Search for flavour-changing neutral current interactions of the top quark and the Higgs boson in events with a pair of $\tau$-leptons in pp collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for flavour-changing neutral current (FCNC) $tqH$ interactions involving a top quark, another up-type quark ($q=u$, $c$), and a Standard Model (SM) Higgs boson decaying into a $\tau$-lepton pair ($H\to {\tau }^{+}{\tau }^{-}$) is presented. The search is based on a dataset of $pp$ collisions at $\sqrt{s}=13$ TeV that corresponds to an integrated luminosity of 139 fb${}^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson ($pp\to tH$), and top quark pair production in which one of the top quarks decays into $Wb$ and the other decays into $qH$ through the FCNC interactions. The search selects events with two hadronically decaying $\tau$-lepton candidates (${\tau }_{\text{had}}$) or at least one ${\tau }_{\text{had}}$ with an additional lepton ($e$, $\mu$), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3$\sigma$ above the expected SM background, and 95% CL upper limits on the $t\to qH$ branching ratios are derived. The observed (expected) 95% CL upper limits set on the $t\to cH$ and $t\to uH$ branching ratios are $9.4\times 10^{-4}$ $(4.8_{-1.4}^{+2.2}\times 10^{-4})$ and $6.9\times 10^{-4}$ $(3.5_{-1.0}^{+1.5}\times 10^{-4})$, respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective $tqH$ couplings are $C_{c\varphi }<1.35$ $(0.97)$ and $C_{u\varphi }<1.16$ $(0.82)$.

arXiv comment56 pages in total, author list starting page 39, 7 figures, 10 tables, published in JHEP. All figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2019-17

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• Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV $pp$ collisions with the ATLAS detector
• Generic tests of CP-violation in high-$p_{\text{T}}$ multi-lepton signals at the LHC and beyond
• Exploring Anomalous Flavor-Changing Neutral tqh Transitions at Future Muon Colliders: Insights from ${\mu }^{+}{\mu }^{-}\to t\bar{q}h$ Interactions
• Single Higgs boson production in association with a top quark through FCNSI