Search for Dark Sector by Repurposing the UVX Brazilian Synchrotron

We propose the first Search for Dark Sector at the Brazilian Synchrotron Light Laboratory, site of Sirius, a fourth-generation storage ring. We show that UVX, Sirius predecessor, can be a promising dark sector detector, SeDS, with unprecedented sensitivity. The search is based on a 1-3 GeV positron beam impinging on a thick target leading the $e^+ e^- \rightarrow \gamma A'$ reaction, followed by a missing mass spectrum event reconstruction. We show that SeDS has the potential to probe dark photons with masses up to 55 MeV and kinetic coupling down to $\epsilon \sim 10^{-14}$ within months of data. Therefore, such experiment would constitute the best dark photon probe worldwide in the 10-55 MeV mass range, being able to probe an unexplored region of parameter space.