Searching for dark photons at the LHeC and FCC-he
M. D’Onofrio, Oliver Fischer, Zeren Simon Wang
Abstract
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 $\ensuremath{\epsilon}$, the ensuing so-called dark photons ${\ensuremath{\gamma}}^{\ensuremath{'}}$, 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 ${m}_{{\ensuremath{\gamma}}^{\ensuremath{'}}}$. 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.