Search for excited leptons in llgamma final states in proton-proton collisions at the HL-LHC

A search for excited leptons (electrons and muons) is presented, using simulation of the upgraded CMS detector at the High-Luminosity LHC (HL-LHC). Excited leptons are predicted by many theories beyond the standard model (SM) where quarks and leptons are not elementary but instead are themselves composite objects. Excited leptons ($\ell^* = e^*, \mu^*$) in $\ell\ell\gamma$ ($\ell= e, \mu$) final states where the excited lepton decays to a SM lepton and a photon ($\ell^*\rightarrow\ell\gamma$) are studied. The main background is Drell-Yan production in association with a photon. The HL-LHC environment (a centre of mass energy of 14 TeV and an integrated luminosity of 3000 fb$^{-1}$) allows for an extension of the discovery potential for excited leptons. The analysis is optimised for HL-LHC conditions, and indicates that $5\sigma$ discovery of excited leptons is possible for masses up to 5.1 TeV. If no significant discrepancies are seen in the data, excited leptons with masses below 5.8 TeV could be excluded at 95$\%$ confidence level. $$\textit{This document has been revised with respect to the version dated December 17, 2018.}$$