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Dicembre 10, 2025  
In this seminar, I present and discuss the recent ATLAS search for associated production of a Higgs boson with a single top quark (tH). This represents the first dedicated measurement of tH using the ATLAS detector and uses the full Run II data, corresponding to 140/fb. This measurement is of particular interest because the tH production process is uniquely sensitive to the CP structure and sign of the top-quark Yukawa coupling, owing to the interference between diagrams involving the top–Higgs and W–Higgs couplings. Unlike the dominant ttH process, tH provides a clean probe of potential CP-odd admixtures or sign flips in the Yukawa interaction. The analysis targets Higgs decays into bb, WW*, ZZ*, and ττ, in events with one up to three leptons (muons and electrons) in the final state. The observed signal strength, μ(tH)=8.1±2.6 (stat)±2.0 (syst), corresponds to a significance of 2.8σ above the background-only hypothesis (expected 0.4σ). A 95% confidence-level upper limit on the tH cross-section is set at 13.9× the Standard Model expectation (expected limit: 6.1×). The result is also interpreted under the scenario of an inverted top-quark Yukawa coupling sign. Notably, several recent measurements across different channels have shown a consistent upward trend relative to the Standard Model prediction, making this result especially intriguing in the broader context of Higgs–top coupling <a href="http://studies.
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