arXiv2020
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 $κ_λ$, 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 79.8 fb$^{-1}$. Constraints on the Higgs self-coupling are presented considering the most sensitive double-Higgs channels (HH), $b\bar{b}τ^+τ^-$, $b\bar{b}γγ$ and $b\bar{b}b\bar{b}$, considering single-Higgs (H) production modes, $ggF$, $VBF$, $ZH$, $WH$ and $t\bar{t}H$, together with $WW^*$, $ZZ^*$, $τ^+τ^-$, $γγ$ and $b\bar{b}$ decay channels, and combining the aforementioned analyses (H+HH) to improve the sensitivity on $κ_λ$. 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: $κ_λ= 4.6^{+3.2}_{-3.8}$, excluding values outside the interval $-2.3<κ_λ<10.3$ at 95% confidence level. Results with less stringent assumptions are also provided, decoupling the Higgs-boson self-coupling and the other Standard Model couplings. The final results of this thesis provide the most stringent constraint on $κ_λ$ from experimental measurements to date.