Searching for dark photons at the LHeC and FCC-he

Monica D’Onofrio,Oliver Fischer,Zeren Simon Wang

doi:10.1103/physrevd.101.015020

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 $\epsilon$, the ensuing so-called dark photons $\gamma'$, 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 $m_{\gamma'}$. In this work, we discuss searches for dark photons in the mass range around and below one 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 fermions. 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.

Highlights

In the class of hidden sector theories, new particles are predicted to interact with the Standard Model (SM) field content via feebly coupled mediator particles
This is important, as we limit ourselves to the deep inelastic scattering (DIS) regime, which means that the squared momentum transfer has to be much larger than the proton mass: Q2 ≫ m2p ≃ 1 GeV2
Extending the SM gauge group with an additional Uð1ÞX factor gives rise to a dark photon that interacts with the SM fermions via kinetic mixing

Summary

INTRODUCTION

In the class of hidden sector theories, new particles are predicted to interact with the Standard Model (SM) field content via feebly coupled mediator particles. This mixing is that the SM quarks and charged leptons acquire a millicharge under the new gauge group Depending on their mass, dark photons have different decay channels. It is important to notice that the dark photon models presently studied in the literature can be categorized loosely into two classes: minimal and nonminimal Models from the former class, so-called minimal models, consider only gauge mixing, and dark photons can be produced from decays of light mesons such as the pion and η, from bremsstrahlung processes, or directly from interactions of particle beams at colliders. Dark photons are being searched for in collider experiments, with LHCb targeting minimal models while ATLAS and CMS are mostly considering nonminimal ones. Details on the proposed detector layout and expected performance can be found, for instance, in Ref. [30]

THE MODEL

ANALYSIS AND RESULTS

CONCLUSIONS