NUC In the literature: contested

Existence of a 17 MeV boson coupled to electrons

In plain words

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 statement

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.

What would settle it

A further PADME run at higher statistics and an independent resonant-production or beam-dump experiment covering the full coupling range.

Status in the literature

Unverified 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.