Search for Associated Production of a Higgs Boson with a Single Top Quark at √ s = 13\,TeV with the ATLAS Detector

An analysis is presented concerning the search for the associated production of a Higgs boson with a single top quark (tH), a process highly sensitive to the complex phase of the top-quark Yukawa coupling. The analysis uses the full 140 fb–1 of ATLAS data at √ s = 13\,\,TeV , targeting H → bb , $$WW$$ , $$ZZ$$ , ττ decays. Advanced multivariate analysis techniques were employed to achieve optimal sensitivity and to separate the tiny tH signal from the large background. The measured signal strength is μ _tH = 8.1 ± 2.6\,(stat.) ± 2.0\,(syst.) . The observed (expected) significance of the tH signal over the background-only hypothesis is 2.8 σ (0.4 σ ). The observed (expected) upper limit on the tH production cross-section is 13.9 (6.1) times the SM prediction at the 95% confidence level. Interpretations for both Standard Model and inverted top-quark Yukawa coupling scenarios also are presented.