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 paper details the search for naturally long-lived “dark” vector mesons (𝑉 𝐷 ) arising from a dark sector of beyond-Standard-Model strongly interacting massive particles, 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′. The results are based on an integrated luminosity of 10 608 nb −1 collected during the 2016 HPS engineering run. The displaced vertex search for 𝑉 𝐷 → 𝑒 + 𝑒 − in the 𝑒 + 𝑒 − invariant mass range 39–179 MeV showed no statistically significant evidence for signal above the QED background.