Prospects for vectorlike leptons at future proton-proton colliders

Vectorlike leptons are an intriguing possibility for physics beyond the Standard Model. We study the reach for discovering (at 5 sigma significance) or excluding (at 95% confidence) models of charged vectorlike leptons that mix predominantly with the tau, using multilepton signatures at various future proton-proton collider options: a high-luminosity LHC with root s = 14 TeV, a high-energy LHC with root s = 27 TeV, and possible new longer-tunnel colliders with root s = 70 or 100 TeV. For weak-isodoublet vectorlike leptons, we estimate that a 27 TeV high-energy LHC with 15 ab(-1) could exclude masses up to about 2300 GeV, or discover them if the mass is less than about 1700 GeV, while a 100 TeV collider with 30 ab(-1) could exclude masses up to about 5750 GeV or make a discovery if the mass is less than about 4000 GeV. However, we find that weak-isosinglet vectorlike leptons present a much more difficult challenge, with some reach for exclusion, but not for discovery at any of the collider options considered.