Observation Of H?bb Decays And VH Production With The ATLAS Detector

A search for the St.andard Model Higgs boson produced in association with a W or Z boson,and decaying to a bb pair has been performed with the ATLAS detector.The data were collected in proton-proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV,and correspond to an integrated luminosity of 79.8 fb-1.Three channels containing zero,one and two charged leptons(electrons or muons)have been considered to target each of the leptonic decays of the W or Z boson,Z ? vv,W?lv and Z?ll,referred to as as the 0-lepton,1-lepton and 2-lepton channels,respectively.A data-driven method has been developed to estimate the multijet background in the 1-lepton channel,along with the detailed studies to assign proper uncertainties on the estimation.Extensive studies have been carried out to improve the sensitivity and robustness of the analysis,such as a study of the 1-lepton channel medium pTV region and a study of pile-up jets suppression cuts.These have been combined with the works on validating and updating the boosted decision tree training and extensive fit studies,to ensure the robustness of the final results.For a Higgs boson mass of 125 GeV,an excess of events over the expected background from other Standard Model processes is found with an observed significance of 4.9 standard deviations,compared to an expectation of 4.3 standard deviations.The ratio of the measured signal events to the Standard Model expectation equals to 1.16+0.270.25= 1.16± 0.16(stat.)+0.21-0.19(syst.).The result is also combined with the other results from the searches for the Higgs boson in the bb decay mode in Run 1 and Run 2.the combination yields an observed(expected)significance of 5.4(5.5)standard deviations,and therefore provides a direct observation of the Higgs boson decay into a bb pair.In addition,a combination of Run 2 results searching for the Higgs boson produced in association with a W or Z boson yields an observed(expected)significance of 5.3(4.8)standard deviations,and therefore provides a direct observation of Higgs boson being produced in association with a W or Z boson.