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 S U ( 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.
Anonymous et al. (Thu,) studied this question.