Evidence for the Higgs boson decaying to a pair of $b$ quarks

The Higgs boson was predicted in the Standard Model (SM) to explain the masses of elementary particles. The Large Hadron Collider (LHC) is used to test the SM in proton-proton collisions at unprecedented center of mass energy. The results are important to search for new physics beyond the SM, which can potentially explain unsolved problems such as hierarchy problem, baryon number, dark matter, neutrino mass and so on. ATLAS and CMS experiments at the LHC discovered the Higgs boson in 2012 (Run1). The mass is about 125 GeV and the properties are consistent with the SM so far. However, the Higgs boson decaying to a pair of $b$ quarks ($H \rightarrow bb$) was not observed in Run1, because of large amount of background and limited statistics of data. Therefore, the $H \rightarrow bb$ is followed with great attention in the 2nd run from 2015 (Run2). This thesis presents a piece of evidence for the $H \rightarrow bb$. The LHC Run2 proton-proton collisions data collected with the ATLAS detector is used. The center of mass energy is 13 TeV, and the integrated luminosity is 36.1 fb$^{-1}$. ATLAS level-1 endcap muon trigger achieved about 90% trigger efficiency. A $b$-jet energy correction improved the Higgs mass resolution about 40% at maximum. An excess over background is found in the Vector boson associated production ($VH$) 2-lepton channel. The observed (expected) significance is 3.6 (1.9) standard deviations. The signal strength compared to the SM is $2.11\ ^{+0.50}_{-0.48}\ \mathrm{(stat.)}\ ^{+0.65}_{-0.47}\ \mathrm{(syst.)}$. In ATLAS $VH$ 0, 1, 2-lepton, Run1 + Run2 combined results, the observed (expected) significance is 3.6 (4.0) standard deviations. The signal strength is $0.90\ ^{+0.18}_{-0.18}\ \mathrm{(stat.)}\ ^{+0.21}_{-0.19}\ \mathrm{(syst.)}$. The results are consistent with the SM.