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Higgs limits at the Tevatron: <1.4 x SM !! August 23, 2007

Posted by dorigo in news, physics, science.

With pleasure I visited the public web page of the CDF experiment today, finding a fresh new combination of most of the searches for Standard Model Higgs bosons ongoing at the CDF and D0 experiments. And it is an exciting piece of news!

As I discussed recently here, CDF has produced a combination of its results. D0 did the same, and then, with a technically very complicated but conceptually very simple operation, the two results have been combined.

So let us have a look at the combination, and then discuss it briefly. It is shown in the plot below:

As usual, the y axis shows the 95% CL limit in units of Standard Model cross section for Higgs production, for each value of the Higgs mass (the x axis). A limit at a value y=2.5 for a value x=145 GeV means that the Tevatron experiments exclude, with 95% confidence, that the Higgs boson has a production cross section at the Tevatron larger than 2.5 times the SM prediction IF its mass is 145 GeV.

The yellow band (yellow has become a classic for LEP exclusion regions) “cuts away” the part of the plane where the Higgs cannot be, having been excluded by the LEP II experiments.

Then there are the results. Let us forget about individual expected limits by CDF and D0 (red and blue dashed lines) and concentrate instead on the black ones. The dashed black line shows the 95% CL limit that the Tevatron experiments were supposed to be able to set, given the datasets analyzed in each analysis and every small feature of each. You get that black dashed line only after combining the results of combinations of pseudoexperiments, taking care of nuisance parameters, integrated luminosities, cross correlations, correlated efficiencies, statistical effects of any kind, and other devilish details. But that is only an expected limit: it tells you what is the real power of the experiments.

The real limit is the one shown with the continuous black line. And here we get to see that, had the Lepton-Photon conference (for which this plot was finalized) been held in october or november this year, the Tevatron could have had a chance to start crying “No higgs at 160 GeV” on newspapers throughout the world. In fact, the integrated luminosity of the analyses most contributing at 160 GeV (H->WW decay searches) were of about 1.9/fb in each experiment, but both CDF and D0 have about 60% more data already in their pockets, and only lacked the time to process them, validate them, and include them in the analyses.

The Tevatron has been a bit lucky there though: as you can see in the plot, the limit set at 160 GeV is about twice as good as the one they expected to set. Lucky ? A debatable point indeed. However, the first point to bring home is definitely that the CDF and D0 experiments are starting to bite in the Higgs hunt business.

Then, you get to see something strange. What is going on at 120 GeV ? The limit there is twice as bad as pseudoexperiments predict!

That fact only highlights the amount of variability of the results that the Tevatron can display, with respect to their “expected limits”. Indeed, the thin dashed black line (and the other dashed lines too) is a misrepresentation of reality: instead of a thin line you should be looking at a wide band – a 1-sigma contour of expectations. The lack of the band is certainly only due to lack of time and hurry in producing the plot in time for summer 2007 conferences, and I think this plot will get more detail in a month or two. In any case, in the absence of the band, I am unable to speculate on how likely it was to get such a lousy limit at 120 GeV if the Higgs was not there. 

Indeed, a unwitty soul could be looking at the discrepancy between dashed and continuous black lines above the mark at 120 GeV and claim that what the Tevatron  experiments are observing at 120 GeV is exactly what they would be seeing if there was a Standard Model Higgs boson produced with a larger-than expected cross section sitting right there. Because the lousier limit there means that the experiments saw more candidate events at 120 GeV than they expected to find from background sources…. An excess ? No way! A fluctuation, and be sure, a mild one.

When the  +-1-sigma bands of expected results will be released, we will be able to quantify just how unlucky – or unlikely – the result is at 120 GeV, if the Higgs boson is not there. In the meantime, the second point to bring home from the plot is that that 95% CL line at 120 GeV is still at 8.5 times the expected Higgs cross section.

That underlines how hard the Higgs boson search is at low mass – something the LHC experiments have already started to dread. The ATLAS and CMS experiments, I wish to remind you, will be in great embarassment if the Higgs mass happens to be just a few GeV higher than what LEP II excluded. And that not because they would be taking data next year only by virtue of having shut down the LEP II experiment only months short of discovering the Higgs boson: but because the Higgs at 120 GeV is tough, very tough, and at the LHC life is even tougher than at the Tevatron. Proof be the wagons of money the experiments have invested in exquisite electromagnetic calorimeters, to detect a blip in the diphoton invariant mass from the H->gamma gamma decay!

UPDATE: in a comment below, an anonymous visitor asks why the actual limit at 120 GeV is so much worse than the expected one, given that the combination by CDF (which I posted a few days ago) is not terribly above their own expectation. I mentioned that the combination of limits is a tricky business, and that one could not really guess until one had all the data available. So here is the D0 combination by itself, the missing piece:

One clearly sees that D0, too, got a larger exclusion at 120-140 GeV than they expected. If you combine this with the CDF limit, I have no reason to doubt that the combination will be worse than expectations in that range.

But let us look at numbers a bit more carefully: at 120 GeV, D0 expected to set a limit at 7xSM and got 10xSM, and CDF did exactly the same: 7xSM expected, 10xSM obtained. At 140 GeV, though, D0 expected 7.5xSM and got 10xSM, CDF expected to set limits at 7.5xSM and actually got 5.8xSM (or so I read off the plot – I am too lazy to fish out the numbers from internal CDF documentation, and I would still be unable to diffuse it here!).

What we get is that a combination of those numbers yields, according to the CDF-D0 group who did the exercise, at 120 GeV a 5xSM expected limit and x8 obtained, while at 140 GeV a expected 4xSM and an obtained 3xSM. Do these numbers make sense ? I think that simple statistical arguments justify those figures, and I thus have to dismiss the rather annoying hypothesis put forward by the commenter that “for the _expected_ overall curve, WHlvbb accidentally used its 68% CL expected, rather than 95%, in its contribution to overall, resulting in a lower than intended overall expected curve“.

UPDATE II: as I posted the above update, a suspicion arose in my mind. Indeed, the numbers for 120 GeV make more sense to me than those at 140 GeV, when a 10xSM combined with a 5.8xSM yield a 3xSM – suspicious. So I take back what I wrote above for the case of the point at 140 GeV. Rather, let us give a closer look at the “point” at 140 GeV in the combination plot, which I paste here magnified for your convenience:

One observes something weird: while there appear to be kinks in the red and blue dashed lines at 140 GeV (the red one is evident, the blue one less so, but still is there), one sees no kink for the black dashed line at the same abscissa.

A suspicion might arise that the black dash was only computed at 130 and 160 GeV, and a straight line drawn between those points. That would explain the slightly better-than-expected combined result at 140!

I will investigate.

UPDATE III (yes, they keep coming): I just revised the title to “1.4xSM”, since that is the actual value at 160 GeV.

UPDATE IV: I checked the issue with the Higgs conveners in CDF and basically, what I can do here is mention a few facts:

  • the combination is preliminary
  • it is based on preliminary results from the individual analyses
  • a better combination, with 1- and 2-sigma bands on the expected limit, and with more data points will be produced soon.

All the above is to say that there is no real mystery, and that one should be looking at the combined plot with a grain of salt. Let us wait for a better version of the plot, which is red hot from the press …. 



1. Peter Woit - August 23, 2007

Very impressive results. Congratulations!

2. Anonymous - August 24, 2007

Very nice. However there’s something I don’t understand. When you look at the individual channels (at least for CDF — see the plot you poster earlier), the only one for which the expected is really much lower than the observed at < 130 GeV is ZHvvbb (and note that that channel doesn’t contribute an enormous amount to the overall). The other individual channels appear to have their expected about equal to observed, even in the range 110-130 GeV. Yet the overall result has expected much below observed. Why? Perhaps D0 is the main source of the difference, but the large and (to my mind) unexplained difference is there even just for CDF overall — see the plot you posted below. Any ideas why?

3. Kea - August 24, 2007

Tommaso, it’s not directly clear (to me at least) how 60% more data translates into a (quantitatively significant) downward shift of the black curve, but I’m guessing that since no Higgs has yet been observed (via some trigger) even in the extra 60% of data, that we expect a significant downward shift. Is this the rough idea? Anyway, fantastic result, and thanks!!

4. Kea - August 24, 2007

Um, isn’t Connes’ Higgs about 170 GeV?

5. dorigo - August 24, 2007

Hi Peter, thank you – although I admittedly contributed little to these results.

Anonymous, the point you raise is a good one. Refraining from quoting the old sentence about people liking sausage, following laws, and asking how these are made, I just tell you I believe the combination of different results contains some subtle points, but I am unable to speculate on what exactly brought the limit up when I do not know the ingredients (D0 combination by itself). I will look into the matter and update the post or produce a new one when I have a better answer.

Kea, you are right: it is not easy to read off how a 60% increase of data translates into a downward shift of the black curve, especially given the fact that the black line is significantly on the lucky side already. In general, 60% more data in statistically dominated searches becomes a 27% improvement. However, I said the additional data might – just might – allow the line to cross the curve.
As for the data taken with other triggers, I believe neither CDF nor D0 has really looked at it, for any analysis (the most analyzed, for a simpler b-physics search, is 2.2/fb in CDF, and I think something alike in D0).
As for Connes, I do not know much about his work. Maybe you want to post about it in your blog ? I’d be glad to read it and link it…

Cheers all,

6. Anonymous - August 24, 2007

Regarding my question above — I’m going to take a guess at why this is so. Note that the expected CDF WHlvbb closely matches the +1 sigma band of the overall CDF expected at around ~115 GeV (where WHlvbb dominates). I’m going to guess that, for the _expected_ overall curve, WHlvbb accidentally used its 68% CL expected, rather than 95%, in its contribution to overall, resulting in a lower than intended overall expected curve. Clearly that would be a bit of an error, if by some very small chance I’ve dead reckoned the issue here ….

On the other hand, it could just be a non-trivial correlation between the channels that happens to differ between expected and observed, in which case of course not a problem at all. Seems like a large difference to me, however.

7. Tripitaka - August 24, 2007

Very cool post T, and keep those updates coming…

8. island - August 24, 2007

Wow, those fluctuations can really keep you guessing. Great job, great news… Keep the heat on em, and may all your results be null… 😉

9. Euclidistheway - August 24, 2007

Does this anticipate a flurry (an excitation) of papers explaining the the new physics of a Higgs at 115 GeV ?

10. dorigo - August 24, 2007

Hi Tripi, thank you! I will.

Island, 🙂 I am unsure what to hope for. I have been sitting and waiting for a while now.

Euclid, I don’t really think so!

Cheers all,

11. Anonymous - August 24, 2007

Thanks very much for looking it over, and of course congratulations to the very large number of people involved in these results (trust me 😉 I know these things are like). Looking forward to the CDF, D0, & combination updated plots. My concerns stemmed not really from the CDF-D0 combination, but from CDF individual channels vs. CDF overall (since D0 doesn’t show their individual channels, at least not on that plot, one can’t easily do the same comparison for D0 — I hope D0 includes their individual channels on any updated plot that they do). No intent to be annoying, it’s clearly very important that everything be perfect on these plots, so one sometimes has to be a pest (and make pesky suggestions 😉 when something looks like it has a chance of possibly being statistically amiss.

Good work to all involved — and thanks Tommaso for giving the public, as well as particle physicists not in Tevatron collaborations (like me), a chance to ask questions of results as soon as they come out and not have to either wait for speakers to make their way to my university, or lab or conference I happen to be at — or to bother some colleague on CDF or D0 by e-mail.

12. Alejandro Rivero - August 24, 2007

Kea, consider Connes higgs more as a proof of concept than a strong prediction.

13. anomalous cowherd - August 25, 2007

Dear Professor Dorigo

You wrote:
“The ATLAS and CMS experiments, I wish to remind you, will be in great embarassment if the Higgs mass happens to be just a few GeV higher than what LEP II excluded. And that not because they would be taking data next year only by virtue of having shut down the LEP II experiment only months short of discovering the Higgs boson:…”

I think that this statement is inaccurate, and unfair to the management of CERN, and the scientific leadership of the LEP experiments. You should remember that unlike hadron collider exeriments, which are essentially broad band beams of quarks and gluons, LEP annihilated electrons and positrons, and in each event essentially dumped the entire centre of mass energy into the produced final state. This means that while at the Tevatron your limits on the Higgs are at a small fraction of your nominal centre of mass energy, and can be improved by running with larger statistics, the LEP II limit was essentially at the end of their kinematic range [remember that it was an H + Z final state, so that you run out of energy at the sum of those masses]. To produce a 120 GeV Higgs, the threshold for production would be at 211 Gev, and you need to get somewhat over the root s threshold turn-on to have any rate. If I recall correctly they ran up to about 208 GeV [by putting as much power into the RF as possible]. Given this limitation of their root s, I think that the LEP limit of 115 GeV is an outstanding accomplishment. It is also completely clear that given their limitation on root s, there is no possible way they could have discovered a 120 GeV Higgs regardless of how long they ran.

This is one of the reasons why CERN management chose to terminate the LEP program when it did. And by so accelerating the timetable for the LHC it brings us closer to the day when we’ll be sure to have data in hand that will be able to convincingly decide the issue. [For reference, if LHC produces its expected (reduced!) luminosity during the first two years of running, that will be enough to enable 5 sigma discovery of a Standard Model Higgs over the entire mass range where the Standard Model makes sense. And yes, that is one of the reasons that both ATLAS and CMS have such high performance electromagnetic calorimetry, but if you look at the search channels in the 120 GeV mass range I believe that channels with Higgs -> tau + tau are also statistically important. So important work will have to go into understanding the detectors and the data set in those first two year of running, but barring unforseen disasters we can resonable look forward to the prospect of a 5 sigma discovery by the time of ICHEP 2010].

So please treat the work of other laboratories and communities with respect; Europe has put 15 years of financial investment into making this a possibility (with limited financial help from outside, though HEP experimentalists from the entire world are lining up to join LHC experiments… about 6,000 in total when I last looked). CERN management has been planning for this since their first LHC studies 25 years ago, and many experimentalists have been working on these experiments for 15 years already to make them a reality. On the whole I think that they have handled this in a professional, considered, manner that I wish we could emulate on this side of the Atlantic.

14. Anonymous - August 25, 2007

cowherd — in a small defense of Tommaso, your point is correct if m_Higgs is 120 GeV, but at least in hindsight a little bit less so if it ends up 117 — it depends on the exact value of “a few” — the decision to shut down was a necessary and, given the info at the time, a correct one. Not sure what any of this has to do with the professionalism of FNAL or North American HEP(?). Blogs are hardly restricted to FNAL or western hemisphere experiments(?), and they are, at least on balance, a useful tool that is here to stay, at least certainly when a few general rules are respected — that’s both my personal view and what has become a fairly general consensus. They will unavoidably be a (small) part of life at the LHC too when results start coming out.

15. dorigo - August 25, 2007

Dear anomalous cowherd,

I agree with most of what you say, however let me make a few points.

1) I spend 80% of my research time in CMS, and I know that the LHC experiments are supposed to cover the whole low mass region in a couple of years of running. However, I also have worked for 15 years at the Tevatron, and I happen to know that one thing are MC predictions and another is the maelstrom of a hadronic collision environment and the extraction of results from it. So I am careful about the H->gamma gamma and H->tau tau channels providing a definitive result with “2-years worth of luminosity”. Also, lots of things can go wrong…
2) Although in my post I use 120 GeV as a reference value, I only said LEP II would have been able to see a Higgs with more running time (and maybe inventing some way to squeeze a couple more RF cavities here or there in the tunnel, although I heard swearing that the beams could not be kicked a mev higher) at 115 or 116 GeV, which is a few GeV above the 114 GeV limit set by the LEP II experiments. You of course know that quite a few of our colleagues who where in the LEP experiments believe they have been stripped of the chance of a discovery by the decommissioning.


16. Thomas Larsson - August 25, 2007

Wow! Another fantastic post about real physics. Thank you Tommaso for keeping us updated on this exciting progress.

17. Louise - August 25, 2007

Great stuff! I’ve been following your posts, and feel that you are hot on on the trail of the Higgs or some other discovery. You’ve started to get jealous comments from “anonymous,” which is a sure sign of success.

18. dorigo - August 25, 2007

Thomas, Louise, thank you for the encouragement. I will indeed keep updating my readers on these searches…


19. anomalous cowherd - August 25, 2007

Anonymous writes:
” Not sure what any of this has to do with the professionalism of FNAL or North American HEP(?). ”

Just to be unambiguous, My comment was not a criticism of “the professionalism of FNAL”; indeed, given their restricted financial circumstances I am really impressed with the physics results now flowing from D0 and CDF. I just wish that the financial resources to develop the accelerator complex, and upgrade the detectors to
produce these results, had been provided to them a decade earlier.
For fifteen (15) years we have had the highest energy collider in the
world, in the Tevatron, and we are only really getting to explore its physics potential now, just when we are on the verge of LHC era.

At the time of the cancellation of the SSC project over a billion dollars a year (a Giga$) was being spent on its construction. If after its cancellation only 10% of that amount (a hundred million $ a year) had been retained and committed to developing the Tevatron collider program, both the facility and its detectors would have been much better much earlier, and it would have fulfilled its physics potential, much better much earlier. The experiments and the lab did the best with the resources which they were provided; one only wonders what they might have accomplished had that support been more generous and more timely.

What my comment did allude to was a national failure at the level of science funding and management in the north american program. We have never recovered from the SSC fiasco. We are now in the situation where our workhorse experiments will be shut on mass by the end of 2010 (at best). CLEO, BABAR, DO and CDF will all be memories in the next decade. The AGS particle program is already a memory, and funding for RHIC running is so restricted that a private donation from the mathematician and philanthropist James Simons [of Chern-Simons fame] was what funded a significant chunk of RHIC running last year. While working at CERN on the LHC is a fantastic opportunity for many of our young people, surely we owe it to them [and to the global health of HEP physics] to have a viable program on more than one continent.

20. Anonymous - August 26, 2007

I agree. Write your congressperson. Actually I think the best tactic will reveal itself when the Higgs and physics beyond the SM are discovered at the LHC, and it becomes clear that the U.S. lost the chance to host such stunning discoveries purely due to its own stupid politics. Then Congress will probably overreact as usual — but perhaps the U.S. will get a good ILC out of that, which would be the right ending to the story. (Or new beginning.)

21. dorigo - August 26, 2007

Dear anomalous and anonymous,

while I think the points you both raise are sensible, I cannot help smiling… Being an internationalist, I have no idea why one should prefer an advancement of science if it is achieved in this rather than that set of geographical coordinates.

Let’s just work in order to ensure that money keeps flowing and sensible political decisions are made in the future with respect to “how” (rather than the less interesting “where”) to continue pushing further our investigations in basic science.


22. Anonymous - August 26, 2007

oh I completely agree, but the only way to get money these days is to appeal to countries, because they are what taxes our income (europe is just starting to grow out of this, but it’s the exception so far). looking forward to the day when the UN or some other world body controls scientific funding, but not holding my breath. 😉

23. anomalous cowherd - August 26, 2007

T. Dorigo writes:

“while I think the points you both raise are sensible, I cannot help smiling… Being an internationalist, I have no idea why one should prefer an advancement of science if it is achieved in this rather than that set of geographical coordinates.”

Umm…. actually if you look back at my original post, its point was to make a request:

“So please treat the work of other laboratories and communities with respect; Europe has put 15 years of financial investment into making this a possibility (with limited financial help from outside, though HEP experimentalists from the entire world are lining up to join LHC experiments… about 6,000 in total when I last looked). CERN management has been planning for this since their first LHC studies 25 years ago, and many experimentalists have been working on these experiments for 15 years already to make them a reality. On the whole I think that they have handled this in a professional, considered, manner that I wish we could emulate on this side of the Atlantic.”

Rather than advocating that we “should prefer an advancement of science if it is achieved in this rather than that set of geographical coordinates” my point was exactly the opposite; I felt that the original article had been unfairly critical of CERN management, and the scientific leadership of the LEP experiments, and I was writing to ask that their contributions be shown the respect that they deserve. CERN has suffered severely under the burden of trying to build the LHC within its budget and without much international help [I visit the theory division there semi-regularly, and the deteriorating physical plant, staff cuts, overworked secretariat, reduced number of fellows, etc. are all prices they have had to pay to help bring the LHC to completion]. They’ve cut everything they reasonably could, and forward borrowed on their budget through 2010, in order to construct this facility that will be used by scientists worldwide. So it’s hard to let pass comments criticizing the necessary decisions that had to be taken to complete the LHC on a feasible schedule, especially since physicists from all over the world are now lining up to work there, even though their own funding agencies have spent the last decade doing their best to contribute as little as possible to LHC construction.

P.S. I remain of the view that the global health of HEP physics depends on having “a viable program on more than one continent.” European taxpayers can’t be expected to subsidize the field worldwide indefinitely. It’s clear from the LHC that it represents the limit of what is financially possible for one continent [and technically, LHC construction saturated the capacity for high-tech fabrication in some sectors of european industry]. The LHC and its luminosity upgrade will saturate the CERN budget for two decades [1995 – 2015]. To have the possibility of results from other than the LHC as well, on a timescale that will provide opportunities to young scientists entering the field now, it’s important that concrete plans for complementary facilities be undertaken in Asia and North America. Asia is taking the lead on this, with JPARC coming on line soon after the LHC, and plans for Super-BELLE in progress. I think that it’s time for North America to get its act together.

P.P.S. Just to be unambiguous, when I use the personal pronoun “we” in my posts above, “we” means the international community of particle physicists.

24. dorigo - August 27, 2007

Hi Anomalous,

well, I do not think I would disrespect the CERN or LEP managements if I wrote in a blog what many still think (and beware, I am not among them), i.e., that closing LEP on the verge of a giant discovery was a giant blunder. I did not put it that way though, and I do not think along those lines. But you act as if I wrote something of that kind…. Blogs tend to mix correct information with sarcasm, fallacies, and random opinions, and as a blogger I am not a white fly. I still do not believe I went too far above.

I actually think that the CERN management did the right thing when they shut LEP down. Is that more clear ?

In any case, you write down things in your comment above as if CERN management was a good father of five, trying to make ends meet, and I was the neighbor criticizing the lease of a new car. It is not exactly a correct picture. CERN receives its money by the nations supporting it, and Italy – the country of which I am citizen and where I work – contributes with loads of funding. INFN, the institute for which I work, and which has built critical parts of the Atlas and CMS detectors (the 100 square meters of silicon sensors of the CMS detector, to give an example), in the face of it has kept hundreds of winners of positions on hold for the last three years. So when you write
“even though their own funding agencies have spent the last decade doing their best to contribute as little as possible to LHC construction”
you are close to being unfair to INFN… But I am not interested in criticizing you for that. As seen from different points on this planet, the drawbacks of a giant project appear different.

I myself do not have a permanent position yet, only because of the budget choices made by INFN and the italian government in their funding of science in Italy in the last few years.

So, yes, building the LHC is a giant effort. Yes, CERN did the best they could with the money we supplied it with. I do not think it is a matter of respect. Scientific decisions were made, and I subscribe to them.


25. anomalous cowherd - August 27, 2007

Dear Professor Dorigo
I agree with your point. It is not only CERN which has suffered penury as a result of LHC construction, but all of European particle physics. The CERN contributions provided by all the European member states have built a facility that will advance world science. And the INFN has been a strong supporter of the LHC experiments, at considerable cost as you correctly point out.
When I write:
“even though their own funding agencies have spent the last decade doing their best to contribute as little as possible to LHC construction”
I am specifically referring to the non-member states, who are not bound by agreed CERN contributions, and who have done relatively little to help share the burden. There was relatively little contribution to the actual machine construction from non-member states, leaving the vast majority to Europe. And in my own country, the funding agency is even dragging its feet about fully paying their agreed common fund contributions to the LHC experiments.
In sum: I agree with you; the construction of the LHC has required hard sacrifices right across the European member nation states, not just at CERN itself. I am not criticizing Europe, nor the INFN, who have shouldered a very large burden over the past decade of LHC construction. I just think that it’s time that other nations, whose scientists will work at the LHC, assume more of their responsability.
With Best regards

26. chris - August 29, 2007

please excuse this question from an ignorant theorist, but i would be really interested to know.

when the lep search for the higgs was abandoned, there were some candidate events – especially from aleph. if memory serves me well, the strict lower limit set by LEPII was therefore slightly lower than expected.

assuming that this was not a fluctuation but the onset of a resonance, would have increasing the luminosity been enough to establish the existence of a higgs? let’s say it hides really close to the limit – at 116GeV or so. and also assuming that the beam energy was maxed out already. wouldn’t it have been enough to reach the increasing side of the resonance and sample there with enough luminosity without ever going over the peak?

also, since i am at it, i really can’t resist asking an experimentalist who has some insight what their gut feeling is. was LEP close to discovery or was it just another (2 sigma?) fluctuation?

thanks alot in advance.

27. dorigo - August 29, 2007

Dear chris,

as an ignorant experimentalist, I think I have some answer for you. The limit set by LEP II was indeed slightly – but we are talking about tenths of a GeV – worse than what on average the experiments would have been expected to set (I will fetch the exact number later if I have time).
If the H is at 116 GeV, indeed LEP II with no energy increase would only see the “lower tail” of th resonance. But that would still mean observing an increased rate, which – with enough luminosity – is bound to be an observation in its own right. Maybe the mass would have not been determined with high precision, but the signal would have been established by running three times longer at the highest energy. The excess was a 1.7-sigma effect.

I think there is a sizable chance that indeed, the higgs is at 115 GeV.
But it is really just a toss-up.


28. Euclidistheway - August 29, 2007

I predict a (MSSM) Higgs will be found at 114.56 GeV. This is dependent on top mass= 170.97 GeV.

29. chris - August 30, 2007

dear dorigo,

thanks alot for the explanation. that must really have been a tough decision.

and in general thanks for this nice blog. the only quality hep blog around it you ask me.


30. dorigo - August 30, 2007

Hi Chris,

thank you so much for your appreciation. It is that kind of feedback that keeps me going, because I am not immune to flattery.


Euclid, before we get a precision on the Higgs mass to the second decimal point in GeV, we will all be retired.


31. Updated Higgs search from D0: WW final state, 1.7/fb « A Quantum Diaries Survivor - December 2, 2007

[…] After seeing this plot, which reaches a x2.4 SM value at 160 GeV, I am starting to be very curious to see the combination with CDF results (which stood at x1.9 SM at 160 GeV already last August). I think we will have to wait for winter conferences to get that plot, but I smell a x1.1 limit at 160 GeV: not yet any mass exclusion for winter 2008 (for the latest combination, yielding x1.4 SM, see here). […]

32. 115 GeV Higgs: is evidence piling up ? « A Quantum Diaries Survivor - March 29, 2008

[…] piling up ? March 29, 2008 Posted by dorigo in news, physics, science. trackback Ever since the Tevatron Run II experiments have started to produce results of the search for a Standard Model […]

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