First Displaced Vertex Search for Electroproduced Dark-Sector Strongly Interacting Massive Particles by the HPS Experiment

The Heavy Photon Search experiment (HPS) is a fixed-target, electron beam experiment designed to search for $e^+e^-$ mass resonances and displaced decays using a forward acceptance spectrometer. This article details the search for naturally long-lived ``dark" vector mesons ($V_D$) arising from a dark sector of beyond-Standard-Model SIMP, characterized by a QCD-like $SU(3)_D$ symmetry and coupled to the Standard Model photon via a new $U(1)_D$ gauge interaction mediated by the ``heavy photon", or $A^\prime$. The results are based on an integrated luminosity of \SI{10608}{nb^{-1}} collected during the 2016 HPS Engineering Run. The displaced vertex search for $V_D \rightarrow e^+e^-$ in the $e^+e^-$ invariant mass range 39-179 MeV showed no statistically significant evidence for signal above the QED background.