Measurement of Higgs-boson self-coupling with single-Higgs and double-Higgs production channels

One of the most important targets of the LHC is to improve the experimental results of the Run 1 and the complete exploration of the properties of the Higgs boson, in particular the Higgs-boson self-coupling. The self-coupling is very loosely constrained by electroweak precision measurements therefore new physics effects could induce large deviations from its Standard Model expectation. The trilinear self-coupling can be measured directly using the Higgs-boson-pair production cross section, or indirectly through the measurement of single-Higgs-boson production and decay modes. In fact, at next-to-leading order in electroweak interaction, the Higgs-decay partial widths and the cross sections of the main single-Higgs production processes depend on the Higgs-boson self-coupling via weak loops. Measurements of $\kappa_\lambda$, i.e. the rescaling of the trilinear Higgs self-coupling, are presented in this dissertation. Results are obtained exploiting proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in 2015, 2016 and 2017, corresponding to a luminosity of up to 80 fb$^{-1}$. Constraints on the Higgs self-coupling are presented considering the most sensitive double-Higgs channels (HH), $b\bar{b}\tau^+\tau^-$, $b\bar{b}\gamma\gamma$ and $b\bar{b}b\bar{b}$, considering single-Higgs (H) production modes, $ggF$, $VBF$, $ZH$, $WH$ and $t\bar{t}H$, together with $WW^*$, $ZZ^*$, $\tau^+\tau^-$, $\gamma \gamma$ and $b\bar{b}$ decay channels, and combining the aforementioned analyses (H+HH) to improve the sensitivity on $\kappa_\lambda$. Under the assumption that new physics affects only the Higgs-boson self-coupling, the combined H+HH best-fit value of the coupling modifier is: $\kappa_\lambda = 4.6^{+3.2}_{-3.8}$, excluding values outside the interval $-2.3<\kappa_\lambda<10.3$ at 95% confidence level. Results with less stringent assumptions are also provided, decoupling the Higgs self-coupling and the other Standard Model couplings. The final results of this thesis provide the most stringent constraints on $\kappa_\lambda$ from experimental measurements.