Long-lived vectors from electromagnetic cascades at SHiP

We simulate dark-vector, $V$, production from electromagnetic cascades at the recently approved SHiP experiment. The cascades (initiated by photons from $\pi^0\rightarrow \gamma \gamma$) can lead to 3-4 orders of magnitude increase of the event rate relative to using primary production alone. We provide new SHiP sensitivity projections for dark photons and electrophilic gauge bosons, which are significantly improved compared to previous literature. The main gain in sensitivity occurs for long-lived dark vectors with masses below $\sim 50-300~{\rm MeV}$. The dominant production mode in this parameter space is low-energy annihilation $e^+ e^- \rightarrow V(\gamma)$. This motivates a detailed study of backgrounds and efficiencies in the SHiP experiment for sub-GeV signals.