{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"e3feedc0ea0ead479fdb0d11e3d32f7fc9803b499f92302f811a43f3a4fc0abb","created":"2026-10-03T07:18:07Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["32374ef8644e111942e945441cdc3d16ba405fbc4f99d22ad2df20e300814dd3","5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"5953a4280edefbed40ae71643d657ea9e836ed873db15b9b8673d37c1788f503","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"nuc.precision-anomalies.x17-anomaly-origin","kind":"phenomenon","literature_status":"contested","n":"1","parents":[{"note":"","parent_revision":"32374ef8644e111942e945441cdc3d16ba405fbc4f99d22ad2df20e300814dd3","relation":"special_case"}],"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.","posed_since":"2016","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.","problem_ref":null,"references":"","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.","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.","title":"Origin of the ATOMKI 17 MeV pair-emission 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