Paper 3.3: "A Search for the Electric Dipole Moment of the Neutron," K. F. Smith, N. Crampin, J. M. Pendlebury, D. J. Richardson, D. Shiers, K. Green, A. I. Kilvington, J. Moir, H. B. Prosper, D. Thompson, N. F. Ramsey, B. R. Heckel, S. K. Lamoreaux, P. Ageron, W. Mampe and A. Steyerl, Phys. Lett. B234, 191–196 (1990)

As mentioned in the commentary on Paper 3.2, our apparatus for the neutron electric dipole moment was moved in 1974 to the much more intense cold neutron beam from the liquid deuterium moderator at the Institut Laue-Langevin (ILL) at Grenoble, France. With that beam and other improvements, W. B. Dress, P. D. Miller, J. M. Pendlebury, P. Perrin and I [Phys. Rev. D15, 9 (1977)] reduced the upper limit to 3 × 10^-24 e cm. Our principal source of error was from B and E not being exactly parallel to each other so that the effective magnetic field v × E/c produced by the motion of the neutron would have a component parallel to B which would change the neutron resonance frequency and therefore falsely look like an electric dipole moment.

We had realized for many years that trapping the neutrons by total reflection inside a suitable bottle would overcome this problem since the average value of v/c would then be very small. However, the neutrons must be slower than 7 m/s. We had tried unsuccessfully at Oak Ridge many years earlier to make a bottle for neutrons, but there were too few slow neutrons. At the ILL there were enough low energy neutrons, even from a water converter, at 320 K to make a successful trap, so Pendlebury, Smith, Golub, Byrne, McComb, Sumner, Taylor, Heckel, Green, Morse, Kilvington, Baker, Clark, Mampe, Ageron, Miranda and I [Phys. Lett. 136B, 327–330 (1984)] constructed a bottle in which the neutrons could be stored and the electric dipole moment measured. We found that the neutron electric dipole moment was less than 1 × 10^-24 e cm.

Paper 3.3 is the latest publication on our searches. We used essentially the same apparatus as in the previous paper, but took advantage of a hundredfold increase in the neutron intensity from the ILL liquid deuterium moderator. We showed that the neutron electric dipole moment must be less than 12 × 10^-26 e cm to a 95% confidence level and by combining with the latest St. Petersburg result we conclude that the neutron electric dipole moment is less than 9 × 10^-26 e cm to a 95% confidence level.

Although none of these searches have found a nonzero electric dipole moment, in lowering the upper limit by a factor of more than 10⁷, we have eliminated the theories that predicted larger electric dipole moments. Currently (1997) a new and more sensitive search is being started with a ¹⁹⁹Hg magnetic field monitor in the same storage volume as the neutrons. This experiment may be able to test different predictions by the standard model, supersymmetric theories and superstring theories.