Ok, having heard the talks, had a lot of conversations with colleagues, and looked a bit more carefully at the plots from today’s talks at CERN, I have a few revised comments, somewhat more optimistic than my initial reaction, with more details to follow.
Exclusion in new regions:
- The CMS experiment has excluded a new chunk of the Higgs mass range, from 127 or 129 GeV (depending on how aggressive you want to be) to about 600 (previously the two experiments together excluded 141 to 476. [Look to CMS for the official numbers]
- ATLAS did not change their result for high mass but excludes the lightweight Higgs down to 131.
- Both experiments are seeing effects in the 120-130 GeV range that are preventing them from excluding as much as they would have expected to do on average.
Hints from ATLAS:
- ATLAS, using Higgs –> 2 photons and Higgs –> ZZ –> two leptons and two antileptons, shows an excess at about 126 GeV, of a local significance of 3.6 sigma (but only 2.3 sigma with most conservative form of the look elsewhere effect; the more complete number lies somewhere between).
- If there were a Standard Model Higgs particle at 125-126 GeV, the expected local significance would be 2.4 sigma. The signal seen is therefore compatible with a Higgs signal of this type.
- It is still compatible with a large fluctuation in the background.
Hints from CMS:
- CMS, using 5 different measurements — Higgs –> 2 photons, Higgs –> ZZ –> two leptons and two antileptons, Higgs –> WW –> lepton, antilepton, neutrino, antinuetrino, Higgs –> taus AND Higgs –> bottom quark/antiquark pairs — claims an excess at about 120 GeV and another at 124 GeVwith a local significance of 2.6 sigma (but only 1.9 sigma with the most conservative form of the look elsewhere effect; the better number lies somewhere between).
- If there were a Standard Model Higgs particle at 125-126 GeV, the expected local significance would be 2.6 sigma. The signal seen is therefore very compatible with a Higgs signal of this type.
- The claim CMS wants to make is that the ratios of the excesses in the 5 different channels is quite consistent with a Higgs signal at around 124 GeV. I am not sure I believe this argument, but it is certainly a legitimate one to make. To believe it, I would need a much more careful explanation of how the three more subtle analyses were done.
- It is still quite compatible with a moderate fluctuation in the background.
Are they compatible?
- There are some weird features in the data that could be due to low statistics, but roughly speaking the answer seems to be that the hints at ATLAS and CMS are reasonably compatible with each other.
- What we saw today is probably compatible with a Standard Model-like Higgs at about 125 GeV.
- What we saw today is also probably compatible with a large but not extraordinary fluctuation in the backgrounds, perhaps combined with a subtle technical problem in one or another analysis.
- And the only way to find out which of these two is the truth is to gather a lot more data in 2012. Period.