Long-lived searches of vectorlike lepton and its accompanying scalar at colliders

Recently, the vectorlike leptons (VLLs) as a simple extension to the standard model (SM) have attracted widespread attention both in theory and experiments. The present collider searches mainly focus on the studies of their prompt decays, which prefer a relatively large coupling. In this paper, we concentrate on searches for long-lived signatures of the singlet VLLs F or their accompanying scalar particles φ both in the hadronic and electronic colliders. The long-lived signatures are naturally induced from small chiral mass mixing between VLLs and SM leptons. Two specific models distinguished by whether the VLLs couple to scalar particles are introduced to realize the aforementioned features. For the long-lived VLLs case, we find that with the kink track method the sensitivities at future HL-LHC with s=14 TeV can reach the regions for VLL mass mF∈[200,1100] GeV and the mass mixing parameter θL∈[10-10,3×10-8]. For the long-lived accompanying scalar particle case, by fixing VLLs or scalar mass, or the mass ratio between VLL and the accompanying scalar, we explore the projected sensitivities through the time delay and displaced vertex strategies, which can probe the regions for mF∈[200,1200] GeV and coupling yθL∈[10-11,10-6]. Furthermore, we also explore the long-lived accompanying scalars at the future CEPC provided that the VLLs can couple to the SM first-generation leptons. We find that CEPC has good performances for mφ<120 GeV and mF<1200 GeV. These long-lived searches are complementary to previous studies, which opens the door toward the smaller coupling regions.