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

TL;DR

This work searches for long-lived dark-sector vector mesons V_D produced via a dark photon A' in a SIMP framework, using the 2016 HPS Engineering Run data. The analysis develops a two-category displaced-vertex selection (L1L1, L1L2), optimizes a vertical-impact-parameter cut (y_{0,min}) as a discriminant, and employs an ABCD-like background estimation in $(y_{0,min}, m_{reco})$ space to hunt for V_D -> e^+e^- decays within a narrow mass window. No significant excess is found; a 90% CL upper limit on the signal yield translates into exclusions in the $(m_{A'}, \epsilon)$ plane for a fixed $m_{\pi_D}/f_{\pi_D}=4\pi$, extending previous constraints and being consistent with BaBar. The study demonstrates sensitivity to SIMP-like visible decays and outlines prospects for future gains with larger data samples and an additional vertex category to probe longer lifetimes.

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.

Figures (13)

• Figure 1: Production of $e^{+}e^{-}$ from the decay of a dark vector meson $V_D$ via a virtual dark photon $A^\prime$.
• Figure 2: A cutaway view of the HPS detector showing the svt in a vacuum chamber inside the bore of the spectrometer magnet and the downstream ecal. The positions of the target and the front portions of the svt are controlled by a set of linear positioning motors upstream of the detector.
• Figure 3: Radiative (left) and Bethe-Heitler tridents (right) have the same final state particles as the $e^+$$e^-$ production from a dark vector decay shown in \ref{['fig:intro:darkdecay']}.
• Figure 4: Diagram showing the two mutually exclusive categories based on the track hit content within a vertex "L1L1" (black) and "L1L2" (blue).
• Figure 5: Illustrations of the vertical track impact parameters, $y_0$, at the target for truly-displaced events (top), not-displaced events (middle), and fake-displaced events (bottom) due to scattering or reconstruction errors.