Discovery, but is it a Higgs? - ePepys
4th Jul 2012
12:52 pm - Discovery, but is it a Higgs?
After months of equivocation ("hints"), we can finally say that we have discovered something.
It was all very exciting. I'm not used to hearing applause in the middle of a physics presentation. And the second time (ATLAS), they were applauding the number ("5.0σ") that I helped determine.
ATLAS see a 5.0σ excess, and CMS see 4.9σ. Each alone would probably be enough to claim a discovery. (According to particle physics convention, one needs >5σ. It is arguable whether one needs to include the "look-elsewhere effect", which reduces the significance to 4.2σ-ish, but one can either combine ATLAS+CMS (as some blogs are already doing), or use CMS to limit the ATLAS search range (or vice versa) so each doesn't need to look elsewhere.)
There certainly was an observation of a Higgs, but that was Peter Higgs, who attended the presentation. But we can't say that what ATLAS and CMS have discovered is a Standard Model Higgs boson. The observations are compatible with a Higgs (though both ATLAS and CMS see a few more decays to two photons than might be expected, which is what pushed us over the 5σ line), but now we need to measure its properties to see whether it's a SM Higgs, some other sort of Higgs, or something entirely new and different.