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Accepted · first revision · record

Title: Do missing-momentum experiments find MeV-GeV dark sector production?
Plain: If a hidden particle of mass between about an electron and a proton mass was produced thermally in the early universe, a predicted coupling follows. Electron beams on targets can test that prediction directly.
Precise: Test the thermal-freeze-out target y = epsilon^2 alpha_D (m_chi/m_A')^4 for scalar and fermion dark matter with $m_{\chi} \sim 1\,\mathrm{MeV} \text{ to } 1\,\mathrm{GeV}$ coupled through a dark photon, using missing energy and momentum in $e- N \to e- N A'$, A' -> chi chi-bar. An answer is a signal or exclusion of the thermal target for the main benchmark models.
Settled by: LDMX and NA64 (electron and muon modes) reaching the thermal targets across 1 MeV to $\sim 1\,\mathrm{GeV}$.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: NA64 excludes thermal targets for some benchmarks at the lowest masses as of 2024 to 2025.
Posed since: (empty)