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Dedicated experiments are scanning this energy.","posed_since":"2016","precise":"Test for a vector or axial-vector boson with $m_X = 16.5 \\text{ to } 17.2\\,\\mathrm{MeV}$ and electron coupling $\\epsilon$ of roughly $2 \\times 10^{-4} \\text{ to } 1.4 \\times 10^{-3}$ through resonant $e+ e- \\to X \\to e+ e-$ production (PADME), electron beam-dump and pair-spectrometer experiments. Answer: discovery, or exclusion at 90 percent confidence of the full coupling range implied by the ATOMKI 8Be and 4He rates.","problem_ref":"054b71499eff153a847d31139e97bb5a0e4e13efb05917c9e497d675261ebd38","references":"","settled_by":"A further PADME run at higher statistics and an independent resonant-production or beam-dump experiment covering the full coupling range.","status_note":"PADME (arXiv 2505.24797, 2025) found an excess near 16.90 MeV with a global significance of about 2 standard deviations; no follow-up result had been published, to our knowledge, as of mid-2026.","title":"Existence of a 17 MeV boson coupled to 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