Search for top+photon production in pp collisions at 13 TeV in the muon+jets channel

A search for events with a single top quark produced in association with a photon is performed. The analysis is based on data corresponding to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$ of pp collisions at $\sqrt{s} = 13~\mathrm{TeV}$ recorded by the CMS experiment at the LHC in 2016. The events are selected by requiring the presence of a muon ($\mu$), a photon ($\gamma$), an undetected neutrino ($\nu$), and at least two jets (j) from which one is identified as a b jet. A multivariate analysis based on topological and kinematic properties is employed to discriminate the signal from the background processes. An excess above the background-only hypothesis is observed, with a significance corresponding to 4.4 standard deviations. A fiducial cross section is measured for photons with transverse momentum greater than 25 GeV and in the central region of the detector that are well separated from muons and jets. The measured cross section times branching fraction is $\mathcal{B}$(t$\rightarrow\mu\nu$b)$\sigma$(t$\gamma$j) $=115\pm17(\mathrm{stat})_{-27}^{+33}(\mathrm{syst})~\mathrm{fb}$, which agrees with the standard model prediction.