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 π + β’π β mass resonances and displaced decays using a forward acceptance spectrometer. This paper details the search for naturally long-lived βdarkβ vector mesons (π π· ) arising from a dark sector of beyond-Standard-Model strongly interacting massive particles, characterized by a QCD-like πβ’πβ’(3) π· symmetry and coupled to the Standard Model photon via a new πβ’(1) π· gauge interaction mediated by the βheavy photon,β or π΄β². The results are based on an integrated luminosity of 10 608 nb β1 collected during the 2016 HPS engineering run. The displaced vertex search for π π· β π +β’ π β in the π +β’ π β invariant mass range 39β179 MeV showed no statistically significant evidence for signal above the QED background.