New Physics Opportunities for Long-Lived Particles at Electron-Proton Colliders

Future electron-proton collider proposals like the LHeC or the FCC-eh can supply 1/ab of collisions with a center-of-mass energy in the TeV range, while maintaining a clean experimental environment more commonly associated with lepton colliders. We point out that this makes electron-proton colliders ideally suited to probe BSM signatures with final states that look like "hadronic noise" in the high-energy, pile-up-rich environment of hadron colliders. We focus on the generic vector boson fusion production mechanism, which is available for all BSM particles with electroweak charges at mass scales far above the reach of most lepton colliders. This is in contrast to previous BSM studies at these machines, which focused on BSM processes with large production rates from the asymmetric initial state. We propose to exploit the unique experimental environment in the search for long-lived particle signals arising from Higgsinos or exotic Higgs decays. At electron-proton colliders, the soft decay products of long-lived Higgsinos can be explicitly reconstructed ("displaced single pion"), and very short lifetimes can be probed. We find that electron-proton colliders can explore significant regions of BSM parameter space inaccessible to other collider searches, with important implications for the design of such machines.