Charged Fermions Below 100 GeV

TL;DR

This paper reevaluates LEP bounds on the mass of charged fermions below 100 GeV, showing that new interactions can allow lighter fermions to evade previous constraints, and highlights future search prospects at the LHC.

Contribution

It demonstrates that charged fermions as light as 75 GeV can evade LEP bounds if coupled to a new scalar, revising previous mass exclusion limits.

Findings

LEP excludes charged fermions lighter than 90 GeV under certain assumptions

Coupling to a new scalar can reduce production cross section, allowing lighter fermions

Future LHC searches can target 75-100 GeV mass range for charged fermions

Abstract

How light can a fermion be if it has unit electric charge? We revisit the lore that LEP robustly excludes charged fermions lighter than about 100 GeV. We review LEP chargino searches, and find them to exclude charged fermions lighter than 90 GeV, assuming a higgsino-like cross section. However, if the charged fermion couples to a new scalar, destructive interference among production channels can lower the LEP cross section by a factor of 3. In this case, we find that charged fermions as light as 75 GeV can evade LEP bounds, while remaining consistent with constraints from the LHC. As the LHC collects more data, charged fermions in the 75-100 GeV mass range serve as a target for future monojet and disappearing track searches.