In physics a nucleon is a collective name for two baryons: the neutron and the proton. They are constituents of the atomic nucleus and until the 1960s were thought to be elementary particles. In those days their interactions (now called internucleon interactions) defined strong interactions. Now they are known to be composite particles, made of quarks. Understanding the nucleons' properties is one of the major goals of quantum chromodynamics, the modern theory of strong interactions.
The proton is the lightest baryon and its stability is a measure of baryon number conservation. The proton's lifetime thus puts strong constraints on speculative theories which try to extend the Standard Model of particle physics. The neutron decays into a proton through the weak decay. The two are members of an isospin I = 1⁄2 doublet.
With spin and parity 1⁄2+, charge +1, and rest mass of 938 MeV, the proton is the nucleus of a hydrogen atom (1H). It has a magnetic moment of 2.79 nuclear magnetons (μN). The electric dipole moment is consistent with zero; the bound on it is that it is less than 0.54 × 10–23 e·cm.
A proton is made up of three quarks (two up quarks and one down quark), held together by the strong force, which is mediated by gluons. In some speculative grand unified theories it may decay. The half-life for this decay has been limited to be greater than 2.1 × 1029 years. The charge radius is measured mainly through elastic electron-proton scattering and is 0.870 fm. For specific decay modes, into leptons or antileptons and a meson, the bound is often better than 1032 years. The proton is therefore taken to be a stable particle, and baryon number is assumed to be conserved.The neutron has no charge, has spin and parity of 1⁄2+, and rest mass of 940 MeV/c2. Like the proton, a neutron is made up of three quarks (in this case one up quark and two down quarks) held together by the strong force. It decays weakly through the process
n0 → p+ + e− + νe
The most precise measurements of its decay lifetime are mainly from traps of various kinds and in beams. The lifetime of a free neutron outside the nucleus is 885.7 ± 0.8 s (about 15 minutes).
Its magnetic moment is −1.91 μN. Both time reversal and parity invariance of the strong interactions implies that the neutron's electric dipole moment must be zero; the current observational bound is that it is less than 6.3 × 10−24 e·cm. The mean-square charge radius related to the scattering length measured in low energy electron-neutron scattering for the neutron is −0.116 fm2.
Violation of baryon number conservation may give rise to oscillations between the neutron and antineutron, through processes which change B by two units. Using free neutrons from nuclear reactors, as well as neutrons bound inside nuclei, the mean time for these transitions is found to be greater than 1.3 × 108 s. The much poorer bound, as compared to protons, is related to the difficulty of the observations.
A limit on electric charge non-conservation comes from the observed lack of the decay
n0 → p+ + νe + νe
The observations which limit the branching fraction of the neutron in this decay channel to less than 8×10−27 are all done looking for appropriate decays of nuclei (A→A and Z→Z+1).
nucleon
