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

This note presents a search for a possible compositness of electrons and muons using $2.7~\mathrm{fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13~\mathrm{TeV}$ collected with the CMS detector in 2015. Excited leptons ($\ell^*$) are assumed to be produced via contact interactions in conjunction with the corresponding standard model lepton. The decays considered here are $\ell^* \to \ell \gamma$ with $\ell = e,\mu$. The number of events observed in data is consistent with background processes from the standard model of particle interactions and exclusion limits on the excited lepton mass, and the compositness scale $\Lambda$, are set. For the case $M_{\ell^*} = \Lambda$, the existence of excited electrons (muons) up to masses of 2.8 (3.0) TeV is excluded. This analysis sets the most stringent limits on $\ell^*$ to date.