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

A search for charged-lepton-flavour violating μτqt (q=u,c) interactions is presented, considering both top-quark production and decay. The data analysed correspond to 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of √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 τ-lepton and at least one jet, with exactly one b-tagged jet, produced by a μτqt interaction. Agreement with the Standard Model expectation within 1.6σ is observed, and limits are set at the 95% CL on the charged-lepton-flavour violation branching ratio of B(t→μτq)<8.7×10⁻⁷. 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^3(2313)_lequ|/Λ²<0.10 TeV⁻² for μτut to |c^1(2323)_lequ|/Λ²<1.8 TeV−2 for μτct. 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 λ^LQ=1.3 to λ^LQ=3.7 for leptoquark masses between 0.5 and 2.0 TeV.