proton

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

unit positive electric charge
spin = ½
mass of 1.671 621 637 × 10⁻²⁷ kilogram, with a standard uncertainty of 0.000 000 083 × 10⁻²⁷ kg.¹
A proton is not point-like, but has a “diameter.” A proton contains 3 quarks (2 up and 1 down), and the proton's diameter is the average distance between the quarks. Its size is usually described by tbe root-mean-square charge radius, which according to the 2006 CODATA values, was 0.8768 femtometers with a standard uncertainty of 0.0069 femtometers.
Then in 2010 a new study² using a novel technique found a value of 0.84184 femtometers, with a standard uncertainty of 0.0067 femtometers. Further studies using new techniques, published in 2013³ and 2016⁴, also found a smaller proton.
As of mid-2016, no convincing explanation of the discrepancy had been found.

1. 2006 CODATA value.

2. Randolf Pohl et al.
The size of the proton.
Nature, vol 466, pages 213-216 (8 July 2010)
doi:10.1038/nature09250.

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.
online

Sources of the proton's mass

As estimated by Yang et al.¹:

quarks, 9%
energy of quarks motion within proton, 32%
gluons, 36%
quark-gluon interactions, 23%

1.Yi-Bo Yang, Jian Liang, Yu-Jiang Bi, Ying Chen, Terrence Draper, Keh-Fei Liu and Zhaofeng Liu.
Proton mass decomposition from the QCD energy momentum tensor.
Physical Review Letters, vol 121, issue 21. Published 19 November 2018.
doi:10.1103/PhysRevLett.121.212001

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)

want more?

The Particle Data Group offers authoritative data, an interesting chart and links to related sites: http://pdg.lbl.gov

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