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Title: Does a dark photon exist with MeV to GeV mass?
Plain: A dark photon would be a heavy cousin of the photon that mixes slightly with ordinary light. It would appear as a narrow bump in electron-positron pair masses or as missing energy.
Precise: Determine whether a $U(1)'$ gauge boson A' with mass 1 MeV to 10 GeV and kinetic mixing $\epsilon \sim 1e-6\ \text{to}\ 1e-3$ exists, in visible ($A' \to \ell+ \ell-$) or invisible decay modes. An answer is a resonance detection or exclusion of the remaining parameter space down to $\epsilon \sim 1e-6\ \text{to}\ 1e-5$.
Settled by: Bump hunts and displaced-vertex searches at LHCb, Belle II, HPS, NA64 and future beam dumps (SHiP).
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1986