Measuring Higgs Boson Couplings, including to the Top Quark, in the Diphoton Decay Channel with Run 2 Data Collected by the ATLAS Detector

This work presents the measurement of Higgs boson couplings through the Higgs boson to diphoton decay ($H \rightarrow \gamma\gamma$) channel using pp collision data with a center-of-mass energy of $\sqrt{s}=13$~TeV recorded by the ATLAS detector. In order to probe the coupling of the Higgs boson to the fermions and vector bosons, the inclusive cross sections times the branching ratio of the Higgs boson to two photons are measured for four primary production modes: gluon-gluon fusion ($ggF$), vector boson fusion ($VBF$), vector boson associated production ($VH$), and top quark associated production ($t\bar{t}H$). The measurement of the $t\bar{t}H$ production mode was performed using the full Run 2 dataset taken between 2015 and 2018, amounting to 139 fb$^{-1}$ of pp collision data. Measurements of the remaining production modes were performed with the subset of Run 2 data taken between 2015 and 2017, amounting to 79.8 fb$^{-1}$. Two significant sources of uncertainty in the analysis are that relating to photon isolation and that from background modeling. A new methodology for calculating corrections to photon isolation in the Electromagnetic calorimeter from a high statistics, low purity data sample is presented in detail. The method utilizes a multistep template fit to remove fake photons from the data sample, which allows for a direct comparison of the isolation shape in data and simulation. The background modeling uncertainty is found to be inflated in many analysis categories due to insufficient simulation statistics. A novel technique using Gaussian Processes to smooth out fluctuations in low statistics simulation samples is explored, along with extensive validation studies. The technique shows significant promise in reducing the inflation of the background modeling uncertainty due to statistical fluctuations in simulation samples. The observed signal strength of the $t\bar{t}H$ production mode is $\mu_{t\bar{t}H}=1.38\;^{+0.41}_{-0.36}$, which is compatible with the Standard Model prediction. The observed signal strength of the $ggF$ production mode was found to be $\mu_{ggF} = 0.97^{+0.17}_{-0.15} $, that of the $VBF$ production mode $\mu_{VBF} = 1.4^{+0.44}_{-0.37}$, and that of the combined $VH$ processes ($W^{\pm}H$, $q\overline{q}ZH$, and $ggZH$) is measured to be $\mu_{VH} = 1.09^{+0.61}_{-0.55}$. These results are all consistent with the Standard Model predictions, and so no direct evidence of new physics is observed.