Combined measurements of Higgs boson properties using $\sqrt{s}$ = 13 TeV proton–proton collision data collected with the ATLAS detector

In July 2012 the ATLAS and CMS collaborations announced the discovery of the Higgs boson with a mass of approximately 125 GeV using proton–proton collisions collected at center-of-mass energies $\sqrt{s} = $ 7 TeV and 8 TeV at the Large Hadron Collider (LHC). It is one of the most significant achievements in physics and leads to the Nobel Prize in Physics in 2013. Following the discovery of the Higgs boson, a broad range of measurements of its properties has been performed. To probe the Higgs sector in the Standard Model (SM) more deeply, it is important to have measurements of the Higgs productions, decay branching ratios and couplings with a higher precision by using more data accumulated in LHC Run2 at $\sqrt{s} =$ 13 TeV. This dissertation presents my research in High Energy Physics at the LHC with the ATLAS experiment, including the measurements of the Higgs boson properties in the diphoton decay channel, and the measurement of the Higgs boson production and decay rates, as well as constraints on its couplings to SM particles by combining all available Higgs production modes and decay channels measured by ATLAS. $H\rightarrow\gamma\gamma$ is one of the most important channels to study the property of Higgs boson. The measurements in this channel are performed with data collected in 2015 and 2016, corresponding to an integrated luminosity 36.1 $fb^{-1}$. 31 reconstructed categories are developed to maximize the sensitivity, to measure the inclusive production cross-sections-times-branching-ratio of $B(H\rightarrow\gamma\gamma)$ and the individual production processes: gluon–gluon fusion (ggF), vector-boson fusion (VBF), Higgs boson production in association with a vector boson (VH), and production of a Higgs boson in association with a top–antitop quark pair or a single top quark (ttH and tH). Results are also interpreted using the Simplified Template Cross Section framework (STXS) and the Higgs coupling strength framework (``$\kappa$"-framework). The inclusive signal strength in this channel is measured to be $0.99 \pm 0.14$, with an improvement on precision at a level of 40\% comparing to the ATLAS Run~1 result. To further improve the precision of the Higgs property measurement, the combination of Higgs properties measurement employs inputs from many individual Higgs channels, including $H\rightarrow\gamma\gamma$, $H\rightarrow ZZ$, $H\rightarrow WW$, $VH(H\rightarrow bb)$, $H\rightarrow\tau\tau$. $ttH(H\rightarrow \text{Multi-Lepton})$, $ttH(H\rightarrow bb)$, $H\rightarrow\mu\mu$, $VBF(H\rightarrow bb)$, $H\rightarrow \text{invisible}$ and $H\rightarrow ZZ (\text{offshell})$， with integrated luminosity 24.5 - 79.8 $fb^{-1}$. After building a likelihood-based combination mechanism, the production cross sections, decay branching ratios, STXS and coupling strength results are measured. The Higgs coupling strength results are with improvements on precision from 20\% to 30\% comparing to the ATLAS + CMS Run~1 results. The observed (expected) significance for the VBF process is $6.5\sigma$ ($5.3\sigma$), giving the first single-experiment observation of the VBF production mode. All results are compatible with the SM predictions.