An elementary particle found in the nuclei of atoms. It belongs to the class of particles called baryons.

1. 2006 CODATA value.

2. Randolf Pohl et al.
The size of the proton.
Nature, vol 466, pages 213-216 (8 July 2010)

3. Antognini et al.
Proton structure from the measurement of 2S-2P transition frequencies of muonic hydrogen.
Science, vol. 339, issue 6118, pages 417-20 (25 January 2013).
doi: 10.1126/science.1230016.

And see:

Randolf Pohl.
Das Proton bleibt zu klein.
Physik in Unserer Zeit, vol. 44, no. 3, pages 110-111 (2 May 2013).
doi: 10.1002/piuz.201390041

4. Pohl et al.
Laser spectroscopy of muonic deuterium.
Science, vol. 353, issue 6300, pages 669-673 (12 Aug 2016)
doi: 10.1126/science.aaf2468

Natalie Wolchover.
New Measurement Deepens Proton Puzzle.
Quanta Magazine, 11 August 2016.

Do protons decay?

The question of whether protons spontaneously change into another particle (a positron, probably) remains one of the great unanswered questions of physics. Experiments have shown that, if protons do decay, they must have a half life of more than 1.6 × 10²⁵ years.¹

1. Evans and Steinberg.
Science, volume 197, page 989. (1977)

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