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 eþe− mass resonances and displaced decays using a forward acceptance spectrometer. This paper details the search for naturally long-lived “dark” vector mesons (VD) arising from a dark sector of beyond-Standard-Model strongly interacting massive particles, characterized by a QCD-like SUð3ÞD symmetry and coupled to the Standard Model photon via a new Uð1ÞD gauge interaction mediated by the “heavy photon,” or A0. 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 VD → eþe− in the eþe− invariant mass range 39–179 MeV showed no statistically significant evidence for signal above the QED background.