Search for Charged-Lepton Flavour Violation in Top-Quark Decays in the $\mu \tau$ Channel using the ATLAS Detector at the LHC

This analysis presents a first direct search for charged-lepton flavour violation in top-quark decays to the $\mu\tau$ final state using data collected from proton-proton collisions at the LHC, during the years 2015 and 2016, at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 36.1 fb$^{-1}$, as detected by the ATLAS experiment. The search is performed by studying top-quark pair events where one quark undergoes a flavour-violating decay $t(\bar{t}) \rightarrow l^{\pm} l'^{\mp} q(\bar{q})$, where $q \in \{u, c\}$ and $l \in \{\mu, \tau\}$ while the other top decays according to the SM into a $b$-quark and a leptonically-decaying $W$-boson. Data is found to be compatible with the absence of a signal and a fit to the background is performed using the profile likelihood technique. The observed (expected) upper limit on the branching ratio of $t_{\textrm{LFV}} \rightarrow \mu \tau q$ is set to $0.96 (1.00) \times 10^{-4}$ at the 95% confidence level using the modified frequentist approach.