X17 particle

The X17 particle 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 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 atoms and in stable helium atoms. 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 $17.01 MeV$.

History
In 2015, Krasznahorkay and his colleagues at ATOMKI, the Hungarian Institute for Nuclear Research, posited the existence of a new, light boson with a mass of about $17 MeV$ (i.e., 34 times heavier than the electron). In an effort to find a dark photon, the team fired protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that then decayed and produced pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the electron+ and electron- particles and a combined energy of approximately $17 MeV$. This indicated that a small fraction of beryllium-8 might shed its excess energy in the form of a new particle. The result was successfully repeated by the team.

Feng et al. (2016) proposed that a "protophobic" X boson, with a mass of $17 MeV$, suppressed couplings to protons relative to neutrons and electrons at femtometer range, could explain the data. The force may explain the $16.7 MeV$ − 2 muon anomaly and provide a dark matter candidate. , several research experiments are underway to attempt to validate or refute these results.

Krasznahorkay (2019) posted a preprint announcing that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium-8, strengthening the case for the existence of the X17 particle.

This was covered in science journalism, focusing largely on the implications that the existence of the X17 particle and a corresponding fifth force would have in the search for dark matter.

In 2021 the workshop "Shedding light on X17" was held at Centro Enrico Fermi in Rome, Italy. The workshop discussed the ATOMKI anomaly and its theoretical interpretation and future experiments to confirm and explain it. See the report of the workshop: Report of "Shedding light on X17". One of the experiments that plans to repeat the original ATOMKI lithium-beryllium experiment is MEG II at PSI institute; the measurement was planned (in 2021) to be completed in 2022. A presentation about MEG II in October 2022: Presentation. Also Universite de Montreal's 6MV (6 megavolt) Tandem Van de Graaff Facility in Montreal has an experiment attempting to reproduce the ATOMKI measurement; data taking should take place in early 2023.

In 2022, another preprint was published by Krasznahorkay et al. supporting the X17 particle hypothesis.

CERN's NA64 experiment and NA62 experiment have reported in 2021 and 2023  respectively results of conducted searches that have put stringent limits for the existence of the X17 particle.

In early 2023 the MEG II experiment performed its replication of the ATOMKI lithium-beryllium experiment; as of January 2024 the results have not yet been published (although the measurements were made in early 2023).

Skepticism
, the ATOMKI paper describing the particle has not been peer reviewed and should therefore be considered preliminary. In late 2019, a follow-up paper was published in Acta Physica Polonica B. Efforts by CERN and other groups to independently detect the particle have been unsuccessful so far.

The ATOMKI group had claimed to find various other new particles earlier in 2016 but abandoned these claims later, without an explanation of what caused the spurious signals. The group has also been accused of cherry-picking results that support new particles while discarding null results.

The X‑17 particle is not consistent with the Standard Model, so its existence would need to be explained by another theory.