Search for charged lepton-flavour violation in top-quark decays at the LHC with the ATLAS detector

A direct search for charged lepton-flavour violation in top-quark decays is presented. The data analysed correspond to $79.8\ \text{fb}^{-1}$ of proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=13\ \text{TeV}$ delivered by the LHC and recorded by the ATLAS experiment. The process studied is the production of top-quark pairs, where one top quark decays into a pair of opposite-sign different-flavour charged leptons and an up-type quark, while the other decays semileptonically according to the Standard Model. The signature of the signal is thus characterised by the presence of three charged leptons, a light jet and a $b$-jet. A multivariate discriminant is deployed and its distribution used as input to extract the signal strength. In the absence of a signal, an upper limit on the branching ratio of ${\cal B} (t \to \ell \ell' q) < 1.86 \times 10^{-5}$ is set at the 95\% confidence level.