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Title: Existence of a 17 MeV boson coupled to electrons
Plain: If the X17 particle exists, it should also appear when positrons hit electrons at just the right energy. Dedicated experiments are scanning this energy.
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 $2e-4 \text{ to } 1.4e-3$ 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.
Settled by: A further PADME run at higher statistics and an independent resonant-production or beam-dump experiment covering the full coupling range.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: contested
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.
Posed since: 2016