Abstract
Searches in ep collisions for heavy excited fermions have been performed with the ZEUS detector at HERA. Excited states of electrons and quarks have been searched for in e+p collisions at a centre-of-mass energy of 300 GeV using an integrated luminosity of 47.7 pb−1. Excited electrons have been sought via the decays e∗→eγ, e∗→eZ and e∗→νW. Excited quarks have been sought via the decays q∗→qγ and q∗→qW. A search for excited neutrinos decaying via ν∗→νγ, ν∗→νZ and ν∗→eW is presented using e−p collisions at 318 GeV centre-of-mass energy, corresponding to an integrated luminosity of 16.7 pb−1. No evidence for any excited fermion is found, and limits on the characteristic couplings are derived for masses ≲250 GeV.
Highlights
The large number of quarks and leptons in the Standard Model suggests the possibility that they may be composite particles, consisting of combinations of more fundamental entities
This Letter reports on searches for excited electrons and quarks in e+p collisions and for excited neutrinos in e−p collisions at HERA
Charged particles were tracked in the central tracking detector (CTD) [9], which operates in a magnetic field of 1.43 T provided by a thin superconducting coil
Summary
The large number of quarks and leptons in the Standard Model suggests the possibility that they may be composite particles, consisting of combinations of more fundamental entities. At the electron49–proton collider HERA, excited electrons, quarks and neutrinos (e∗, q∗, ν∗) with masses up to the kinematic limit of 318 GeV could be produced directly via t-channel exchange of a gauge boson as shown in Fig. 1: for e∗, via γ /Z exchange; for q∗, via γ /Z/W exchange; and for ν∗, via W exchange. 20.1 ± 1.2 13.9 ± 1.1 32.9 ± 1.1 4.1 ± 0.6 q∗ → qγ Prompt γ , PHP, NC DIS q∗ → qW → qeν. If its polar angle was less than 0.3 rad, the cluster was not required to have a matching track; such clusters may be photon candidates; the following variables were calculated using CAL cells but excluding those with polar angles below 10◦, to avoid a contribution from the proton remnant: ◦ the total invariant mass, M; ◦ the hadronic invariant mass, Mhad, and transverse energy, EThad, calculated excluding those CAL cells belonging to electron or photon candidates; ◦ a second missing-transverse-momentum variable, P/ t (θ > 10◦). Pattern-recognition algorithms were used to suppress non-ep backgrounds such as cosmic rays and beam–halo muons
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