Neutrons in time

25 February 1999

A new limit on the electric dipole moment of the neutron comes from an experiment using ultra-cold neutrons at the research reactor at the international Institut Laue-Langevin, Grenoble, France. For many years, the team has been refining its techniques to push the limit progressively lower.

Viewed as an electrically neutral particle, the neutron should have no electric dipole moment, but an effect could arise due to its constituent electrically charged quarks.

An electric dipole moment (the product of spin and electric field) violates time reversal symmetry (T), and any non-zero value for the neutron electric dipole moment would signal a dependence on the arrow of time.

This would be no surprise to physicists. The neutral kaons violate the combined CP reflection which changes particle to antiparticle as well as producing a mirror image. Because the combined CPT has to be good, T symmetry has to be broken in sympathy.

This “natural” level of T violation should give a value for the neutron electric dipole moment of some 10­30. Meanwhile, imaginative physicists have proposed models which predict a larger value. The latest measurement of 6.3 x 10-26, although still a long way from the naturally expected value, does start to restrict theoretical speculation.

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