The Higgs Boson discovery

The results presented by the ATLAS Collaboration (2012) and the CMS Collaboration (2012) summarised above, provided conclusive evidence for the discovery of a new particle with a mass around 125 GeV in the data taken by the ATLAS and CMS experiments. The measured signal strength was found to be consistent with the one expected for the production of the Higgs boson of the Standard Model. The decays to pairs of vector bosons with a net electric charge of zero identified the new particle as a neutral boson. The observation in the diphoton channel excluded the spin-1 hypothesis for the production of an on-shell resonance by virtue of the Landau-Yang theorem. Although these results were compatible with the interpretation in terms of the Standard Model Higgs boson, a wide range of other interpretations was possible as well. At the end of the first LHC operation period (2010 – 2012) about 2.5 times more data had been collected by each of the experiments than was used for the discovery of the Higgs boson. Using this data, evidence for the Higgs boson was seen in many individual production and decay modes, including the Standard Model predicted decays into fermions. Extensive searches were made for deviations from the Standard Model predictions, but in general the measured signal strengths were broadly consistent with expectations, and tests of angular distributions indicated that the particle discovered is a scalar, as expected. A combined measurement of the mass of the Higgs boson by the two experiments (ATLAS and CMS Collaborations, 2015) obtained the result mH = 125.09 ± 0.21 (stat.) ± 0.11 (syst.) GeV.