We were all transfixed by the Higgs seminars on July 4, but the work was nowhere near over for the experimentalists — they had to actually write up papers describing the results. And of course taking the opportunity to do a little more analysis along the way.
The ATLAS Collaboration
(Submitted on 31 Jul 2012)
A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb^-1 collected at sqrt(s) = 7 TeV in 2011 and 5.8 fb^-1 at sqrt(s) = 8 TeV in 2012. Individual searches in the channels H->ZZ^(*)->llll, H->gamma gamma and H->WW->e nu mu nu in the 8 TeV data are combined with previously published results of searches for H->ZZ^(*), WW^(*), bbbar and tau^+tau^- in the 7 TeV data and results from improved analyses of the H->ZZ^(*)->llll and H->gamma gamma channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0 +/- 0.4(stat) +/- 0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10^-9, is compatible with the production and decay of the Standard Model Higgs boson.
And here’s CMS:
The CMS Collaboration
(Submitted on 31 Jul 2012)
Results are presented from searches for the standard model Higgs boson in proton-proton collisions at sqrt(s)=7 and 8 TeV in the CMS experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at 7 TeV and 5.3 inverse femtobarns at 8 TeV. The search is performed in five decay modes: gamma gamma, ZZ, WW, tau tau, and b b-bar. An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, gamma gamma and ZZ; a fit to these signals gives a mass of 125.3 +/- 0.4 (stat.) +/- 0.5 (syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.
No huge surprises, I would say, but a few tiny tweaks to the earlier announcements. ATLAS now includes an analysis of decays into two W bosons, which wasn’t discussed in the July 4 seminar. Their overall significance is now 5.9 sigma, and CMS has 5.0 sigma, both a touch higher than before. (And at this point the search for more sigmas becomes less urgent; nobody really doubts that they’ve seen something.) The seminar results hinted at a discrepancy between the observed rate of decays into two photons, and the predicted rate in the Standard Model; sadly, the significance of this discrepancy has gone down just a bit. “Sadly,” of course, because such a discrepancy could be a sign of new particles contributing to the decays. They could still be there, but we’ll need more data to say anything for sure.
For the book I’m writing (did I mention that?) I fought to include two relatively technical-looking plots — the “bumps” in the two-photon events near the mass of 125 GeV. My winning argument was simple: “This is what we paid $9 billion for!” So here are the relevant updated plots from these papers, three for each experiment.
CMS two-photon events:
ATLAS two-photon events:
CMS 4-charged-lepton events (via ZZ):
ATLAS 4-charged-lepton events (via ZZ):
CMS 2-tau events:
ATLAS 2-charged-lepton events (via WW):
We’re certainly not sure that this is “the” Higgs boson as predicted by the Standard Model, and both papers are careful not to fall into that trap. In my book I was intentionally not so careful. I did take care to explain that, while we’ve found a new particle, we don’t know for certain that it’s the SM Higgs. But it’s certainly Higgs-like, and if further investigation reveals that it differs in interesting ways, so much the better. So I’m happy to call it “the discovery of the Higgs boson” until we get firm evidence that it’s something else.