Archive for December, 2011
and CMS has the better analysis.
Fabiola Gianotti made a masterful presentation of the ATLAS Higgs search, and Guido Tonelli made a highly insightful and detailed one for CMS. The presentations were both excellent with a wealth of information that will take time to digest. The bottom line, though, is that both collaborations observe a modest excess around 125 GeV which may – or may not – be the first signs of a standard mode-like Higgs boson.
This is an extremely exciting time, despite all the sober and cautious statements by the speakers, DG, audience and by nearly all colleagues – people are excited by these results. Few people would claim that the Higgs has been found, but its definitely not not there, if you know what I mean.
I will share some personal first impressions here – but I should state clearly that I am a member of the CMS Collaboration, not involved in any of the Higgs searches but heavily involved in electroweak topics, some of which have a direct bearing on the Higgs results. Anyway…
It is plain that the statistical significance of the Higgs results from ATLAS is higher, if interpreted as evidence (not really the right word…) for a signal. They report a combined significance of 3.6σ compared to the CMS combined significance of 2.4σ. To be clear – this does not mean that ATLAS has done a better job or that their analyses are more sensitive. Indeed, they used less integrated luminosity, employed fewer channels and did not utilize any multi-variate techniques. Monte Carlo studies indicate that the CMS search is more sensitive, a priori, but the ATLAS Collaboration was luckier with the roll of the dice – they were dealt a luckier set of cards.
Both Fabiola and Guido made interesting comments within the context of their talks.
Fabiola pointed out that 120 – 130 GeV is a very nice region for the Higgs boson. At first this may seem peculiar, because it is a most challenging one at the LHC. But her point is a very good one: only in this low-mass region can the LHC collaborations expect to see a signal in several different final states (WW, ZZ, 2-photon, ττ, bb, etc.) which would, eventually, allow us to learn a lot about relative branching ratios, mass, spin and couplings. This would not be the case if a Higgs boson showed up at 250 GeV, in which case only WW and ZZ channels would be fruitful. So if the present hint is confirmed, LHC physicists can enjoy years of data analysis and slowly reveal the properties of the Higgs boson signal (if, indeed, that is what they would be observing).
Guido made the clear point that the CMS constraints on the Higgs boson are distinctly weaker than they should be due to the excess in the data – that there is essentially no difference in the mass limit at 95% Cl and 99% CL. This “difficulty” is precisely what one would expect if a new signal indeed is present. In his words, “the excess in the CMS data prevent to go below 127 GeV.” The same is true, of course, for the ATLAS data. He also showed how there is a broad excess in the WW + ττ + bb channels “modulated” by the γγ + ZZ channels. This is a pithy way of describing the current preliminary result.
Anyway, to return to my opening statement (which is tongue-in-cheek of course). I can phrase my impression another way:
- CMS did more with less, (the apparatus is much smaller than ATLAS) while
- ATLAS got more for less (the results are “luckier”).
Now let’s see what next year brings!
Update: A very complete set of documents, plots, videos, etc. from CMS is available here: http://cms.web.cern.ch/news/cms-search-standard-model-higgs-boson-lhc-data-2010-and-2011.