Abstract
A search for compositeness of electrons and muons is presented using a data sample of proton-proton collisions at a center-of-mass energy of sqrt(s) = 8 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 inverse femtobarns. Excited leptons (l*) produced via contact interactions in conjunction with a standard model lepton are considered, and a search is made for their gauge decay modes. The decays considered are l* to l + gamma and l* to l + Z, which give final states of two leptons and a photon or, depending on the Z-boson decay mode, four leptons or two leptons and two jets. The number of events observed in data is consistent with the standard model prediction. Exclusion limits are set on the excited lepton mass, and the compositeness scale Lambda. For the case M(l*) = Lambda the existence of excited electrons (muons) is excluded up to masses of 2.45 (2.47) TeV at 95% confidence level. Neutral current decays of excited leptons are considered for the first time, and limits are extended to include the possibility that the weight factors f and f', which determine the couplings between standard model leptons and excited leptons via gauge mediated interactions, have opposite sign.
Highlights
The standard model (SM) of particle physics describes the observed phenomena very successfully, it provides no explanation for the three generations of the fermion families
Neutral current decays of excited leptons are considered for the first time, and limits are extended to include the possibility that the weight factors f and f, which determine the couplings between standard model leptons and excited leptons via gauge mediated interactions, have opposite sign
At the Large Hadron Collider (LHC) at CERN, previous searches performed by the CMS [19] and the ATLAS collaborations [20] have found no evidence of excited leptons, obtaining a lower limit on the mass M ∗ < 2.2 TeV for the case M ∗ = Λ
Summary
The standard model (SM) of particle physics describes the observed phenomena very successfully, it provides no explanation for the three generations of the fermion families. Compositeness models predict the existence of excited states of quarks (q∗) and leptons ( ∗) at the characteristic scale of the new binding interaction. Since these excited fermions couple to the ordinary SM fermions, they could be produced via contact interactions (CI) in collider experiments, with subsequent decay to ordinary fermions through the emission of a W/Z/γ boson, or via CI to other fermions. At the Large Hadron Collider (LHC) at CERN, previous searches performed by the CMS [19] and the ATLAS collaborations [20] have found no evidence of excited leptons, obtaining a lower limit on the mass M ∗ < 2.2 TeV for the case M ∗ = Λ. We consider the production of an excited lepton in association with an oppositely charged lepton of the same flavor, with subsequent radiative decays ( ∗ → γ) or neutral current decays ( ∗ → Z)
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