A Quantum Diaries Survivor

Just a few days ago D0 released their new results from searches of the   decay process. That result has now been combined with all their other searches, in particular those for a lighter Higgs boson, which mostly decays to a b-quark pair if as the following graph explains.

In the graph, the line marked “bb” shows the fraction of times a H boson disintegrates into a pair of b-quarks, which subsequently - given the large energy each is endowed with, equal to - produces two hadronic jets in the D0 detector. The line marked “WW” shows instead the dominance of the WW final state if the Higgs mass is larger, an effect due to the fact that the elusive particle decays to the heaviest bodies available: as soon as two W bosons are energetically possible (and even a little before that, when one W is produced with less energy than its rest mass) they become the dominant decay. Note also the three regions with background color yellow, pale blue, and bluer blue: yellow marks the region where LEP II has excluded a Higgs boson, while pale blue marks the region where the bb final state is dominant, the so-called “light-higgs” region. 

To find a decay one needs some extra handle, because two b-jets are a quite common occurrence at the Tevatron - one event in a hundred thousand, as opposed to one in a hundred billion. The extra handle is provided by requiring that the H comes together with a W or Z boson, in a process called associated production, or Higgs-strahlung from vector bosons - the latter adapted from the german word bremsstrahlung, which means braking radiation, the one produced by a charged particle forced to slow down. Electrons (as well as any other charged body)produce real photons by bremsstrahlung when they are curved or decelerated, while W and Z bosons “emit” real H bosons when they have enough energy to do so.

D0’s combination is a pleasure to look at, because for once I do not need to wrestle too much into gory details, and can concentrate on describing the results. And you, dear reader, are presented with a series of very nice and clear plots, where everything is tidy and whose message is clear.

The D0 plots are made all with the same style: a neural network classifier is used to discriminate signal from background, and you get to see the data (black points) in a histogram of the NN output, compared to the expected sum of backgrounds (all together in a single distribution, marked with a red line).

The agreement of data and background speaks by itself, but in addition you get to see how the Higgs signal would show up in the distribution. To see it better, it is multiplied by a factor of x15. Thank god, a common factor across the board (for all WH/ZH searches, while the higher S/N of searches forces the removal of the enhance factor), so that one can easily compare visually the statistical power of different searches. Kudos to D0 for their excellence in data display!

So here are the individual search plots. They are based on data samples ranging from 0.6 to 1.7 inverse femtobarns.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the   search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the search results (l stands for a charged lepton, either electron or muon). The expected signal is in blue, multiplied by a factor of 15. This plot is made with events containing two b-tagged jets, for maximum signal purity.

Above, the search results (l stands for a charged lepton, either electron or muon). The expected signal is in blue, multiplied by a factor of 15. This plot is made with events containing only one b-tagged jet, for a larger signal efficiency.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15. This analysis searches for the spectacular signal of three W bosons, which is difficult to see because of the price to pay in branching fraction of W bosons to leptons. Backgrounds, however, are smaller than in most other searches. Here, instead than a NN output, a discriminant is plot on the x axis. Its meaning, however, is exactly the same: a value close to 1 is more likely for signal-like events.

Above, the search result from the latest chunk of 0.6/fb data analyzed by D0. Here, the signal (still in blue) is not multiplied by 15, but the y axis is logarithmic.

Finally, all searches are combined to extract a global 95% confidence level limit. You can see it in the plot below. The limit, as usual with Higgs searches at the Tevatron, is on the number of times the Higgs production could exceed the predicted Standard Model rate without having been seen by D0.

The black curve is the limit found by D0, while the hatched red line is the limit expected given the sensitivity of the searches and the data available. You can see that a big progress has been made in the searches at low Higgs mass, where the limit is sitting at only a few times the SM. This leaves room to hopes that the Tevatron may say something meaningful also in the low mass region before LHC becomes the only game in town.

For more information, please visit the D0 site of this analysis.

This just in… The CMS tracker, a mindboggingly complex barrel composed of multiple layers of silicon detectors, equipped with readout electronics, high voltage cables, cooling pipes, and the occasional screwdriver forgotten by a technician inside (just kidding), has been inserted in its housing in the core of the CMS detector. See the pictures below…

The anonymous comment left yesterday on the guest post by Tony Smith annoyed me quite a bit. The visitor, instead than discussing the topic presented in the post, decided to produce a personal attack on Tony, guilty of having a web site of his own, containing personal theories and ideas. At the heart of the comment was a question directed to me: why do you give credit to this person, who is evidently a crackpot ?

That kind of behavior is in my opinion to be strongly discouraged, because of the coward nature of the censor who expresses his views anonymously, attacking the post author personally without getting his hands dirty with the matter being discussed. Such a comment does not deserve an answer, so much so that I am providing an articulated one below.

So, why is this site not crackpot-free ? Why does a “respectable” physicist give room and voice to people who are not professionals like him, who work at their very own theories on their free time, who go against the mainstream ?

First of all, I would like to make it clear that, despite my explicit invitation to anybody who thinks he or she has something interesting to say in the field of science, I am not willing to publish just anything. The judgement is with me, since this is a personal blog before it is a public place.

Second, my choice is a principled one. This site would have no meaning if it was for scientists only. Physicists know where to find the information they need, and while they may enjoy reading a colleague’s viewpoint on this or that scientific result, well, that does not provide sufficient motivation for me. No, this site not just for scientists. It is actually mostly aimed at people with an interest in science, who are willing to spend some time to learn about particle physics through my posts, and most of all through the comments other knowledgeable people leave in the posts.

“But why then”, would our anonymous friend interrupt me here “do you think it is a good service for people willing to learn about science to provide them with untested theories, steep speculations, ungodly ideas that have no room in serious knowledge repositories such as the Arxiv or scientific journals ? Why diverting them from the correct path, why suggesting that mainstream theories might be wrong ?”

Because, my friend, science is made by human beings. And humans err. As beautiful and well-tested as the Standard Model is, it still is a theory: it is not god-given truth, but a human-made mathematical construct, subject to disproof, liable to be modified, and certainly incomplete. Science is a process, not a state. Doing science entails taking wrong turns, spending years on a silly idea, making untestable hypotheses. For instance, string theory is certainly science, although possibly a very expensive waste of human resources.

By giving voice to so-called crackpots, I am providing a richer panorama of the investigations of the theories of nature than the one you can find by checking new hep-th or hep-ph papers. And people, even non scientists, have a mind of their own, and can discern what has a red label from what is back-yard bricolage. This site is crackpot-rich, and everybody is allowed to speak, even anonymous commenters - the lowest-ranking caste in the web society.

The CMS collaboration is having its last “CMS week” of the year this week. This evening at 6PM many of the attendees gathered in the big industrial building where the detector components have been assembled and lowered bit by bit in the pit, down into the cavern where the whole thing is being put together. Below you can see one of the few pieces still waiting to join the rest: a wheel of muon chambers, designed to detect forward-aiming charged particles penetrating enough to punch the whole central structure of the CMS detector - muons, that is. One cannot avoid feeling awed while walking under these giant structures.

There was good food and drinks available to the participants. Too good food - it speaks of an army not aggressive enough. But anyway, things unrolled easily and as I left to go back to work everybody seemed to be having a good time.

Yes, I said I am back to work… I have been fighting with some analysis code this afternoon, and I promised it I would come back in the evening to finish it off. Tomorrow I will be flying back to Venice, so I have to see the results of the code tonight or wait next Monday for them.

Tony Smith needs no presentation to the readers of this blog, since he often contributes to the discussion of physics posts here.  His web site can be found at tony5m17h.net . I received yesterday, and am glad to publish, the following interesting discussion of the properties of the E8 group, which has attracted a lot of attention since the recent paper by Lisi. Enjoy!

Garrett Lisi at hep-th/0711.0770 describes a physics model based
on the 248-dimensional rank 8 exceptional Lie algebra E8 in which
each of the 240 root vectors of E8 are given a physical
interpretation.

The Lie algebra root vectors of E8 form a polytope (called the
Witting polytope) in 8-dimensional Euclidean space. 8 of the 248
generators of E8 are used to form the 8-dimensional root vector
space, and the remaining 248 - 8 = 240 generators of E8 correspond to
the 240 vertices of the E8 root vector Witting polytope.

Garrett Lisi shows a projection of the 240 vertices down into
2-dimensional space

A youtube movie based on a New Scientist article describes some of Garrett Lisi’s physical interpretations of the vertices, and shows how the patterns of vertices transform under rotations.

In this guest post, I want to describe an alternative set of physical interpretations of the 240 E8 root vector vertices and present a movie of how they transform under rotations, so that E8 physics might be more intuitively visualized. In this post, I will abuse notations by using E8 , Spin(16), etc., for both Lie algebra and Lie group, and I will not be careful about group / algebra distinctions, factors of Z2, and other technical matters that might get in the way of exposition.

Like Garrett Lisi’s E8 physics model, this E8 physics model is based on seeing E8 in terms of

248-dimensional E8( = EVIII = 120-dimensional adjoint Spin(16) + 128-dimensional half-spinor Spin(16)

and on seeing 120-dimensional Spin(16) as

120-dimensional Spin(16) = 28-dimensional D4 + 28-dimensional D4* + 64-dimensional 8v x 8g

and on seeing 128-dimensional half-spinor Spin(16) as

128-dimensional half-spinor Spin(16) = 64-dimensional 8s’ x 8g + 64-dimensional 8s” x 8g

and on seeing the 240 root vectors of E8 and the 120 - 8 = 112
root vectors of rank 8 Spin(16) as

240 E8 root vectors = 112 adjoint Spin(16) root vectors + 128 half-spinor Spin(16) root vectors =

= 24 D4 root vectors + 24 D4* root vectors + 64-dimensional 8v x 8g + 64-dimensional 8s’ x 8g + 64-dimensional 8s” x 8g

However, in this E8 physics model the physical interpretations of the 240 root vectors are not exactly the same as in Garrett Lisi’s model. Here is how they look in this model.

In this image of this model there are two sets of 24 vertices each:

24 yellow points correspond to the 24 root vectors of D4 which is used to construct Gravity by a generalized MacDowell-Mansouri mechanism based on the 15-dimensional D3 = A3 Conformal Group Spin(2,4) = SU(2,2). To help get started with visualization, here are the 24 yellow points

in the image. Note that the 24 yellow points form three sets:

6 near the top, in a 1 4 1 pattern corrresponding to the 6 vertices of an octahedron;

12 in the middle, in a 4 4 4 pattern corresponding to the 12 vertices of a cuboctahedron;

6 near the bottom, in a 1 4 1 pattern corresponding to the 6 vertices of a second octahedron.

Note also that a 24-cell can be seen as being made up of a cuboctahedron and two octahedra as in this stereo image:

in which the cuboctahdron is green and the two octahedra are red and blue. So, it is clear that the 24 yellow points form a 24-cell, which is the root vector polytope of the D4 Lie algebra.

24 purple points correspond to the 24 root vectors of D4* which is used to construct the U(3) x SU(2) x U(1) Standard Model based on the 15-dimensional D3 = A3 group SU(4) and its 9-dimensional subgroup U(3) and the 6-dimensional SU(4) / U(3) = CP3 Twistor space, with the U(3) giving the SU(3) x U(1) of the Standard Model and the CP3 Twistor space giving (via relation to quaternionic structure) the SU(2) of the Standard Model. Note that the 24 purple points form a pattern similar to that of the 24 yellow points shown above.

Each of the remaining three sets of 64 vertices is of the form 8 x 8g, where 8g denotes the 8 Dirac gamma basis elements of the Dirac gammas of an 8-dimensional Kaluza-Klein spacetime.

64 blue points correspond to 8v x 8g, where 8v corresponds to the 8 basis elements of an 8-dimensional Kaluza-Klein spacetime, so that the 64 blue points correspond to an 8×8 matrix of the 8 spacetime basis elements with respect to 8 Dirac gammas.

64 red points correspond to 8s’ x 8g, where 8s’ corresponds to D4 +half-spinors and to the 8 first-generation fermion particles (electron, neutrino, red up quark, green up quark, blue up quark, red down quark, green down quark, blue down quark), so that the 64 red points correspond to an 8×8 matrix of the 8 first-generation fermion particles with respect to 8 Dirac gammas.

64 green points correspond to 8s” x 8g, where 8s” corresponds to D4 -half-spinors (mirror image to +half-spinors) and to the 8 first-generation fermion antiparticles,, so that the 64 green points correspond to an 8×8 matrix of the 8 first-generation fermion antiparticles with respect to 8 Dirac gammas.

Note that the 24 yellow D4 + 24 purple D4* + 64 blue = 112 adjoint Spin(16) vertices are in some sense fundamentally bosonic, physically corresponding to gauge bosons or spacetime vectors,

while

the 64 red and 64 green = 128 half-spinor Spin(16) vertices are in some sense fundamentally fermionic, physically corresponding to fermion particles and antiparticles.

which is characteristic of exceptional Lie algebras being constructed by combining adjoint-type and spinor-type repesentations.

To see how the 240 root vectors of E8 transform under rotation, I used a root vector rotation web applet by Carl Brannen and took a bunch of screen shots and used them to make an image-sequence movie. There may be a little glitch about half-way through the 34 second movie (I may have messed up by hitting a reset button, or by taking screen shots a little off center, or etc), but to me it seems that, even so, the movie gives interesting visualization insights into how the 240 root vectors of E8 fit together to describe physics.

Click here to see the .mov movie.

Using the basic components described above, it is natural to construct a Lagrangian

with the 64 blue points (8-dimensional Kaluza-Klein spacetime) as base manifold

with the 24 D4 and 24 D4* yellow and purple points (Gravity and the Standard Model gauge groups) forming curvature terms

with the 64 red fermion particle and 64 green fermion antiparticle points forming fermion terms.

The blue 64 and red 64 and green 64 are related by Triality inherited from the Spin( triality among vectors, +half-spinors, and -half-spinors. Instead of using the triality for fermion generations, this model uses Triality to show a subtle supersymmetry between fermions and gauge bosons, seeing the gauge bosons as related to bivectors constructed from the blue 64 vectors, and using the Triality to relate them to the red 64 fermion particles and the green 64 fermion antiparticles.

In the interest of keeping this expository guest post somewhat simple, I will only mention in passing such things as that the 8-dimensional Kaluza-Klein is motivated by the work of Batakis, the second and third generations of fermions are composites of the first generation fermions, the Higgs mechanism comes from a geometric construction due to Meinhard Mayer, the force strengths as particle masses are calculated using structures related to bounded complex domains in the spirit of Armand Wyler, etc. For such details and more, as well as references, see my web page entitled E8, Cl(16) = Cl( (x) Cl(8), and Physics Calculation or the corresponding 82-page pdf version.

I will try to reply to comments here not only about the visualization movie, but also about any questions that might arise from the 82-page detailed paper.

I read today on the italian newspaper “La Repubblica” about an ongoing investigation against Silvio Berlusconi (right), former italian premier and now leader of the coalition opposing the Prodi government.

It appears that the man has recently made several phone calls, offering economical advantages to Agostino Saccà,  president of Rai Fiction - a branch of the italian national TV network- in exchange for some favors. Berlusconi also is investigated for offering two million euros plus many other benefits to a few senators of the center-left coalition, in exchange for their vote to force the fall of the Prodi government.

I have nothing against the investigations or the judges that are producing them, but I have to say I feel underwhelmed. Is that all we have to show for three decades of Berlusconi’s alleged systematic violations of italian law ? He owns and controls three major TV networks, several newspapers and magazines, publishing firms. He is the center of the largest conflict of interests ever seen in a “democratic” country. And we discuss his pathetic attempts at buying a few votes in the Senate ?

Come on, we knew he would have done it since day one of the Prodi government. Ever since we saw that at the Senate the center-left majority had a ridiculously small margin (only a few votes) everybody knew Berlusconi would buy a few extra votes from as many avid center-left senators and kill the legislature. Now we know for a fact that he made a few phone calls (we also know he owns Dini’s party votes already, but that is stuff for another post) . If that is all judges have to show, I would really rather see these judges work on organized crime instead -we sorely need more judiciary action in that area.

I have been rooting for judges to demonstrate that the guy is a felon for fourteen years now. He surely is one, but it proved harder than many had predicted to actually frame him and tie him to his responsibilities. He dodged all the bullets -and most were of a larger caliber than these last ones. At this point, I am rather bothered by the ineffective action of the center-left government, which could have finally passed a law to prevent media owners to do politics - effectively putting him in off-side. They could have, in the honeymoon after the electoral victory in May 2006. Now, it is much harder to obtain the result, because of the fragility of the center-left coalition.

Judges… They were a last-ditch hope one day, when it was clear that Berlusconi had violated several laws, and he looked too strong to be defeated politically. In 1994 I was really scared by the radical right-wing turn our country had taken, and I hoped Berlusconi would be tried for corruption, for tax fraud, and for a long list of other illegal activities connected to his many off-shore companies where he distributed the profit of his businesses. Now, I see him more like a depressed clown and less like a dangerous politician. I think wiring phones and taping conversations is a very retrograde, iron-curtain-fashion way of repressing the jolly fraudolent activities that are such a normal practice in today’s Italy. Silvio, I am with you on this one.

My wife teaches in a Liceo Classico - a high school where students learn latin and greek, and where the focus is on human sciences; her school is one which has a long, prestigious history - but despite of that, misdemeanor reigns. Through her tales, I sometimes get to know about common fraudolent practices and trends of teenager students in Italy.

One of the most important criteria for evaluation of a student’s knowledge in Latin and Greek is the written exam, where a short piece taken from the classics is translated into Italian. The students are not supposed to have seen the piece beforehand, but that is not a problem given the vast amount of literature from which the teacher can choose the subject of the exam. The teacher gives the text to the class at the beginning of the exam, and the students have typically two hours to complete the task.

Now, my wife reports that it is becoming common practice for some students to obtain outside help through their cell phones. Usually, before the exam starts the teacher collects all cell-phones from the students: unfortunately, many youngsters in Italy have a second cell-phone, and some use to conceal it somewhere. These rascals are thus able to paste the text to be translated into a message, send it to somebody outside the school, and wait until the text is fetched from the internet along with a perfect translation. They then copy the resulting translation received through an instant message into their translation. A high score is guaranteed. The teacher usually has no chance to discover the fraud, because students have become really skilled with typing on cell phones without looking at the keyboard.

I knew about this practice - it is not applied only in high school, but -even more annoying- in selections for new positions in public administration or other white collar jobs. But my wife tells me in some cases it is the very mothers of the students who stay home in front of a computer to do an internet search. I find the thought simply unbearable. A parent that teaches his son or daughter that the only important thing is the result - a good grade - and not the process, nor the capability to study, is sending a clear message: just be smarter than your peer and cheat if you need to. Quite in line with today’s world, in truth.

The final piece of a long process has been placed yesterday, with the blessing of the cross section of Z production times branching fraction to b-quark pairs at 1.96 TeV by CDF. Julien, my colleague in Padova, took the burden of bringing to a completion the analysis we produced of decays, by producing for the first time a measurement of in hadronic collisions. The signal is shown in the plot below.

In the plot you see the data (black points) fit as the sum of QCD background (green) and a small signal (red) sitting a bit below 90 GeV (b-jets lose energy to leptons, as I explain below). In the inset you see the background-subtracted signal, amounting to about 6500 events.

In our long-lasting analysis of Z decays, our main goal has always been to demonstrate the possibility of extracting from the signal a precise number for the b-jet energy scale, the ratio between energy measured by the CDF detector for real and simulated hadronic jets produced by b-quark fragmentation. That number is a calibration which is in principle needed by all analyses aiming at measuring precisely the top quark mass by kinematical fitting of jet and lepton four-vectors. In principle -yes. Because it took us a long time to obtain a precise number for the b-JES, in the meantime top mass fitting techniques have become so fantastically precise that they now would not gain very much by including that input.

Well, I realize the above paragraph is a bit too handwaving for anybody’s taste, so let us see a bit more quantitatively what we are talking about here. In general, the jet energy scale is set for generic jets by studying the energy measurement in QCD two-jet events. The methods are very refined, and they allow to determine the systematic uncertainty on the jet energy measurement with high precision. CDF has currently an uncertainty of about 2.5% on the generic JES. That number, however, does not directly apply to b-jets. That is because b-quarks are quite special:

• They have a large mass (of the order of 5 GeV), which implies that the jet hadronization products receive more energy transversely to the jet direction than other lighter partons impart to their products;
• They have a hard fragmentation, which implies that the decay products have a momentum spectrum different from that of light parton jets. That, in turn, may cause a different energy response in the calorimeter, due to a non-linearity in the latter’s reponse to low and high momentum hadrons;
• They produce leptons in their decay: b-quarks produce 12% of the times an electron and 12% of the times a muon. Then, most of the times the charm quark that the b has decayed into also decays producing electrons and muons. Overall, about a third of b-jets contain an electron or a muon. The calorimeter responds too much to electrons (if they are mistaken for hadrons, which surely happens unless one explicitly searches for them) and far too little to muons; but most importantly, together with the charged lepton the b-jet will originate neutrinos, which will carry away all of their momentum, producing a deficit in the estimated parton energy;
• b-quarks are also special because of their color-connection to the top quark in top decay. That is the rule in QCD interactions, but on the contrary hadronic W decays, which are used to check or help set the scale of jets in single lepton top pair decays, have no color connection to the initial state.

All in all, b-jets are genuinely special. One can estimate that all these effects overall cause only a minor systematic uncertainty on the jet energy, but an estimate based on simulations cannot be trusted completely. So, a real measurement of the b-jet energy scale, independent from other determinations, is an important input in top mass measurements.

Jet energy scale systematics in CDF are shown in the (slightly outdated, but I could not find a more updated version) plot below. As you can see, for 30-40 GeV jets the total systematic uncertainty is largish, due to the combination of several effects. 

The black line is the total uncertainty, obtained from the sum in quadrature of all the other effects - among which the largest at low jet Pt is the out-of-cone uncertainty, due to the insufficient knowledge of soft effects of final state radiation.

Let us compare the above situation with the most precise top mass result by CDF, which now uses 1.8/fb of data: …, where the first uncertainty is statistical and the second is the JES systematic uncertainty resulting from a calibration using the signal in the same top decays used to extract the mass measurement. To that 1.2 GeV, one must add in quadrature the residual scale uncertainty and the systematic due to the unknown difference with b-jets, which amount respectively to 0.5 and 0.4 GeV. These numbers have collectively an impact of less than 1% on the top mass measurement by now!

A precise b-JES determination cannot help much the measurement of top quark mass in the single lepton final state, because there the decay is good enough for the purpose. But in dilepton final states no W bosons decay to jets, and there are only b-jets to determine there. The best dilepton measurement in CDF obtains currently , where the first uncertainty is statistical and the second is systematic. To the latter the JES contributes for a hefty 2.6 GeV: there is room of improvement there, from a precise b-JES determination. 

Now, let me discuss what we found with the analysis of Z boson decays. We measured a b-JES using about 6000 decays selected by a tight kinematical selection and double secodary vertex tagging, in 584/pb of Run II data. That means an error of less than 2% on the b-JES, which could indeed help the dilepton top mass decays.

UPDATE: for lack of time, in my first publication of this post I could not include an additional information. The 2.6 GeV systematic uncertainty due to the JES in the dilepton top mass measurement quoted above must be interpreted as a roughly 2.6% contribution from jet energy scale (my own estimate). That is because in the top mass measurement the jet energy scale typically accounts for 1 GeV uncertainty for each 1% in JES uncertainty. So, a 2% error from the signal fit would indeed help reducing significantly that uncertainty. A back of the envelope calculation (beware of them!) would predict that combining a determination with 2% error and one with 2.6% would correspond to one of , or 1.6 GeV on the top mass - a reduction of 1 GeV. Despite its inaccurate nature, this computation shows that it is indeed useful to calibrate b-jets with the Z peak. In our case, the 2% uncertainty on b-JES was extracted with less than 600/pb, a dataset three times smaller in luminosity than those which produced the mentioned top mass measurements.

Yesteday Julien presented for approval, and obtained blessing for, a complementary measurement obtained from the same data sample: the cross section for Z production. Of course this is simply a check, since we know very well the Z cross section from measurements involving the background-free , decays. However, it is a very nice check at that, and technically it indeed is a first-timer: nobody measured the cross section of Z bosons in hadronic collisions before - the UA2 collaboration in 1987 produced a combined W/Z signal to hadronic jets, but did not disentangle the two.

So what is our measurement ? We find  (the first uncertainty is statistical, the second systematic, and the third is due to the background modeling), in agreement with NLO calculations (A.D.Martin et al.), predicting .

A paper is going to be submitted to NIM quite soon… Congratulations to Julien, and of course to all the other members of our group:  K. Hatakeyama, M.Shochet, S.Kwang, C.Neu, T.Tomura, D.Whiteson - plus, of course, myself.

The italian way. How else to define the regularization process that is going to take place inside INFN in the forthcoming months ?

INFN suffered in the past four years from a blockade of the hiring process of new personnel with permanent positions. People who had won a selection to become a researcher, a technician, or an accountant were put in stand-by, in a kind of limbo. There simply were no funds to hire the new personnel. Berlusconi’s government had cut funds to research and the result was a total freezing of the institute.

At the end of 2005, something started moving again. A big selection of national scope was called for researcher positions. In particle physics, 25 people were selected with a tough exam in Rome -among them, yours truly. They were hired with a 5-year contract and the promise that the position would become permanent without the need of passing a further selection. So these were temporary positions which would become permanent. Indeed, one year after the selection, the new government now led by Prodi’s center-left coalition gave more funds to INFN, and the institute started a procedure to “stabilize” the selection winners, i.e. hire them permanently: but, to make things just a bit more interesting, they inserted the clause that they had to total three years of service before being eligible.

Now, italian bureaucracy is a perfect case study for Murphy’s law: anything that can go wrong usually will. So the new procedure started by INFN looked to the eyes of the least gullible observers like a necessary but by no means sufficient step toward the coveted permanent position. After a few more months, it now transpires that INFN has more funds than they expected to a few months ago. They could make the winners of the 2005 selection permanent, but this could be seen as a undue favor to few. The outsiders -those who did not win the 2005 selection, or who were distracted by other obligations then, would question the procedure, having no chance to get hired themselves.

So instead, what will INFN choose to do ? They will call a new selection for immediate hiring of permanent researchers, and give a large bonus  in points to the winners of the 2005 selection. These poor souls, once assured that they would never have to pass a selection to get a permanent position, have to get on theory books again, and pass yet one more exam. Or, they could abstain from participating, in the faith that INFN already promised to hire them permanently upon completing three years of service…

Nobody will choose the second course of action, for fear that INFN changes rules yet over again, or that funds disappear, that Prodi’s government falls and the new premier votes some law freezing INFN hiring again. So that proves that really, life is a continuous exam… And it also shows that Italy is a really funny country.

W production in association with heavy flavor jets is a very important process at hadron colliders. The presence of heavy quarks in the final state may mimic the signature of important physics processes such as top production, Higgs bremsstrahlung off W bosons, and other more exotic mechanisms. Until recently, the various components of W+heavy flavor were only estimated with Monte Carlo simulations using theoretical predictions which had little experimental verification. Then, production has been measured with good precision at the Tevatron. Even more recently, the elusive production process has been accurately measured by CDF.

W+charm production occurs via interactions whereby a strange quark from the proton sea is converted into a charm by charged weak interaction, with a direct manifestation of its carrier, the W. The two leading order diagrams are shown on the right here and below. One clearly sees that the production only involves positive-charged W bosons and anticharm quarks, or negative-charged W bosons and charm quarks: there is therefore a very striking sign correlation, which can be readily exploited if the charge of the charm quark is measured.

Indeed, with the use of a soft lepton tagging algorithm - which finds the lepton originated from the semileptonic decay of the charmed hadron in the jet - it is possible to verify the charge correlation with the primary lepton from W decay, thus measuring the cross section and determining some interesting kinematical properties of the process.

The analysis is straightforward: from a W sample collected by requiring missing Et and a high-Pt electron or muon, events with one or two additional hadronic jets of are selected. In 1.8/fb of Run II data CDF finds 1822 events where one of the two jets is tagged by the SLT algorithm, which searches for a soft electron or muon inside the jets. Of these, 1059 jets have the soft lepton charge opposite to the W lepton charge: that alone indicates a large asymmetry.

To measure W+charm cross section, the difference of opposite sign lepton pairs minus same charge lepton pairs in the data is subtracted by the same difference estimated from background processes. Backgrounds are mostly charge-symmetric, with the exception of non-W QCD events where the two leptons have a charge correlation due to their common origin (for instance, in the decay of a pair), and Drell-Yan production, where the leptons also have opposite charge. Accounting for backgrounds and systematic effects, CDF measures ,  where the first uncertainty is statistical, the second systematical, and the third is due to luminosity uncertainty of the dataset. This favourably compares with theoretical predictions obtained at leading order with the ALPGEN generator, equal to . Below you can see some kinematical distributions which show the good understanding of the data. The Wc process is in good agreement with its expected properties.

Above, the transverse momentum of the SLT muon contained in the charm jet has the expected characteristics from charm quark decay: it peaks at the low Pt threshold, as it should, while other backgrounds have harder spectra.

The azimuthal angle between missing Et and soft muon is another variable capable of distinguishing Wc production from the main backgrounds. In fact, the angle is large for the former on average, while it is very small for non-W backgrounds. Notice how there are negative entries in the plot, due to the OS-SS subtraction procedure by which the data is obtained bin by bin in the histogram.

Finally, the missing transverse energy, signalling the escape of a energetic neutrino from W decay, is another good indicator of the nature of the selected processes. Hats off to another nice analysis by CDF.