Search for charged lepton flavor violation in top quark interaction with an up-type quark, a muon, and a $\tau$ lepton with the CMS experiment

A search for charged-lepton flavor violation (CLFV) in top quark (t) interactions is presented. Proton-proton collision data collected with the CMS experiment corresponding to an integrated luminosity of 138 fb$^{-1}$ are used. The analysis selects events containing a single muon, a hadronically decaying $\tau$ lepton, and three jets where one has been identified to originate from the fragmentation of a bottom quark. Machine learning multiclass 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 $\mathcal{B}$ for CLFV top quark decays to a muon, a $\tau$ lepton, and an up or a charm quark are $\mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{u}) < (0.04$, $0.078$, and $0.118) \times 10^{-6}$, and $\mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{c}) < (0.81$, $1.71$, and $2.05) \times 10^{-6}$ for scalar, vector, and tensor-like operators, respectively.