Revision 1

Superseded · first revision · record

Title: Origin of the ATOMKI 17 MeV pair-emission anomaly
Plain: 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: 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.
Settled by: 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.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status note: MEG II (2024) found no signal in $7\mathrm{Li}(p, e+ e-)8\mathrm{Be}$ and set 90 percent CL limits $R < 1.8e-6$ ($17.6\,\mathrm{MeV}$) and $R < 1.2e-5$ ($18.1\,\mathrm{MeV}$) 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.
Posed since: 2016