Search for charged-lepton flavour violation in top quark interactions with an up-type quark, a muon, and a τ lepton in proton-proton collisions at TeV

A search for charged-lepton flavour violation (CLFV) in top quark (t) production and decay is presented. The search uses proton-proton collision data corresponding to 138 fb⁻¹ collected with the CMS experiment at TeV. The signal consists of the production of a single top quark via a CLFV interaction or top quark pair production followed by a CLFV decay. The analysis selects events containing a hadronically decaying τ lepton and a muon of opposite electric charge, as well as at least three jets, one of which is identified as originating from the fragmentation of a bottom quark. Machine learning classification techniques are used to distinguish signal from standard model background events. The results of this search are consistent with the standard model expectations. The upper limits at 95% confidence level on the branching fraction for CLFV top quark decays to a muon, a τ lepton, and an up or a charm quark are set at < (0.04, 0.08, and 0.12) × 10⁻⁶, and < (0.81, 1.71, and 2.05) × 10⁻⁶ for scalar, vector, and tensor-like operators, respectively.