Search for Excited Leptons in pp collisions at \sqrt s = 7 TeV

Abstract

Keywords: CMS PhysicsA search for excited leptons is carried out with the CMS detector at the LHC, using 36 pb −1 of pp collision data recorded atThe search is performed for associated production of a lepton and an oppositely charged excited lepton pp → * , followed by the decay * → γ , resulting in the γ final state, where = e, μ. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of * production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for * production at this collision energy.The exclusion region in the compositeness scale Λ and excited lepton mass M * parameter space is extended beyond previously established limits. For Λ = M * , excited lepton masses are excluded below 1070 GeV/c 2 for e * and 1090 GeV/c 2 for μ * at the 95% confidence level.© 2011 CERN. Published by Elsevier B.V. All rights reserved. IntroductionA fundamental question in the standard model (SM) of particle physics concerns the source of the mass hierarchy of quarks and leptons. A possible explanation for the three generations is a compositeness model in which the known leptons and quarks are bound states of either three fermions or a fermion-boson pair [1]. This substructure, if it exists, implies a large spectrum of excited states. Novel strong interactions would couple the excited fermions to ordinary quarks and leptons. These contact interactions can be described with an effective Lagrangian [2]:where Λ is the compositeness or contact interaction scale, g * 2 represents a coupling constant chosen to be 4π , and j μ is the fermion current:The SM and excited fermions are denoted by f and f * , respectively. The subscripts L (R) refer to left-(right-) handed fermions.The η factors for left-handed currents are conventionally set to one, and those for right-handed currents are set to zero. The excited fermions are assumed to have spin and isospin of 1/2. Gaugemediated transitions between ordinary and excited fermions are ✩ © CERN, for the benefit of the CMS Collaboration.E-mail address: cms-publication-committee-chair@cern.ch.described by an effective Lagrangian [2,3]:where G a μν , W μν , and B μν are the field strength tensors of the gluon, SU(2), and U(1) gauge fields, respectively, and g s , g, and g are the corresponding gauge couplings. The scaling parameters f s , f , and f are assumed to be equal to one. Previous searches at LEP [4][5][6][7], HERA [8,9], and the Tevatron [10-13] have found no evidence for such excited leptons. Searches for qq contact interactions performed by studying the dilepton production at the Tevatron [14,15] This Letter presents a search for excited leptons in pp collision data collected in 2010 at a centre-of-mass energy of 7 TeV with the Compact Muon Solenoid (CMS) [17] detector at the Large Hadron Collider (LHC). The data sample corresponds to an integrated luminosity of 36 pb −1 . The production of an excited lepton * (μ * or e * ) in association with an oppositely c...