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.[2][3] 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.[4] The X17 particle could be the force carrier for a postulated fifth force, possibly connected with dark matter,[4] and has been described as a protophobic (i.e., ignoring protons)[5] vector boson with a mass near 17 MeV.[4]

X17 particle
CompositionElementary particle
FamilyBoson
InteractionsFifth force
StatusUnconfirmed
SymbolX17
Theorized2015
Mass17.01±0.16 MeV[1]
Mean lifetime10−14 s[2]
Decays intoone electron and one positron
Electric charge0 e

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).[6] 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.[2] Excess decays were observed at an opening angle of 140° between the
e+
 and
e−
 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.[4]

Feng et al. (2016)[7] proposed that a "protophobic" X boson, with a mass of 16.7 MeV, suppressed couplings to protons relative to neutrons and electrons at femtometer range, could explain the data. The force may explain the g − 2 muon anomaly and provide a dark matter candidate. As of 2019, several research experiments are underway to attempt to validate or refute these results.[6][7]

Krasznahorkay (2019)[8] 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.[8]

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.[9][10][11]

In 2021 the workshop "Shedding light on X17" was held at Centro Enrico Fermi in Rome. 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.[12][13] 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.[14]

In 2022, another preprint was published by Krasznahorkay et al. supporting the X17 particle hypothesis.[15]

The NA64 experiment and NA62 experiment have reported in 2021[16][17] and 2023[18][19] respectively results of conducted searches that have put stringent limits for the existence of the X17 particle.

Skepticism

As of December 2019, the ATOMKI paper describing the particle has not been peer reviewed and should therefore be considered preliminary.[20] In late 2019, a follow-up paper was published in Acta Physica Polonica B.[1] Efforts by CERN and other groups to independently detect the particle have been unsuccessful so far.[21]

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.[5][22]

The X‑17 particle is not consistent with the Standard Model, so its existence would need to be explained by another theory.[3]

See also

References
  1. Krasznahorkay, A.J.; Csatlós, M.; Csige, L.; Firak, D.; Gulyás, J.; Nagy, Á.; Sas, N.; Timár, J.; Tornyi, T.G. (2019). "On the X(17) light-particle candidate observed in nuclear transitions". Acta Physica Polonica B. 50 (3): 675. Bibcode:2019AcPPB..50..675K. doi:10.5506/APhysPolB.50.675. S2CID 126936875.
  2. Krasznahorkay, A.J.; et al. (26 January 2016). "Observation of anomalous internal pair creation in 8Be: A possible indication of a light, neutral boson". Physical Review Letters. 116 (42501): 042501. arXiv:1504.01527. Bibcode:2016PhRvL.116d2501K. doi:10.1103/PhysRevLett.116.042501. PMID 26871324. S2CID 206268170.
  3. O'Callaghan, Jonathan (9 December 2019). "Evidence of new X‑17 particle reported, but scientists are wary". Scientific American. Retrieved 9 December 2019.
  4. Cockburn, Harry (21 November 2019). "Scientists may have discovered fifth force of nature, laboratory announces". The Independent. Archived from the original on 7 May 2022. Retrieved 21 November 2019.
  5. Wolchover, Natalie (7 June 2016). "Evidence of a 'fifth force' faces scrutiny". Quanta Magazine. Retrieved 20 November 2019. A lab in Hungary has reported an anomaly that could lead to a physics revolution. But even as excitement builds, closer scrutiny has unearthed a troubling backstory.
  6. Cartlidge, Edwin (2016). "Has a Hungarian physics lab found a fifth force of nature?". Nature. doi:10.1038/nature.2016.19957. S2CID 124347962.
  7. Feng, Jonathan L.; et al. (2016). "Protophobic fifth force interpretation of the observed anomaly in 8Be nuclear transitions". Physical Review Letters. 117 (7): 071803. arXiv:1604.07411. Bibcode:2016PhRvL.117g1803F. doi:10.1103/PhysRevLett.117.071803. PMID 27563952. S2CID 206279817.
  8. Krasznahorkay, A.J.; et al. (23 October 2019). "New evidence supporting the existence of the hypothetic X‑17 particle". arXiv:1910.10459v1 [nucl-ex].
  9. McRae, Mike (20 November 2019). "Physicists claim they've found even more evidence of a new force of nature". ScienceAlert.com. Retrieved 20 November 2019.
  10. Prior, Ryan (22 November 2019). "A 'no-brainer Nobel Prize': Hungarian scientists may have found a fifth force of nature". CNN News. Retrieved 22 November 2019.
  11. Malewar, Amit (21 November 2019). "Scientists may have discovered the fifth force of nature – It's not the first time researchers claim to have caught a glimpse of it". TechExplorist.com. Retrieved 23 November 2019.
  12. Chiappini, Marco; Francesconi, Marco; Kobayashi, Satoru; Meucci, Manuel; Onda, Rina; Schwendimann, Patrick (2021). "Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning". Universe. 7 (12): 466. Bibcode:2021Univ....7..466C. doi:10.3390/universe7120466.
  13. Meucci, Manuel (2022). "MEG II experiment status and prospect". The 22nd International Workshop on Neutrinos from Accelerators. p. 120. arXiv:2201.08200. Bibcode:2022iwna.confE.120M.
  14. Azuelos, G.; Bryman, D.; Chen, W.C.; De Luz, H.; Doria, L.; Gupta, A.; Hamel, L-A.; Laurin, M.; Leach, K.; Lefebvre, G.; Martin, J-P.; Robinson, A.; Starinski, N.; Sykora, R.; Tiwari, D.; Wichoski, U.; Zacek, V.; Zacek, V. (2022). "Status of the X17 search in Montreal". Journal of Physics: Conference Series. 2391 (1): 012008. arXiv:2211.11900. Bibcode:2022JPhCS2391a2008A. doi:10.1088/1742-6596/2391/1/012008. S2CID 253761073.
  15. Sas, N. J.; Krasznahorkay, A. J.; Csatlós, M.; Gulyás, J.; Kertész, B.; Krasznahorkay, A.; Molnár, J.; Rajta, I.; Timár, J.; Vajda, I.; Harakeh, M. N. (2022). "Observation of the X17 anomaly in the 7Li(p,e+e−)8Be direct proton-capture reaction". arXiv:2205.07744 [nucl-ex].
  16. Andreev, Yu. M.; Banerjee, D.; Bernhard, J.; Burtsev, V. E.; Charitonidis, N.; Chumakov, A. G.; Cooke, D.; Crivelli, P.; Depero, E.; Dermenev, A. V.; Donskov, S. V.; Dusaev, R. R.; Enik, T.; Feshchenko, A.; Frolov, V. N. (15 December 2021). "Search for pseudoscalar bosons decaying into e+e− pairs in the NA64 experiment at the CERN SPS". Physical Review D. 104 (11). arXiv:2104.13342. Bibcode:2021PhRvD.104k1102A. doi:10.1103/PhysRevD.104.L111102. ISSN 2470-0010. S2CID 233407961.
  17. "'X' boson feels the squeeze at NA64". CERN Courier. 25 June 2021. Retrieved 22 July 2021.
  18. NA62 Collaboration (2023). "Search for K+ decays into the π+e+e−e+e− final state". arXiv:2307.04579 [hep-ex].
  19. "Dark boson searches at CERN's North Area". CERN. 11 August 2023. Retrieved 28 August 2023.
  20. Johnson-Groh, Mara (9 December 2019). "Mysterious 'particle X‑17' could carry a newfound fifth force of nature, but most experts are skeptical". Live Science. Retrieved 9 December 2019.
  21. Banerjee, D.; Burtsev, V.E.; Chumakov, A.G.; Cooke, D.; Crivelli, P.; Depero, E.; Dermenev, A.V.; Donskov, S.V.; Dusaev, R.R.; Enik, T.; Charitonidis, N. (8 June 2018). "Search for a hypothetical 16.7 MeV gauge boson and dark photons in the NA64 Experiment at CERN". Physical Review Letters. 120 (23): 231802. arXiv:1803.07748. Bibcode:2018PhRvL.120w1802B. doi:10.1103/PhysRevLett.120.231802. ISSN 0031-9007. PMID 29932721. S2CID 49380594.
  22. Siegel, Ethan (26 November 2019). "This is why the 'X‑17' particle and a new, fifth force probably don't exist". Forbes. Retrieved 28 November 2019.
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