Search for excited leptons in the $\ell\ell\gamma$ final state in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

A search is presented for excited electrons and muons using a data sample of proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$, collected with the CMS detector in 2016 and corresponding to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$. This is the first search in the $\ell\ell\gamma$ final state in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$. The observed data are consistent with the standard model prediction, and exclusion limits on the excited lepton mass and the compositeness scale are set. Excited electrons (muons) with a mass below $3.9~(3.8)~\mathrm{TeV}$ are excluded under the assumption that the excited lepton mass equals the compositeness scale. The best observed limit on the compositeness scale is obtained in the mass range between 0.5 and $1.0~\mathrm{TeV}$, excluding a compositeness scale below $25~\mathrm{TeV}$ for both excited electrons and muons. These are the most stringent limits to date in the $\ell\ell\gamma$ final state.