Search for heavy and excited leptons at LEP

Abstract

New particle searches are ongoing at the Large Electron Positron Collider (LEP) running yearly at increasing centre-of-mass energies. The searches for heavy stable and unstable leptons performed by the four LEP experiments (ALEPH, DELPHI, L3 and OPAL) are summarised, followed by a description of the searches for excited leptons. Experimental signatures and results, including the most recent data analysed, are presented. Physics Beyond the Standard ModelAlthough very successful in describing experimental observations, the Standard Model (SM) of particle physics is however incomplete. It relies on many assumptions that cannot be explained within its framework. The search for physics beyond the SM is therefore an active field which takes on two different approaches.One approach includes models such as Grand Unified Theories (E 6 ,SO(10)...) and supersymmetry (SUSY). Within this frame work, the known SM particles are regarded as being fundamental and point-like. Some of these models predict the existence of new fundamental particles such as heavy charged (L ± ) and neutral (N) leptons.The second approach to physics beyond the SM assumes that known particles have substructure at a scale Λ > ∼ 1 TeV. Composite models could explain the number of fermion generations (or families) and the mass spectrum observed within the SM. Under the assumption of compositeness, it is natural to expect the presence of excited states of the SM fermions.With the ongoing increase in energy and luminosity at the LEP collider, new particles could be within experimental reach. Various searches are performed by the four LEP experiments. This note reviews the searches for heavy and excited leptons. Experimental signatures and results of the search for heavy stable/long-lived and unstable leptons are presented. Then, a description of the searches for single and pair production of excited leptons follows. Unless stated, the results presented here include data taken in 1999 at a centre-of-mass energy up to 202 GeV, corresponding to an integrated luminosity of about 220 pb −1 per experiment. Searches for Heavy LeptonsHeavy leptons can be sequential, mirror or vector leptons [4]. Neutral heavy leptons could, in addition, be Dirac or Majorana particles.The mixing between heavy and SM leptons is severely constrained by LEP1 data and lowenergy experiments [5]. Therefore, only the searches for pair produced heavy leptons are considered. Charged (L ± ) and neutral (N) leptons could be pair produced in e + e − collisions via the diagrams shown in Figure 1. Unstable leptons