Modern Physics

Particle Physics

Particles

Proton

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

(1)

(2)

which cannot be computed from first principles but must
instead be measured experimentally. The ratio of the proton
mass to the electron
mass is

(3)

which can be approximated quite accurately by

(4)

(5)

(6)

(H. Watson).

The mathematician G. H. Hardy
(1999, p. 17) estimated the number of protons in the
universe
as 1080.

Conservation
of baryon number explains why the decay

(7)

( MeV), which does not violate the conservation
laws of charge, energy, linear momentum, or angular momentum
nonetheless does not occur. It is fortunate that this process
"never" happens, since otherwise all protons in the universe
would gradually change into positrons! However, having
stated that protons do not decay, it must also be noted that
supersymmetric theories predict that protons actually do decay,
although with a half-life of at least 1030 years, which is longer than the age of the
universe.
All attempts to detect the decay of protons have thus far
been unsuccessful. One experimental test looked for Cherenkov
radiation given off by the reaction

(8)

in a 10-meter cube of water. In this way, the half-life of the proton
was found to exceed 1033 years.
Unfortunately, this limit cannot be improved because cosmic
background radiation contributes radiation via the reaction

(9)

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

References

--. 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.

© Eric W. Weisstein