Measurement of the Higgs boson production in association with top quarks in multilepton final states in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A measurement of the associated production of a top-quark pair with the Higgs boson ($t\bar{t}H$) in multilepton final states is presented. The analysis is based on a data sample of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 140 fb$^{-1}$. Six final states defined by the number and flavour of reconstructed charged leptons are combined in a simultaneous likelihood fit to extract the $t\bar{t}H$ signal and constrain the most relevant backgrounds. The measured $t\bar{t}H$ cross-section normalised to Standard Model (SM) prediction is $\sigma_{t\bar tH}/\sigma^{\text{SM}}=0.63^{+0.20}_{-0.19}$. This result corresponds to an observed (expected) significance of 3.3$\sigma$ (5.3$\sigma$). Additionally, two other fits are used to measure the $t\bar{t}H$ cross-section differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, and to extract the associated production cross-section of a single top-quark with the Higgs boson ($tH$) together with the $t\bar{t}H$ one. The $CP$ structure of the top quark-Higgs boson Yukawa coupling is probed through analysis of $t\bar{t}H$ and $tH$ events. The results are compatible with the SM hypothesis, and values of the mixing angle between $CP$-even and $CP$-odd top-Higgs Yukawa couplings of $| \alpha | > 62^\circ$ are excluded at 68$\% $ confidence level.