Search for charged-lepton-flavour violating $\mu \tau qt$ 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 ${\text{fb}}^{-1}$ 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 $|c_{\mathsf{lequ}}^{3(2313)}|/{\Lambda }^2<0.10{\text{ TeV}}^{-2}$ for $\mu \tau ut$ to $|c_{\mathsf{lequ}}^{1(2323)}|/{\Lambda }^2<1.8{\text{ TeV}}^{-2}$ 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 }^{\text{LQ}}=$1.3 to ${\lambda }^{\text{LQ}}=$3.7 for leptoquark masses between 0.5 and 2.0 TeV.

54 pages in total, author list starting page 37, 7 figures, 9 tables. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2023-23

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