NUC On PubPhys: open In the literature: contested Compiled by AI, not yet reviewed by an editor

Origin of the ATOMKI 17 MeV pair-emission anomaly

In plain words

A group in Hungary (ATOMKI) saw too many electron-positron pairs emitted at large angles from excited beryllium, helium and carbon nuclei, as if a new particle of about 17 MeV were produced. Other experiments have not confirmed it.

Precise statement

ATOMKI reported excesses in the e+ e- opening-angle distribution from internal pair creation in the 18.15 MeV 8Be (2016), 20.21 MeV 4He and 17.23 MeV 12C transitions, each consistent with a boson of mass about 16.7 to 17.0 MeV. Determine the origin: a new boson (X17), a standard nuclear-physics effect, or an instrumental artifact. Answer: the identified origin with independent experimental confirmation.

What would settle it

Independent repetition of the 8Be and 4He pair-emission measurements with different detectors, combined with direct searches for a 17 MeV boson coupled to electrons.

Status in the literature

Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.

MEG II (2024) found no signal in $7\mathrm{Li}(p, e+ e-)8\mathrm{Be}$ and set 90 percent CL limits $R < 1.8 \times 10^{-6}$ ($17.6\,\mathrm{MeV}$) and $R < 1.2 \times 10^{-5}$ ($18.1\,\mathrm{MeV}$) on the X17 branching ratio relative to gamma emission, while PADME (2025) reported an excess near $16.90\,\mathrm{MeV}$ with a global significance of about 2 standard deviations.

Parent problems

Special case of Does a dark photon exist with MeV to GeV mass? · open · from revision 1 The parent was revised since revision 1: it now states another question.

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