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

TL;DR

The HPS experiment conducted a search for long-lived dark vector mesons decaying into electron-positron pairs, but found no significant evidence, setting constraints on dark sector models involving a new gauge interaction.

Contribution

First displaced vertex search for dark vector mesons in electron beam fixed-target experiment, exploring a previously untested parameter space for dark sector particles.

Findings

No significant signal observed above background.

Set new limits on dark vector meson production.

Demonstrated the feasibility of displaced vertex searches in fixed-target experiments.

Abstract

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.