Electric Dipole Moment of the Neutron

Abstract

Abstract The existence of the electric dipole moment (EDM) of elementary particles (particularly of the neutron) is associated with their symmetry properties. The discovery of parity violation has aroused a special interest in discrete transformations: charge conjugation (C), inversion of coordinates (P), and time reversal (T). Detailed reviews on the problem are given in Refs 218 and 219. A charge conjugation operation means the replacement of all particles by their antiparticles. A charged or a not truly neutral particle transforms into another, for example an electron into a positron. A truly neutral particle (e.g. a photon or a n° meson) transforms into itself. Each transformation implies a quantum mechanical quantity, which either reverses or preserves its sign. The charge conjugation reverses the sign of the charge (electronic, baryonic, leptonic) of the particle (Q → −Q), magnetic moment (μ → −μ), magnetic field strength (H → −H), and electric field strength (ℰ → −ℰ), but preserves the sign of the momentum (p → p) and spin (σ → σ). An experimentally observed particle-antiparticle symmetry reveals itself in the requirement of the invariant of physical laws under charge conjugation (C): if a certain physical process occurs in nature, the physical process in which the particles have been replaced by antiparticles occurs with the same probability (the law of C parity conservation). Up to 1956 the above formulated law was believed to hold for any interactions of any physical objects. In 1956 the breakdown of invariance under charge conjugation in weak interactions was discovered. For example, the free neutrino was found to be polarized in the opposite direction to its momentum and antineutrino in the direction of the momentum. The charge conjugation operation applied to the neutrino (or antineutrino) leads to a situation having no place in nature.