Searching for Strongly-Interacting Dark Matter with the Heavy Photon Search Experiment
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Searching for Strongly-Interacting Dark Matter with the Heavy Photon Search Experiment

Abstract

The Heavy Photon Search Experiment (HPS) is a fixed-target experiment at Jefferson Lab’s Hall B, designed to explore a \gls{hs} of particles containing dark matter and a new force mediator known as the “heavy photon” (\ap). The \ap is a massive spin-1 gauge boson associated with a new $U(1)_D$ symmetry in the HS that kinetically mixes with the \gls{sm} photon with a weak coupling strength parameterized by \eps, with $\eps^2 \sim 10^{-2}-10^{-10}$.HPS utilizes a high-intensity electron beam on a thin tungsten target to produce heavy photons in the MeV-GeV mass range via “dark bremsstrahlung,” a process analogous to \gls{sm} bremsstrahlung but suppressed by $\eps^2$. The \ap can decay resonantly to \gls{sm} leptons, allowing HPS to conduct both mass resonance searches for prompt decays (large \eps) and displaced vertex searches for long-lived particles (small \eps). In addition to the minimal \ap model, HPS probes more complex extensions such as the QCD-like strongly-interacting massive particles (SIMPs) \gls{hs} containing “dark” pions (\pid) and vector mesons (\vd), with \pid as dark matter candidates. These particles introduce new thermal dark matter freeze-out scenarios and visible signals through long-lived \vd decays to \gls{sm} leptons, which are accessible to HPS.

This analysis conducted a displaced vertex search for $\vd \rightarrow \ele\pos$ in the mass range \SIrange{30}{124}{MeV} and \eps between $10^{-6} < \eps < 10^{-2}$ using data from the 2016 Engineering Run (\SI{10.753}{nb^{-1}}) at \SI{2.3}{GeV}. Unlike the minimal \ap search, SIMP signal kinematics required new approaches to signal normalization and \gls{sm} background rejection. The strongest signal evidence was a local p-value of 0.01317 for $m_{\vd} = \SI{119}{MeV}$, corresponding to a global significance of $0.9\sigma$. Although no signal was found, this search excluded a region of the SIMP parameter space at \SI{90}{\%} confidence. This work demonstrates HPS’s competitive capability to probe SIMP sectors within cosmologically significant parameters and introduces a new method for HPS displaced vertex searches using track vertical impact parameter cuts.

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