SHEDDING LIGHT ON HIGGS BOSON SELF-INTERACTIONS IN THE BBƔƔ CHANNEL. PHOTON AND B-JET CALIBRATIONS, AND SEARCHES FOR HIGGS BOSON PAIRS WITH THE ATLAS EXPERIMENT.

The work presented in this thesis is based on proton-proton collision data of the Large Hadron Collider (LHC), recorded by the ATLAS detector between 2015 and 2018 (Run 2), amounting to a total integrated luminosity of 140 fb−1. This work focuses on two analyses, one in the Standard Model (SM) context, and one related to probing new physics scenarios, whose common theme is the study of the Higgs boson self-interactions through the search for Higgs boson pair production in the final state with two photons and two bottom quarks (bbγγ). The first analysis is a search for SM Higgs boson pairs (HH) in the bbγγ final state. No excess of events with respect to the SM background is observed. An observed upper limit of μHH < 4.0 is set at 95% confidence level (CL) on the di-Higgs signal strength, corresponding to the di-Higgs production cross-section normalised to its SM prediction. The corresponding expected upper limit under the assumption of no HH production (SM HH production) is 5.0 (6.4). The observed 95% confidence intervals for the coupling modifiers are −1.6 < κλ < 6.9 and −0.5 < κ2V < 2.7, while the corresponding expected results are −2.8 < κλ < 7.8 and −1.1 < κ2V < 3.3, yielding the most stringent expected constraints on κλ achieved by a single analysis to date. This analysis is combined with all the other HH searches in various final states, thus providing the most complete picture on HH production with the Run 2 dataset. The combined observed (expected) upper limit on the signal strength is μHH < 2.9 (2.4) at 95% CL. The observed 95% confidence intervals for the coupling modifiers are −1.2 < κλ < 7.2 and 0.6 < κ2V < 1.5, with corresponding expected intervals −1.6 < κλ < 7.2 and 0.4 < κ2V < 1.6. The second analysis is a search for a new heavy scalar resonance X, decaying into a Higgs boson and another scalar particle S (X → SH), in the final state where the Higgs boson decays into two photons and the S particle decays into two bottom quarks. This is the first search for asymmetric Higgs boson pair production in the bbγγ channel using ATLAS data. It searches for a X → SH → bbγγ signal within a wide range of masses for the two resonances X and S (denoted mX and mS), covering 170 ≤ mX ≤ 1000 GeV and 15 ≤ mS ≤ 500 GeV. No significant excess above the expected background is found and 95% CL upper limits are set on the cross section times branching ratio, ranging from 39 fb to 0.09 fb. The largest deviation from the background-only expectation occurs for (mX , mS ) = (575, 200) GeV with a local (global) significance of 3.5 (2.0) standard deviations.