Search for excited electrons singly produced in proton–proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS experiment at the LHC

A search for excited electrons produced in pp collisions at $\sqrt{s}$ = 13 TeV via a contact interaction q$\overline{q}$→ee${}^{\text{â}}\text{ˆ}\text{—}$ is presented. The search uses 36.1 fb${}^{-1}$ of data collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider. Decays of the excited electron into an electron and a pair of quarks (eq$\overline{q}$) are targeted in final states with two electrons and two hadronic jets, and decays via a gauge interaction into a neutrino and a W boson (νW) are probed in final states with an electron, missing transverse momentum, and a large-radius jet consistent with a hadronically decaying W boson. No significant excess is observed over the expected backgrounds. Upper limits are calculated for the pp→ee${}^{\text{â}}\text{ˆ}\text{—}$→eeq$\overline{q}$ and pp→ee${}^{\text{â}}\text{ˆ}\text{—}$→eνW production cross sections as a function of the excited electron mass m${}_{e^{\text{â}}\text{ˆ}\text{—}}$ at 95% confidence level. The limits are translated into lower bounds on the compositeness scale parameter Λ of the model as a function of m${}_{e^{\text{â}}\text{ˆ}\text{—}}$. For m${}_{e^{\text{â}}\text{ˆ}\text{—}}$<0.5 TeV, the lower bound for Λ is 11 TeV. In the special case of m${}_{e^{\text{â}}\text{ˆ}\text{—}}$=Λ, the values of m${}_{e^{\text{â}}\text{ˆ}\text{—}}$<4.8 TeV are excluded. The presented limits on Λ are more stringent than those obtained in previous searches.