Although the neutron is a neutral particle, there are small electric charges deep within it. If the average positions of these charges do not coincide the neutron would have an electric dipole moment (EDM) which would violate one of the fundamental symmetries of nature: time reversal symmetry. The violation of this symmetry is thought to be at the heart of how the Universe has evolved from the Big Bang to be matter dominated at present.

Experiments with ultra-cold neutrons (neutrons with very low energy) can look for a signal that the EDM would give by storing the neutrons in a cell that is submitted to a combination of a weak magnetic and strong electric field. Scientists at ILL thus, step-by-step, push the boundaries of our knowledge of the Universe forward; this search for the neutron EDM may well bring us a step closer to the understanding of why we are living in a material world.

M. van der Grinten, ILL brochure "Neutrons and the Universe", p.13

See also: hepwww.rl.ac.uk/edm/