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

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 $b\bar{b}$, $W{W}^{\ast }$, $Z{Z}^{\ast }$, and $\ddot{I}„\ddot{I}„$, 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, $\hat{I}{¼}_{tH}$, defined as the ratio between the measured cross-section and the predicted Standard Model value, is $\hat{I}{¼}_{tH}=8.1\pm 2.6(stat.)\pm 2.0(syst.)$. 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.

68 pages in total, author list starting page 51, 8 figures, 11 tables, published in JHEP. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGP-2024-03