Searching for dark photons at the LHeC and FCC-he

Extensions of the Standard Model (SM) gauge group with a new U(1)X predict an additional gauge boson. Through kinetic mixing with the SM photons featured by a coupling ε, the ensuing so-called dark photons γ′, which acquire mass as a result of the breaking of the gauge group U(1)X, can interact with the SM field content. These massive dark photons can therefore decay to pairs of leptons, hadrons, or quarks, depending on their mass mγ′. In this work, we discuss searches for dark photons in the mass range around and below 1 GeV at the LHeC and FCC-he colliders. The signal is given by the displaced decays of the long-lived dark photon into two charged particles. We discuss the impact of conceivable irreducible (SM and machine-related) backgrounds and different signal efficiencies. Our estimates show that the LHeC and FCC-he can test a domain that is complementary to other present and planned experiments.