Search for the production of a Higgs boson in association with a single top quark in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This paper reports a search for Higgs boson production with a single top quark at 13 TeV using ATLAS data, employing multivariate techniques, and sets limits on the production cross-section relative to the Standard Model.

Contribution

It introduces a comprehensive search for tH production with multiple Higgs decay channels using advanced analysis techniques and provides the first constraints on this process at 13 TeV.

Findings

Observed signal strength μ_{tH} = 8.1 ± 2.6 (stat.) ± 2.0 (syst.)

Observed upper limit on tH cross-section is 13.9 times the SM prediction

Significance of the observed signal is 2.8 standard deviations

Abstract

A search for the production of a Higgs boson in association with a single top quark, $tH$, is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of $140\mathrm{fb}^{-1}$ at a centre-of-mass energy of $13$ TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into $b\bar{b}$, $WW^{*}$, $ZZ^{*}$, and $\tau\tau$, accompanied by an isolated lepton (electron or muon) from the top-quark decay. Multivariate techniques are employed to enhance the separation between signal and background processes. The observed signal strength, $\mu_{tH}$, defined as the ratio between the measured cross-section and the predicted Standard Model value, is $\mu_{tH}~=~8.1~\pm~2.6~\mathrm{(stat.)}~\pm~2.0~\mathrm{(syst.)}$. The significance of the observed (expected) signal above the background-only expectation is $2.8$ ($0.4$) standard deviations. The corresponding observed (expected) upper limit at the 95% confidence level on the $tH$ cross-section is found to be $13.9$ ($6.1$) times the value predicted by the Standard Model. An interpretation with an inverted sign of the top-quark Yukawa coupling is performed, and the signal strength and corresponding limit are reported.