The seven months that changed physics

Physicists have been hunting the Higgs boson ever since the mid seventies, when the standard model gained widespread acceptance. The most sophisticated experiments preceding LHC were only able to produce limits on its mass. Thanks to the excellent performance of the LHC and by analysing the 2011 and 2012 data, the ATLAS and CMS experiments have discovered a new boson around a mass of 125 GeV and with a spin different from 1. The result is consistent, within uncertainties, with expectations for a standard model Higgs boson. The collection of further data will be necessary to ascertain this conclusion and measuring the quantum numbers of the new particle will be among the first priorities. The most challenging task will be to measure the couplings to other particles. So far, we can only conclude that the Yukawa coupling of the new particle to fermions has not been firmly established by the LHC experiments. By looking instead to the couplings to bosons, the ratio of the couplings to W and Z seems to be consistent with expectations in both experiments. The only possible hint of an anomaly comes from some intriguing difference with respect to the standard model prediction, reported in H → γγ by both experiments. A Higgs particle of mass 125 GeV puts strong constrains on many SUSY models. SUSY could be consistent with a Higgs-like boson but it would prefer it to be lighter. (A.C.)