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Nucleon

From Academic Kids

In physics a nucleon is a collective name for the two baryons the neutron and the proton. They are constituents of the atomic nucleus and till the 1960s were thought to be elementary. In those days their interactions (now called internucleon interactions) defined strong interactions. Now they is known to be a composite particles, and made of quarks and gluons. 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 the protons through the prototype weak decay. The two are members of an isospin I=1/2 doublet.

Contents

The proton

With quantum numbers 1/2+, charge +1, and mass of 938.272029±0.000080 MeV, the proton is the nucleus of a hydrogen atom (1H1). It has a magnetic moment of 2.792847351±0.000000028 nuclear Bohr magnetons. The electric dipole moment is consistent with zero; the bound on it is that it must be less than 0.54×10-23 ecm.

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±0.008 fm. For specific decay modes, into antilepton or lepton 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.

For more details see the article on the proton.

The neutron

The neutron has no charge, has JP of 1/2+, and mass of 939.565360±0.000081. The most precise measurements of its decay lifetime are mainly from traps of various kinds and in beams. The lifetime of a neutron is 885.7±0.8 secs. It has the weak decay

<math>n\to p+e+\overline\nu_e<math>

Its magentic moment is -1.91304273±0.00000045 nuclear Bohr magnetons. Both time reversal and parity invariance of the strong interactions implies that the neutron's electric dipole moment must be zero. Observations have, however, only put a bound on it: that it must be less than 0.63×10-23 ecm. The mean-square charge radius related to the scattering length measured in low energy electron-neutron scattering. For the neutron it is -0.1161±0.0022 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 seconds. 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

<math>n\to p+\nu_e+\overline\nu_e<math>

The observations which limit the branching fraction of the neutron in this decay channel to less than one part in 8×10-27 are all done looking for appropriate decays of nuclei (A→A and Z→Z+1).

For more details see the article on the neutron.

Antinucleons

CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and the antiproton have to be equal. This equality has been tested to one part in 10-8). The equality of their masses is also tested to better than one part in 10-8. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton has been tested to 1 part in 90×10-12. The magnetic moment of the antiproton has been found with error of 8×10-3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton. For the neutron-antineutron system, the masses are equal to one part in (9±5)×10-5.

Quark model classification

In the quark model with SU(2) flavour, the two nucleons are part of the ground state doublet. The proton has quark content of uud, and the neutron, udd. In SU(3) flavour, they are part of the ground state octet (8) of spin 1/2 baryons. The other members of this octet are the hyperons strange isotriplet Σ0,±, the Λ and the strange iso-doublet Ξ0,-. One can extend this multiplet in SU(4) flavour (with the inclusion of the charm quark) to the ground state 20-plet.

See also

References and external links

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