X17 particle
hypothetical subatomic particle as the cause of anomalous measurement results near 17 MeV
The X17 particle (X17 boson) is a hypothetical subatomic particle proposed by Attila Krasznahorkay and his colleagues to explain certain anomalous measurement results; these anomalous measurements are known as ATOMKI anomaly or beryllium (8Be) anomaly or X17 anomaly. The particle has been proposed to explain wide angles observed in the trajectory paths of particles produced during a nuclear transition of beryllium-8 nuclei and in helium nuclei. The X17 particle could be the force carrier for a postulated fifth force, possibly connected with dark matter, and has been described as a protophobic (i.e., ignoring protons) vector boson with a mass near 17 MeV/c2 — which is about 33 times heavier than an electron, but still very light for a boson. This is where it gets the name "X17".
The hypothesis originated from observations of unexpected wide-angle trajectories of electron-positron (e+e−) pairs produced during the nuclear transitions of excited Beryllium-8 nuclei. Similar resonant behavior was later reported by the same team in experiments involving Helium-4 and Carbon-12 nuclei. If confirmed, the X17 particle could represent the force carrier for a previously unknown fifth force of nature, often described in theoretical models as a "protophobic" (proton-ignoring) vector boson.
Because the X17 particle is not predicted by the Standard Model of particle physics, its existence would require significant extensions to current physical theories. It has garnered substantial interest as a potential "portal" to the dark sector, offering a possible candidate for dark matter interactions and a potential solution to other outstanding physics mysteries, such as the muon anomalous magnetic moment.
Multiple independent experiments worldwide—such as the NA64 experiment at CERN, the MEG II experiment at the Paul Scherrer Institute, and the PADME experiment at the Laboratori Nazionali di Frascati—have been searching for evidence of the X17 particle, with recent data placing increasingly stringent constraints on its allowed properties.
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