Measurement of the SM Higgs boson properties in the diphoton decay channel with the ATLAS Run 2 dataset

This manuscript presents the work I carried out during my PhD in particle physics at the Milano University and CERN from Autumn 2018 to March 2021. The analyses presented are based on $pp$ collision data at a center-of-mass energy $\sqrt{s} = 13\,\text{TeV}$ collected by the ATLAS experiment during the Run 2 period of the LHC, corresponding to an integrated luminosity of $139\,\text{fb}^{-1}$. The ATLAS research program ranges from the precise measurement of the parameters of the Standard Model of particle physics to the testing of the plethora of Beyond Standard Model theories. The work reported is fully part of this program, focusing on the precise measurement of the Higgs boson mass and production cross sections in the $H\to\gamma\gamma$ decay channel, one of the best channel these properties: this is due to the excellent photon reconstruction performance, to the clear final state signature and to the robust background description and subtraction. The production cross sections measurement is presented in a number of different parameterisations which deeply explore the full Higgs production phase space and represents the final ATLAS Run 2 results for this decay channel. The analysis features an innovative and fully redesigned categorisation, optimised to measure the Simplified Template Cross Sections (STXS) truth bin with a multivariate approach, which greatly improves the results with respect to partial Run 2 publication. The total production cross section for $|y_H| < 2.5$ is measured to be $\sigma^{\gamma\gamma}_\text{obs} = 121^{+10}_{-9}\,\text{fb}$ with a SM expectation of $\sigma^{\gamma\gamma}_\text{exp} = 116\pm 6 \, \text{fb}$. The measurement of the production cross sections for the main Higgs production modes (ggH, VBF, WH, ZH, ttH) found their signal strengths compatible with the SM predictions, with relative uncertainties ranging from 11% to 58%. An observed (expected) upper limit at 95% CL of 10 (6.8) times the SM prediction is set for the associated production of the Higgs boson with a single top quark process (tH). Cross sections are also presented in 28 and 33 different Higgs production regions, defined based on the STXS framework. No significant deviations from the SM expectation are observed and the compatibility between the measurements and the SM predictions corresponds to a $p$-value of 60% and 70% respectively. Moreover the combination of this analysis with with others performed in the different Higgs decay channel is presented. This permits to obtain more precise results by exploiting the full dataset potential and to disentangle the Higgs production and decay processes. No deviations from the SM expectations have been observed. The preliminary Higgs boson mass measurement in the diphoton decay channel is described as well. A systematic and theoretically motivated investigation of the categorisation design has been carried out. The expected result obtained from an Asimov dataset generated with $m_H = 125.09\,\text{GeV}$ present a $287\,\text{MeV}$ uncertainty, decomposed in $125\,\text{MeV}$ and $259\,\text{MeV}$ for the statistical and systematic counterparts, respectively. The results is improved by 30% with respect to the latest $H\to\gamma\gamma$ Run 2 result, partially due to the increase in dataset statistic, but with sizeable contributions from updated photon energy scale uncertainties (10%) and from the updated categorisation (7%).