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

A search for charged-lepton-flavor violating $\mu \tau qt$ ($q=u$, $c$) interactions is presented, considering both top-quark production and decay. The data analyzed correspond to $140{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ 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 standard deviations is observed, and limits are set at the 95% confidence level (CL) on the charged-lepton-flavor violation branching ratio of $B(t\to \mu \tau q)<8.7\times {10}^{-7}$. An effective field theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavor of the associated light quark and the Lorentz structure of the coupling. These range from $\left|c_{\mathrm{lequ}}^{3\left(2313\right)}\right|/{\Lambda }^2<0.10{\mathrm{TeV}}^{-2}$ for $\mu \tau ut$ to $\left|c_{\mathrm{lequ}}^{1\left(2323\right)}\right|/{\Lambda }^2<1.8{\mathrm{TeV}}^{-2}$ for $\mu \tau ct$. An additional interpretation is performed for scalar leptoquark production inducing charged-lepton-flavor violation, with fixed intergenerational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from ${\lambda }^{\mathrm{LQ}}=1.3$ to ${\lambda }^{\mathrm{LQ}}=3.7$ for leptoquark masses between 0.5 and 2.0 TeV.