Search for excited electrons singly produced in proton–proton collisions at √s = 13 TeV with the ATLAS experiment at the LHC

A search for excited electrons produced in pp collisions at √s = 13 TeV via a contact interaction qq¯ → ee^∗ 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 (eqq¯) 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^∗ → eeqq¯ and pp → ee^∗ → eνW production cross sections as a function of the excited electron mass m_e∗ 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∗. For m_e∗ < 0.5 TeV, the lower bound for Λ is 11 TeV. In the special case of m_e∗ = Λ, the values of m_e∗ < 4.8 TeV are excluded. The presented limits on Λ are more stringent than those obtained in previous searches.