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The proton is a charged elementary particle that can exist freely or an an atomic nucleus. It has charge  , where e is the charge on the electron. It is also a fermion, having spin 1/2. Its mass is
which cannot be computed from first principles but must instead be measured experimentally.
Curiously, quark spins appear to account for only 20-30% of the neutron and proton spins (Jaffe 1995,
Hellemans 1996). The current best two values for the proton's radius disagree:
0.805 ± 0.011 and
0.862 ± 0.012 femtometer (Stein 1995). Weingarten (1993) calculated the mass of the proton to within 6% using
"lattice" quantum chromodynamics.
 Antiproton, Atom, Conservation of Baryon Number, Electron, Neutron, Nucleus, Proton Mass
--. Science, 1077, 21 May 1993.
Caso, C. et al. "1999 WWW Review of Particle Physics." Europ. Phys. J. C3, 1, 1998. http://pdg.lbl.gov/1999/s016.ps.
Hardy, G. H. Ramanujan: Twelve Lectures on Subjects Suggested by his Life and Work. New York: Chelsea, 1999.
Hellemans, A. "Quark Studies Put Theorists in a Spin." Science 271, 911, 1996.
Jaffe, R. L. "Where Does the Proton Really Get Its Spin?" Physics Today 48, 24-30, Sept. 1995.
Stein, B. P. "Physics Update." Physics Today 48, 9, Oct. 1995.
© 1996-2007 Eric W. Weisstein
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