magneton

magneton [′mag·nə‚tän]

(physics)

A unit of magnetic moment used for atomic, molecular, or nuclear magnets, such as the Bohr magneton, Weiss magneton, or nuclear magneton.

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Magneton

A unit of magnetic moment used to describe atomic, molecular, or nuclear magnets. More precisely, one unit, the Bohr magneton, is used at the atomic and molecular levels, and another unit, the nuclear magneton, is used at the nuclear level. Still another unit (which might be called the muon magneton, but is usually not named) is used to describe the magnetic moment of the muon.

The Bohr magneton μ_B is defined and its value given in Eq. (1), where -e and m

(1) 

are the charge and mass of the electron respectively and ℏ is Planck's constant divided by 2&pgr;. In Dirac's theory the magnetic moment of the electron is exactly -μ_B, but according to the theory of quantum electrodynamics the electron has a small anomalous magnetic moment. The experimental value of the electron magnetic moment μ_e is given by Eq. (2), in agreement with the

(2) 

prediction of quantum electrodynamics within the errors.

The unit of magnetic moment to describe the muon is obtained from the Bohr magneton by replacing m in Eq. (1) by the muon mass m_μ. The experimental value of the muon magnetic moment is given in Eq. (3). The deviation

(3) 

of the muon magnetic moment from its Dirac value can also be accounted for by the theory of quantum electrodynamics. See Lepton

The nuclear magneton is obtained from the Bohr magneton by replacing m by the proton mass m_p. The value of the nuclear magneton is given in Eq. (4). The

(4) 

nuclear magneton is used not only as the unit for the magnetic moment of the proton but also for the neutron and other hadrons and for atomic nuclei. If the proton and neutron were Dirac particles, the proton's magnetic moment would be one nuclear magneton (except for a small correction arising from quantum electrodynamics) and the neutron's magnetic moment would be zero (because the neutron is uncharged). However, the proton and neutron have large anomalous

(5) 

magnetic moments, given in Eqs. (5). See Neutron, Nuclear moments, Proton

According to present theory, the proton, neutron, and other hadrons have large anomalous magnetic moments because these particles are not elementary but composite. In the theory of quantum chromodynamics, the principal constituents of a baryon, such as the proton or neutron, are three quarks. See Baryon, Elementary particle, Fundamental constants, Quantum chromodynamics, Quarks