Neutron electric dipole moment

A nonzero electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide.

[1] A permanent electric dipole moment of a fundamental particle violates both parity (P) and time reversal symmetry (T).

CP violation has been observed in weak interactions and is included in the Standard Model of particle physics via the CP-violating phase in the CKM matrix.

Measuring a neutron electric dipole moment at a much higher level than predicted by the Standard Model would therefore directly confirm this suspicion and improve our understanding of CP-violating processes.

Quantum chromodynamics – the theoretical description of the strong force – naturally includes a term that breaks CP-symmetry.

[3][4] As in the case of the strong interaction, the limit on the neutron EDM is already constraining the CP violating phases.

It started in 1980 with an experiment at the Leningrad Nuclear Physics Institute [ru] (LNPI) obtaining a limit of |dn| < 1.6×10−24 e⋅cm .

The Cryogenic neutron EDM experiment or CryoEDM was under development at the Institut Laue-Langevin but its activities were stopped in 2013/2014.

Parity (P) and time-reversal (T) violation due to an electric dipole moment. μ denotes the magnetic dipole moment of the neutron, while d is the electric dipole moment.
The history of neutron EDM limits including the latest best result by the nEDM collaboration at PSI. [ 1 ] The prediction stemming from the Standard Model is also indicated.