A Search for Excited Fermions in e+ p Collisions at HERA

TL;DR

This study searched for excited fermions in electron-proton collisions at HERA but found no evidence, setting limits on their production and excluding certain mass ranges for excited electrons, neutrinos, and quarks.

Contribution

First search for excited fermions at HERA, establishing new mass exclusion limits based on collision data.

Findings

No evidence of excited fermions was observed.

Excluded excited electrons with masses 30-200 GeV.

Excluded excited neutrinos with masses 40-96 GeV.

Abstract

Using the ZEUS detector at HERA, we have searched for heavy excited states of electrons, neutrinos, and quarks in $e^+p$ collisions at a center-of-mass energy of 300~GeV. With an integrated luminosity of 9.4~pb$^{-1}$, no evidence was found for electroweak production and decay of such states. Limits on the production cross section times branching ratio and on the characteristic couplings, $f/\Lambda$, are derived for masses up to 250~GeV. For the particular choice $f/\Lambda = 1/M_{f^*}$, we exclude at the 95% confidence level excited electrons with mass between 30 and 200~GeV, excited electron neutrinos with mass between 40 and 96~GeV, and quarks excited electroweakly with mass between 40 and 169~GeV.