Dark photons from charm mesons at LHCb

We propose a search for dark photons A^' at the LHCb experiment using the charm meson decay D^* (2007) ^0D^0A^'. At nominal luminosity, D^*0D^0 decays will be produced at about 700 kHz within the LHCb acceptance, yielding over 5 trillion such decays during Run 3 of the LHC. Replacing the photon with a kinetically mixed dark photon, LHCb is then sensitive to dark photons that decay as A^'e^+e^-. We pursue two search strategies in this paper. The displaced strategy takes advantage of the large Lorentz boost of the dark photon and the excellent vertex resolution of LHCb, yielding a nearly background-free search when the A^' decay vertex is significantly displaced from the proton-proton primary vertex. The resonant strategy takes advantage of the large event rate for D^*0D^0A^' and the excellent invariant-mass resolution of LHCb, yielding a background-limited search that nevertheless covers a significant portion of the A^' parameter space. Both search strategies rely on the planned upgrade to a triggerless-readout system at LHCb in Run 3, which will permit the identification of low-momentum electron-positron pairs online during data taking. For dark photon masses below about 100 MeV, LHCb can explore nearly all of the dark photon parameter space between existing prompt-A^' and beam-dump limits.