Exotic leptons at future linear colliders

Abstract

Doubly charged excited leptons give rise to interesting signatures for physics beyond the standard model at the present Large Hadron Collider. These exotic states are introduced in extended isospin multiplets which couple to the ordinary leptons and quarks either with gauge or contact effective interactions or a combination of both. In this paper we study the production and the corresponding signatures of doubly charged leptons at the forthcoming linear colliders, and we focus on the electron-electron beam setting. In the framework of gauge interactions, the interference between the t and u channels is evaluated that has been neglected so far. A pure leptonic final state is considered (e-e-_s13_s178594_s19e-e-_s13_s17957_s19e_s13_s17957_s19¯e) that experimentally translates into a like-sign dilepton and missing transverse energy signature. We focus on the standard model irreducible background, and we study the invariant like-sign dilepton mass distribution for both the signal and background processes. Finally, we provide the 3 and 5-sigma statistical significance exclusion curves in the model parameter space. We find that for a doubly charged lepton mass m*_s13_s178776_s192_s13_s178201_s19_s13_s178201_s19TeV the expected lower bound on the compositeness scale at Compact Linear Collider, _s13_s17923_s19>25_s13_s178201_s19_s13_s178201_s19TeV, is much stronger than the current lower bound from the LHC (_s13_s17923_s19>5_s13_s178201_s19_s13_s178201_s19TeV) and remains highly competitive with the bounds expected from run II of the LHC.