X17 particle

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X17 particle
CompositionElementary particle
FamilyBoson
InteractionsFifth force
StatusHypothetical
SymbolX17
Theorized2015
Mass16.84±0.36 MeV[1]​
Mean lifetime10−14 s[2]​
Decays intoone electron and one positron
Electric charge0 e

The X17 particle is a hypothetical subatomic particle proposed by and his colleagues to explain certain anomalous measurement results.[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]​

The NA64 experiment at CERN looks for the proposed X17 particle by striking the electron beams from the Super Proton Synchrotron on fixed target nuclei.[6][7]

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).[8] 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)[9] 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.[8][9]

Krasznahorkay (2019)[1] 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 X‑17 particle.[1] This was covered in science journalism, focusing largely on the implications that the existence of the X‑17 particle and a corresponding fifth force would have in the search for dark matter.[10][11][12]

Skepticism[]

As of December 2019, the ATOMKI paper describing the particle has not been peer reviewed and should therefore be considered preliminary.[13] Furthermore, efforts by CERN and other groups to independently detect the particle have been unsuccessful so far.[14]

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][15]

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. ^ Jump up to: a b c Krasznahorkay, A.J.; et al. (23 October 2019). "New evidence supporting the existence of the hypothetic X‑17 particle". arXiv:1910.10459v1 [nucl-ex].
  2. ^ Jump up to: a b c 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. ^ Jump up to: a b O'Callaghan, Jonathan (9 December 2019). "Evidence of new X‑17 particle reported, but scientists are wary". Scientific American. Retrieved 9 December 2019.
  4. ^ Jump up to: a b c d Cockburn, Harry (21 November 2019). "Scientists may have discovered fifth force of nature, laboratory announces". The Independent. Retrieved 21 November 2019.
  5. ^ Jump up to: a b Wolchover, Natalie (7 June 2016). "Evidence of a 'Fifth Force' Faces Scrutiny - 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". Quanta Magazine. Retrieved 20 November 2019.
  6. ^ Lopes, Ana (27 November 2019). "The plot thickens for a hypothetical "X17" particle". CERN News. Retrieved 3 May 2021. Check |archive-url= value (help)
  7. ^ "'X' boson feels the squeeze at NA64". CERN Courier. 25 June 2021. Retrieved 22 July 2021.
  8. ^ Jump up to: a b Cartlidge, Edwin (2016). "Has a Hungarian physics lab found a fifth force of nature?". Nature. doi:10.1038/nature.2016.19957. S2CID 124347962.
  9. ^ Jump up to: a b 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.
  10. ^ 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.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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.
  15. ^ 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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