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 fb${}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV delivered by the LHC. 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 BF(t $\to$ $\ell {\ell }^{\prime }$ q) < 1.86 $\cdot$ 10${}^{-5}$ is set at 95% confidence level.