HEP
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Does a dark photon exist with MeV to GeV mass?
In plain words
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 statement
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 $\epsilon \sim 1e-6\ \text{to}\ 1e-5$.
What would settle it
Bump hunts and displaced-vertex searches at LHCb, Belle II, HPS, NA64 and future beam dumps (SHiP).
Related problems
- More general than Do missing-momentum experiments find MeV-GeV dark sector production?
- More general than Origin of the ATOMKI 17 MeV pair-emission anomaly