Shedding light on Higgs boson self-interactions in the $b\bar{b}\gamma\gamma$ 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 ($b\bar{b}\gamma\gamma$).
The first analysis is a search for SM Higgs boson pairs ($HH$) in the $b\bar{b}\gamma\gamma$ final state. No excess of events with respect to the SM background is observed. An observed upper limit of $\mu_HH$ < 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 $−1.4 < \kappa_{\lambda} < 6.9$ and $−0.5 < \kappa_{2V} < 2.7$, while the corresponding expected results are $−2.8 < \kappa_{\lambda} < 7.8$ and $−1.1 < \kappa_{2V} < 3.3$, yielding the most stringent expected constraints on $\kappa_{\lambda}$ 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 $\mu_HH < 2.9 (2.4)$ at 95% CL. The observed 95% confidence intervals for the coupling modifiers are $−1.2 < \kappa_{\lambda} < 7.2$ and $0.6 < \kappa_{2V} < 1.5$, with corresponding expected intervals $−1.6< \kappa_{\lambda} <7.2$ and $0.4 < \kappa_{2V} < 1.6$.
The second analysis is a search for a new heavy scalar resonance $X$, decaying into a Higgs boson and another scalar particle $S$ ($X \rightarrow SH$), 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 $b\bar{b}\gamma\gamma$ channel using ATLAS data. It searches for a $X \rightarrow SH \rightarrow b\bar{b}\gamma\gamma$ signal within a wide range of masses for the two resonances $X$ and $S$ (denoted $m_{X}$ and $m_{S}$), covering $170 \leq m_{X} \leq 1000$ GeV and $15 \leq m_{S} \leq 500$ 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 $(m_{X}, m_{S}) = (575, 200)$ GeV with a local (global) significance of 3.5 (2.0) standard deviations.