[This paragraph was my introductory paragraph from this morning, before the presentation. Details of the presentation follow.] IndependHiggs Day has dawned, warm and hazy. Yesterday, at the CERN laboratory that hosts the Large Hadron Collider [LHC], all the discussion at lunch, and over afternoon coffee and evening beer, centered on the Higgs particle (here’s my Higgs FAQ, my video clips for non-experts about how the Higgs search is done, and a recent article for non-experts explaining its larger importance;more detailed background on the Higgs is linked from here), and on today’s presentation to update the search for it. The mood is ebullient and excited; everyone is anticipating a very big step forward, perhaps a definitive one. Gone is the uncertain mood of December, when CERN preceded its presentations with a warning that the results shown then would not be conclusive — a warning that is notably absent this time. It’s a party atmosphere, and a media circus; many luminaries are here, invited by the director to witness the event. The press will fill an entire auditorium all their own; physicists will be spread out over perhaps a dozen auditoriums. It’s been suggested that CERN should have rented a soccer stadium in Geneva; the small size of the main auditorium at the lab reflects how much smaller particle physics experiments were just 30 years ago.
What I will be watching for in the presentations today is the following (to be updated when I know the answers): Here’s what happened in the presentation today:
[8:50 Professor Higgs has entered the room.]
[9:00 CERN Director General Rolf Heuer very brief opening remarks]
First, CMS (presented by spokesperson Joseph Incandela, from the University of California Santa Barbara): (5.2 inverse fb of data, with 5.6 for muon analyses, improvements on many fronts)
- Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons? Yes!! 4.1 sigma, a bit above standard model expectation though not too much
- Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs? Yes! 5 or 6 events above background, 3 sigma, a bit below expectation but quite consistent with standard model
- TOTAL 5 SIGMA!!! LOUD APPLAUSE…
- Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino? (a) not at all, but very low statistics (b) not incompatible, but still low statistics (c) preliminary technique, yes
- TOTAL 4.9 sigma, EXPECTED 5.9 sigma ;
- MASS 125.3 +- 0.6, preliminary
- Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011? Yes
- Does the evidence in favor of a new particle support the idea that this is a Higgs particle? Absolutely!!!! As I have explained elsewhere, an observation of a signal in both photons and leptons, with even roughly the ratio predicted by the Standard Model, is a very strong argument in favor of the new particle being a Higgs particle.
- Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)? No signs of (statistically significant) non-Standard Model behavior
Next, ATLAS (presented by spokesperson Fabiola Gianotti, from CERN): (up to 5.9 inverse fb, and various improvements, some quite important)
- Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons? Yes!! 4.5 sigma , 3.6 sigma with look-elsewhere effect over 110-150 GeV: (cross-section almost twice as big as expected! but only 2 sigma excess); mass about 126.5 GeV
- Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs? Yes!! 6 or 7 events above background, 3.4 sigma (2.5 sigma after look-elsewhere over 110-140), cross-section again a bit large, 1.3 +- 0.6 of Standard Model expectation
- Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino? Nothing from 8 TeV collisions shown today
- TOTAL 5.0 SIGMA!!!!!! LOUD APPLAUSE…
- MASS 126.5 GeV (from photons only; leptons are lower; no error bar given, so measurement of mass is not complete) with 1.2 of standard model production rate
- Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011? Yes
- Does the evidence in favor of a new particle support the idea that this is a Higgs particle? Absolutely!!
- Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)? No significant deviations from Standard Model though two-photon rate is currently 2 sigma high. Two sigma is not enough to be excited about. This is especially since the Standard Model prediction for the overall cross-section is a very tricky calculation; its current precision is estimated at 15%, but perhaps one might worry about that estimate.
Then:
- Is what CMS observes largely consistent with what ATLAS observes? Absolutely!!
- Are there any notable discrepancies between the two experiments that might undermine confidence in their results? None! The discrepancy of about 1.2 GeV in the masses quoted is not official, since ATLAS does not yet have a final mass measurement with a proper uncertainty estimate. Even if I take the ATLAS estimate as if it were a mass measurement, if I put a similar uncertainty on it as CMS has quoted, the discrepancy between the two experiments is below 2 sigma, not enough to cause concern at this time, especially given the strength of the signal in each experiment separately.
WOW!!!! CONGRATULATIONS LHC, ATLAS and CMS !!!
[Heuer celebrates a global success… A standing ovation for all…]
[No one has any questions; the data is so clean and so convincing!]
[Some nice words from the inventors of the idea…]
