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

Abstract

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 us ing 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 κλ, 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 ¯bτ +τ −, b ¯bγγ and b ¯bb¯b, considering single-Higgs (H) production modes, ggF, V BF, ZH, W H and ttH¯ , together with WW∗ , ZZ∗ , τ +τ −, γγ and b ¯b decay channels, and combining the aforementioned analyses (H+HH) to improve the sen sitivity on κλ. 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: κλ = 4.6 +3.2 −3.8 , excluding values outside the interval −2.3 < κλ < 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 constraint on κλ from experi mental measurements.