Is it a Higgs?
Here’s a quick relatively non-technical FAQ on the Higgs results from yesterday. If you have another question send me a tweet or leave a comment below and I’ll answer it if I get time.
Has the Higgs been discovered?
Maybe! Usually ‘the’ Higgs refers to the Standard Model Higgs but many other theories (e.g., Supersymmetry) have a Higgs sector. The searches at ATLAS and CMS are designed to look for the Standard Model Higgs. This means we can get a very good idea of how consistent the data are with what we expect the Standard Model to look like. The current results are largely consistent with these expectations.
Have we reached the 5 sigma benchmark for observation?
Yes and no. The Standard Model Higgs, once its mass is known, has well predicted decay rates. At the LHC we reconstruct 5 decay channels:
- Higgs decays to 2 photons
- Higgs decays to 4 charged leptons via ZZ(*)
- Higgs decays to 2 charged leptons and 2 neutrinos via WW(*)
- Higgs decays to 2 tau leptons
- Higgs decays to a b quark and an anti-b quark
None of these individual channels have been measured at the 5 sigma level, instead when statistically combining the data from each decay channel the individual – smaller – excesses over background reinforce each other and push the combined significance over the 5 sigma level. The goal in the next months will be to see if the 5 sigma measurement can be made in each decay channel. The current measurement is mainly dominated by the 2 photon channel followed by the ZZ channel.
Is there any inconsistency with a Standard Model Higgs?
Possibly. There are not enough data to draw firm conclusions but both experiments see a slightly higher than expected rate in the 2 photon decay channel. Perhaps more interesting and in need of further investigation is the CMS result for the 2 tau lepton decay channel. Currently this updated result comes very close to excluding a 125GeV Standard Model Higgs at the 95% confidence level. ATLAS does not have an updated result yet but if it reinforces the CMS result this would be a very compelling indication that we’re not dealing with a Standard Model Higgs. The next few months could be very interesting!
What can we say about the new particle?
It decays to 2 photons and 4 charged leptons via ZZ and possibly to 2 charged leptons and 2 neutrinos via WW. All of these decays are at a rate close to the expectation from a Standard Model Higgs. The particle has a mass somewhere around 126GeV and the observed decay channels mean it must be a boson with even spin (0 or 2); we expect the Standard Model Higgs to have spin 0. Tevatron and CMS data hints that it also decays to a b quark and an anti-b quark but there is no evidence at the moment that it decays to two tau leptons.