Fatjet Signatures of Quintuplet Fermions at the LHC

Abstract

This paper explores a simplified extension of the standard model featuring a neutral fermion quintuplet and a scalar quadruplet, which together generate neutrino masses through tree and loop level mechanisms. The quintuplet fermions decay into standard model gauge bosons via the scalars, producing unique collider signatures at the LHC characterized by multilepton and multijet final states. The study focuses on the pair production of quintuplet fermions in the 700-1200 GeV mass range, where their decays produce highly boosted W and Z bosons identifiable as fatjets. Emphasis is placed on the production and decay of doubly charged fermions due to their higher cross section. Advanced jet substructure and kinematic techniques are applied to enhance sensitivity by reducing standard model backgrounds. A detailed analysis of signal significance is performed in the two lepton, two fatjet and three lepton, one fatjet channels for different masses of the fermion and the scalars, optimizing selection cuts to maximize signal efficiency over standard model backgrounds. The study found that both channels exhibit excellent performance, with significance exceeding $5\sigma$ under realistic conditions including a 50\% background uncertainty at integrated luminosity up to 3000 fb$^{-1}$.