pieces. One reporter asked me to check the physics of the article she was writing, and upon giving a glance at her terrible draft, I decided I would accept their original offer of writing a piece myself, at the condition that my name would not appear. In fact, some colleagues in CDF were not happy with the blogging John Conway and I had done about the issue, and we were identified as the source of the trouble with the New Scientist and Economist papers. I did apologize with them, despite not feeling guilty of any misdemeanor. However, I wanted to have the newspaper write correct physics, but they were evidently unable to do it by themselves. So, for the sake of correct science popularization, I yielded and wrote a piece. The editor at Il Sole, Armando Massarenti, proposed the pseudonym for me – I did not cook that up myself. I must say I was pleased with it. Democrito has first and last letter equal to my last name, and is considered the inventor of the concept of atomism. Atom comes from him: a-temno, “which cannot be cut”.

The result is translated for you here. The original piece, in italian, can be accessed in pdf format by clicking on the icon above.

A week ago I discussed here the recently approved analysis by which CDF shows a small hint of WW/WZ signal in their Run II data, with one W boson decaying to a lepton-neutrino pair, and the other boson (either a W or a Z) producing a pair of hadronic jets.

Such a process is very hard to put in evidence in hadronic collisions, due to the large irreducible background of events due to one leptonic W decay accompanied by QCD radiation from the initial state of the collision. In fact, despite having been sought by many in Run I, no appreciable signal had surfaced in Tevatron data either from CDF or D0.

Now, D0 has really bagged it. They used a more performant selection method than the one used by CDF, and were a bit more bold in their use of Monte Carlo simulations. The result is that they find a very significant excess, amounting roughly to 960 events, in a total of nearly 27,000.

I encourage those readers who are unfamiliar with the basics of vector boson production at hadron colliders to read the introductory part of the former post on this topic, which I linked above. Here I will avoid repeating that introduction, and concentrate instead on the analysis details.

D0 uses a total of 1.1 inverse femtobarns of 1.96 TeV proton-antiproton collisions, acquired during Run II of the Tevatron. The samples of data are collected by triggers selecting a signal of a high-energy electron or muon, and a further requirement that a transverse energy imbalance of 20 GeV or more is requested, thus characterizing the leptonic decay of one vector boson. Finally, the transverse mass of the lepton-missing transverse energy  system has to be larger than 35 GeV, reducing backgrounds from non-W events.

[The transverse mass is computed by neglecting the z-component of the particles momenta: if both particles are emitted perfectly transverse to the beam direction, transverse and total mass coincide. This is forced by the absence of a z-measurement of the neutrino momentum, since the energy imbalance it creates by escaping the detector cannot be measured along the proton-antiproton axis.]

Besides characterizing the leptonic W decay, two jets with transverse energy above 20 GeV are required. After this selection, the data contain a non-negligible amount of non-W backgrounds, constituted by QCD multijet events where the leptonic W is a fake; but the bulk is due to W+jets production, where the jets arise from QCD radiation off the initial partons participating in the hard interaction. Several Monte Carlo samples are used to model the latter background process, while the former is handled by loosening the lepton identification criteria in the data: the looser the lepton requirement, the larger this contamination, such that for really loose electron and muon candidates the samples are almost purely due to QCD multijet events.

Signal and backgrounds are separated using a multivariate classifier to combine information from several kinematic variables. This is the Random Forest algorithm, which I had the occasion to discuss in the past (two years ago a student of mine used it to discriminate hadronic top events in a similar dataset in CDF). The Random Forest output is highest (close to one) for signal events, while backgrounds are given a value closer to zero. The result of the classification is shown below: the excess for high values of RF output are due to the diboson signal (in red the signal content estimated by the fit).

A fit to the RF output provides the normalization of the signal and the background components, as shown above. Notice the blue “envelope” in part (b) of the plot: it is the systematic uncertainty due to background RF templates. Of course, the level of the blue curve is deceiving, since shape uncertainties are totally correlated among themselves; but the signal does stand out on top of it.

A plot of the dijet mass distribution confirms the interpretation, as shown below. The bottom part shows the data subtracted by background contributions (points with error bars), which compares well with the shape of the expected diboson contribution. D0 finds a combined WV (WW+WZ) cross section of , where the first uncertainty is statistical, the second is systematic, and the third relates to the integrated luminosity uncertainty of the base of data used in the search. This compares well with the theoretical prediction of .

In the plot above, the combined W/Z signal peaks at about 80 GeV, with a resolution of roughly 15 GeV; the background template uncertainty is again in blue, again underlining the difficulty of this measurement, which finds a signal excess exactly where the backgrounds peak.

One question I often hear asked in plots such as the one above is “why do W and Z boson peak at the same mass value ? They have a 10.7 GeV mass difference after all”. True, but the dijet mass resolution of the D0 detector is insufficient to tell the two signals apart, and what one observes is the combined shape. To be more precise, one should also add that the Z contribution in the plot is much smaller than the W one (about one third). Further, one should also point out that the heavy flavors produced by the Z boson will produce a underestimated Z mass reconstruction, due to the neutrinos often produced in the semileptonic decay of b- and c-quark jets. It is a fact that the decay will peak at about 83 GeV after calibration of generic jet response, due to that effect alone…

I like to let the authors point out that “This work further provides a validation of the analytical methods used in searches for Higgs bosons at the Tevatron”, as in the conclusion of their paper. Indeed, the advanced methodologies by which the Tevatron experiments are setting more and more stringent limits on Higgs boson production are perceived by some as a bit uncautious. Things appear to be well under control, it instead transpires, once one can demonstrate that a signal known to be there can be indeed extracted from samples which have a very small signal-to-background ratio, as is the case of all Higgs searches.

The picture above made the headlines today in repubblica.it, one of the news sites I read most often. It shows a little mouse, originally intended as a meal supplied to a viper, managing to invert the food chain, killing the monster. I am stating the obvious when I say we usually rejoice when we see something like that happening: we always root for the weak against the strong, especially if we feel weak ourselves -and don’t we all, in some respect ?

The picture had me thinking about the competition between the Tevatron and LHC particle accelerators. Since the LHC has not produced a single proton-proton collision in the core of the CMS and Atlas detectors yet, while the Tevatron has supplied CDF and D0 with an enormous amount of collisions which are fruiting scores of groundbreaking physical results, the two machines might be argued to not be competing yet. But that would be a myopic assessment, on several levels.

• There is competition for the funding of experiments. Lab directors, experiment heads, and faculty members are very sensitive to this issue of course, since it affects their chances of leadership and power. Funds to particle physics experiments are getting cut these days, and the many experiments have to fight against each other to keep their budget plans intact. The LHC has been competing for funding with the rest of the HEP facilities since the start of its construction, or arguably even before then.

• Then there is of course the fight for a place under the media spotlights. It is a level of competition tightly connected with the funding one, but it has some additional branches, since the media attention can be an important fuel to boost the career of scientists. The recent media hype for the startup of LHC on September 10 was a masterful organization by CERN general director Aymar, and the following incident with the magnets in sector 34 was troublesome to CERN as much for its media impact as it was for the lab schedule. Of course, at the Tevatron and elsewhere many met the global interest for LHC in the former occasion with ill-concealed jealousy, and the latter with more evident satisfaction.

• A third level of competition, in turn somewhat connected to the second, has its ground on the scientific conferences around the world which are now scheduled every second week. Presenting scientific results at conferences is a very important item in the construction of a strong curriculum, and some talks increase the prestige of the speakers. The Tevatron has had a large share of talks allotted at all the major conferences in the recent past, due to the mass of new results it produced; but Atlas and CMS have started being allotted several talks already, which will be used to discuss “Monte Carlo analyses” rather than results on real data. Still, this causes a compression of the benefits of the Fermilab scientists.

• And then there is the fight for the Higgs. The Tevatron experiments are still caressing hopes to find the Higgs boson before LHC does, and are thus squeezing their brains to improve the already excellent analyses they have been producing. This year, for the first time, a direct limit on the Higgs boson mass has been set by CDF and D0 in a joint effort. Although the limit is not stringent yet (a single mass value has been excluded, 170 GeV), the two experiments are working to wipe off the board the whole high-mass region, where the LHC would have no trouble in finding a significant signal with one year worth of data. If the Higgs is proven to be lighter than 130 GeV or so, the fight between the two sides of the Atlantic is promised to become red-hot in the next two-three years, depending on whether the Tevatron gets funded to run for the fiscal year 2010.

• Finally, there might be new physics out there, and it might still be at reach of the Tevatron. The six-months delay of the LHC data taking is making this race even more interesting, especially since CDF and D0 are reaching a level of sensitivity in the SUSY parameter space that might enable them to discover new physics before LHC.

So, as you see, there is competition in HEP between LHC and the Tevatron even if the former has not begun taking data yet. The mouse of the picture above, in my mind, is the Tevatron biting the LHC’s throat, turning the tables when everybody expects the latter to entertain itself with a quiet meal. This is still possible. If CDF or D0 discovered SUSY, such an event would be a defeat of incredible proportions for CERN, echoing the disasters of the sixties and the seventies, when the US were banqueting lavishly with new discoveries, barely leaving bread crumbs for Europe.

“And where do you stand“, you might well ask, since I am working for both CMS at CERN and CDF at the Tevatron ? Well, I am slightly embarassed to answer, but I must say that until CDF closes down, my heart beats for it. The first love is the one you never forget, and to that piece of rethorics today I can add the image of the little mouse. Curious feeling: I am spending 80% of my research time doing my best to help make CMS a success, but I still caress some hope that it will, at least to some extent, fail to the hands of my former love. If you think that is despicable, please consider: what is important is not who gets a Nobel prize here. It is a win-win situation for whom, like me and most of you, only cares for the advancement of science. CMS, Atlas, CDF, D0: who cares ? All I care is to find out the truth!