The DarkLight experiment : searching for the dark photon

Abstract

We describe a setup to search for the existence of a massive gauge boson A', the "dark photon," mediating dark matter interactions with coupling strength [alpha]'. In certain models, the A' decays promptly but invisibly and might be discoverable in high luminosity collider experiments. Following a proposed setup by Wojtsekhowski et al. to use the VEPP-3 electron-positron storage ring at the Budker Institute for Nuclear Physics, we study whether an e+ + e- - A'+ -[gamma] signal could be seen over the quantum electrodynamic background. The proposed VEPP-3 setup is sensitive to an A' within the mass range mA = 5-20 MeV. Out of the two backgrounds, e++e- - 3[gamma] and e+ + e- - e+ + e- + [gamma], we find that the former process provides the dominant background for the A' signal. While positron bremsstrahlung events can be detected and suppressed in Wojtsekhowski's apparatus, the 3[gamma] cross-section has a large cross section in this range, "faking" an A'. We use Monte Carlo numerical integration techniques to calculate the cross sections and obtain reach plots, determining which values of mA' and a' could be discovered at 5[sigma] confidence. This background study can be used to improve the VEPP-3 proposal, and provides a valuable comparison study with the MIT-led DarkLight proposal to search for a dark photon in the same mass range.