Proton radius puzzle

The proton radius puzzle is an unanswered problem in physics that relates to several experiments that measured a changed radius of a proton after an electron and electric charge change.[1]

Problem

In 2010, the known and officially recorded radius of a proton was (8.768±0.069)×10−16 m (or 0.8768±0.0069 fm), with approximately 1% relative uncertainty. That year, Pohl et al. published the results of an experiment to refine this measurement and obtain a more precise radius.

The experiment, performed at the Paul Scherrer Institute, involved forming muonic hydrogen by replacing the electron in the atom with a muon. The much higher mass of a muon causes it to orbit 207 times closer than an electron to the hydrogen nucleus (a single proton), where it is consequently much more sensitive to the size of the proton. The resulting radius was recorded as 0.842±0.001 fm, 5 standard deviations (5σ) smaller than the prior measurements. The experiment does not directly measure the physical size of the radius, but rather an energy difference between two separate energy levels of the atom, known as the Lamb shift, which is sensitive to the radius.[2][3] The newly measured radius is 4% smaller than the prior measurements, which were believed to be accurate within 1%. (The new measurement's uncertainty limit of only 0.1% makes a negligible contribution to the discrepancy.)[4]

Since 2010, additional measurements using electrons have slightly reduced the estimated radius to (8.751±0.061)×10−16 m (0.8751±0.0061 fm),[5] but by reducing the uncertainty even more have worsened the disagreement to over 7σ.

A follow-up experiment by Pohl et al. in August 2016 used a deuterium atom to create muonic deuterium and measured the deuteron radius. This experiment allowed the measurements to be 2.7 times more accurate, but also found a discrepancy of 7.5 standard deviations smaller than the expected value.[6][7]

Proposed resolutions

In 2012, Karr and Hilico attempted to explain the energy difference in the puzzle using the three-body force, but found through calculations that the ions involved did not bind tightly enough to explain the change in energy.[8]

In 2013, Onofrio suggested that the unknown effect may be the result of quantum gravity based on interactions between gravity and the weak force. This would result in a higher binding energy in the nucleus, decreasing the energy difference within it. Onofrio noted that while the energy difference for atomic hydrogen would be two orders of magnitude smaller, it should still be possible to detect it. Otherwise, the inability to detect the difference may be due to a flavour-dependent interaction.[9][3]

In August 2016, Dahia and Lemos proposed that gravity in higher dimensions is the cause. This relates to models based on brane cosmology where the effects of gravitational potential are amplified across shorter distances far more than in Newtonian physics.[10]

A month later in September, a new boson was proposed by Yu-Sheng, McKeen and Miller to resolve both the proton radius puzzle and the muon's anomalous magnetic dipole moment problem.[11]

Randolf Pohl, the original investigator of the puzzle, stated that while it would be "fantastic" if it led to a new discovery, the "most realistic thing is that it’s not new physics". He stated that his personal assumption is that past measurements have misgauged the Rydberg constant and that the official proton size for hydrogen is inaccurate.[12]

References

  1. ↑ Krauth, J. J.; Schuhmann, K.; et al. (2 June 2017). The proton radius puzzle. 52nd Rencontres de Moriond EW 2017. La Thuile, Aosta Valley. arXiv:1706.00696 Freely accessible [physics.atom-ph]. Presentation slides (19 March 2917).
  2. ↑ Pohl R, et al. (July 2010). "The size of the proton" (PDF). Nature. 466 (7303): 213–216. Bibcode:2010Natur.466..213P. PMID 20613837. doi:10.1038/nature09250.
  3. 1 2 Zyga, Lisa (November 26, 2013). "Proton radius puzzle may be solved by quantum gravity". Phys.org. Retrieved September 2, 2016.
  4. ↑ Carlson CE (May 2015). "The proton radius puzzle". Progress in Particle and Nuclear Physics. 82: 59–77. Bibcode:2015PrPNP..82...59C. arXiv:1502.05314 Freely accessible [hep-ph]. doi:10.1016/j.ppnp.2015.01.002.
  5. ↑ "CODATA Internationally recommended 2014 values of the Fundamental Physical Constants: Proton RMS charge radius rp".
  6. ↑ Pohl R, et al. (2016). "Laser spectroscopy of muonic deuterium" (PDF). Science. 353 (6300): 669–673. Bibcode:2016Sci...353..669P. PMID 27516595. doi:10.1126/science.aaf2468.
  7. ↑ "Proton-radius puzzle deepens". CERN Courier. 16 September 2016. After our first study came out in 2010, I was afraid some veteran physicist would get in touch with us and point out our great blunder. But the years have passed, and so far nothing of the kind has happened.
  8. ↑ Karr J, Hilico L (2012). "Why Three-Body Physics Does Not Solve the Proton-Radius Puzzle". Physical Review Letters. 109 (10): 103401. Bibcode:2012PhRvL.109j3401K. PMID 23005286. arXiv:1205.0633 Freely accessible [physics.atom-ph]. doi:10.1103/PhysRevLett.109.103401.
  9. ↑ Onofrio R (2013). "Proton radius puzzle and quantum gravity at the Fermi scale". EPL. 104 (2): 20002. Bibcode:2013EL....10420002O. arXiv:1312.3469 Freely accessible [hep-ph]. doi:10.1209/0295-5075/104/20002.
  10. ↑ Dahia F, Lemos AS (2016). "Is the proton radius puzzle evidence of extra dimensions?". European Physical Journal. 76 (8): 435. Bibcode:2016EPJC...76..435D. arXiv:1509.08735 Freely accessible [hep-ph]. doi:10.1140/epjc/s10052-016-4266-7.
  11. ↑ Liu Y, McKeen D, Miller GA (2016). "Electrophobic Scalar Boson and Muonic Puzzles". Physical Review Letters. 117 (10): 101801. Bibcode:2016PhRvL.117j1801L. PMID 27636468. arXiv:1605.04612 Freely accessible [hep-ph]. doi:10.1103/PhysRevLett.117.101801.
  12. ↑ Wolchover, Natalie (August 11, 2016). "New Measurement Deepens Proton Puzzle". Quanta Magazine. Retrieved September 2, 2016.
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