Search for Excited Leptons in <i>ep</i> Scattering at HERA Using the ZEUS Detector

Abstract

A search for heavy excited electrons and neutrinos has been performed with the ZEUS detector at HERA. The ep scattering data used were taken at a centre-of-mass energy of 318 GeV in the years 1998-2000. Excited electrons have been searched for in e^-p and e⁺p collisions corresponding to integrated luminosities of 16.7 pb^-1 and 66.1 pb^-1, respectively. The decay channels e*->egamma, e*->nuW->nuqq and e*->eZ->eqq have been considered. Excited neutrinos have been sought in the e^-p data set via the decay channels nu*->nugamma, nu*->eW->eqq and nu*->nuZ->nuqq. The event selection has been performed employing both fixed cuts and a probability-based method as alternatives, with the latter yielding better overall performance. No evidence for an e* or nu* signal is observed in any of the channels analysed. Upper limits (95% confidence level) as a function of the excited-lepton mass have been set on the cross section times the branching ratio, sigmaBR, and on the coupling over the compositeness scale, f/Lambda. In many cases, these limits extend up to higher masses and are more stringent than the existing ones.