| The Higgs mechanism causes the spontaneous electroweak symmetry breaking and generates masses for the elementary particles.The predicted particle,the Higgs boson,is the only scalar particle in the Standard Model(SM).The Higgs boson was discovered by the ATLAS and the CMS Collaborations at the Large Hadron Collider(LHC)in 2012.Over the past decade,the ATLAS and the CMS experiments continued to improve the precision of the measurements of the Higgs boson properties.The measurements tested the Standard Model predictions while probing new physics beyond the SM.The Higgs boson decaying to a pair of b-quarks has the largest branching ratio among all the Higgs decay modes,which makes it one of the most sensitive channel to study the Yukawa nature of the Higgs coupling to the fermions.This thesis presents a measurement for a Higgs boson produced in association with a W or Z boson and decaying to a bb pair.The analyzed data samples,corresponding to an integrated luminosity of 139 fb-1,were collected in proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV from 2015 to 2018(Run 2)by the ATLAS detector.The measurement focuses on high vector boson transverse momentum phase spaces where the decay products of the Higgs boson can be effectively reconstructed as a large-radius jet(R=1.0).This thesis also presents the reconstruction of the largeradius jet collections,in particular the jet energy and mass scale calibration.Several large-R jet definitions have been developed at the ATLAS experiment.These jet collections were evaluated based on the performance of reconstruction,calibration and tagging.The jet collection with the best overall performance becomes the baseline recommended to all the physics analyses.For the large-R jet collection used in the VH(H→bb)measurement,the jet energy scale is calibrated to 1%precision level and the jet mass scale is calibrated to 10%precision level.The calibration performance has been improved with the latest large-R jet collection.The non-closure of the calibrated jet energy scale is smaller than 0.5%.The H→bb decay was not observed in Run 1(2011-2012)of the LHC because this channel has an all-hadronic final state through the gluon-gluon fusion production mode.It is very difficult to select signal events from the overwhelming multijet background at the LHC.The most sensitive production mode to observe H→bb decay is the associated production of a Higgs boson and a W or Z boson,where the vector boson decays leptonically.The increased colliding energy and luminosity in Run 2 enabled sufficient statistics of V H production.The evidence of the H→bb decay was first observed in data collected during 2015-2016,corresponding to an integrated luminosity of 36.1 fb-1.The signal was observed with significances of 3.5 and 3.8 standard deviations by the ATLAS and the CMS experiments,respectively.The decay products of the Higgs boson were reconstructed as two b-tagged small-R jets(R=0.4)in the analysis.Further measurements continued to improve the sensitivity of the analysis with full Run 2 dataset.The measurements have been conducted through two independent approaches.One of the approaches mainly targets low to medium transverse momentum phase spaces where the decay products of the Higgs boson can be reconstructed as two small-R jets effectively,referred to as the VH(bb)resolved analysis.The other approach mainly targets medium to high transverse momentum phase spaces where the decay products of the Higgs boson can be reconstructed as a large-R jet,referred to as the V H(bb)boosted analysis.The analysis presented in this thesis follows the second approach.The VH(bb)resolved analysis had the same strategy as the original measurement that observed H→bb decay.The events were selected by requiring two b-tagged small-R jets in the final states.The ZH signal was observed with a significance of 5.3 standard deviations and the W H signal was observed with a significance of 4.1 standard deviations.The high transverse momentum phase spaces can be more sensitive to potential anomalous couplings.Moreover,the resolved topology is not very efficient for reconstruction of the decay products of the Higgs boson in high pT regions.The angle between the two b-jets becomes smaller with larger Higgs boson pT,and the two jets merge into a large-R jet.In the VH(bb)boosted analysis,the events were selected by matching jets reconstructed from tracks(track jets)to the leading large-R jet.Two of the track jets were required to be b-tagged.The V H signal was measured through a maximum profiled likelihood fit to the large-R jet mass distributions.The signal strength,defined as the ratio of the observed cross section and the Standard Model predicted cross section,was measured as 0.72-0.36+0.39.The signal was observed(expected)with a significance of 2.1(2.7)standard deviations.The cross sections of the associated production of a Higgs boson and a W or Z boson were measured in two transverse momentum regions,250-400 GeV and above 400 GeV.All the measurements are compatible with the Standard Model predictions.The results were interpreted in the Effective Field Theory as constraints on anomalous couplings.After the success of the VH(bb)resolved and boosted analysis,the measurement of H→bb decay can still be improved from many aspects.The VH(bb)resolved and boosted analyses have been combined into an analysis that aims to design one coherent,harmonized approach to extract V H(bb)results and extend the physics output.This analysis is in progress at the time of the publish of this thesis.The sensitivity of the measurement has been improved significantly through the multivariate analysis introduced to the boosted topology.The b-tagging algorithms and the Higgs tagging strategy are also optimized together with the multivariate analysis.Furthermore,the measurement of H→bb decay will benefit from the increased luminosity in Run 3 of the LHC(expected 300 fb-1),the large-R jet collection with better resolution and future improvements of b-tagging,object reconstruction. |
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