Search for charged-lepton-flavour violating $μτqt$ interactions in top-quark production and decay in $pp$ collisions at $\sqrt{s}= 13$ TeV with the ATLAS detector at the LHC

Abstract: A search for charged-lepton-flavour violating $\mu\tau qt$ ($q=u,c$) interactions is presented, considering both top-quark production and decay. The data analysed correspond to 140 $\textrm{fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}= $13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying $\tau$-lepton and at least one jet, with exactly one $b$-tagged jet, produced by a $\mu\tau qt$ interaction. Agreement with the Standard Model expectation within $1.6\sigma$ is observed, and limits are set at the 95% CL on the charged-lepton-flavour violation branching ratio of $\mathcal{B}(t \to \mu\tau q) < 8.7 \times 10^{-7}$. An Effective Field Theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavour of the associated light quark and the Lorentz structure of the coupling. These range from $|c_{\mathsf{lequ}}^{3(2313)}| / \Lambda^{2} < 0.10\textrm{ TeV}^{-2}$ for $\mu\tau ut$ to $|c_{\mathsf{ lequ}}^{1(2323)}| / \Lambda^{2} < 1.8\textrm{ TeV}^{-2}$ for $\mu\tau ct$. An additional interpretation is performed for scalar leptoquark production inducing charged lepton flavour violation, with fixed inter-generational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from $\lambda^{\textrm{LQ}} = $1.3 to $\lambda^{\textrm{LQ}} = $3.7 for leptoquark masses between 0.5 and 2.0 TeV.