Paper 3.2: "Experimental Limit to the Electric Dipole Moment of the Neutron," J. H. Smith, E. M. Purcell and N. F. Ramsey, Phys. Rev. 108, 120–122 (1957)
Reprinted with permission from The Physical Review. Copyright 1957, The American Physical Society.
This paper describes our first search for a neutron electric dipole moment as a test of parity. With our graduate student, James Smith, we designed a neutron beam experiment at an Oak Ridge reactor using thermal neutrons, polarized by total reflection from a magnetized mirror and analyzed by transmission through saturated iron. Transitions were induced by the separated oscillatory fields method and the electric dipole moment obtained by comparing the resonance frequencies with the electric and magnetic fields being parallel or antiparallel. In this experiment we showed that the electric dipole moment of the neutron had to be less than 5 × 10-20 e cm, much less than could be inferred from any previous experiment.
The experiment was completed in 1951, but publication in The Physical Review was delayed until 1957. However, the result was well known before the discovery of parity nonconservation in the weak interaction from Smith's 1951 thesis, our colloquia and my book Molecular Beams (Oxford University Press, 1956).
Initially the negative result in this experiment was explained away by theorists as a result of parity conservation. Later, after the failure of parity conservation in the weak interaction, it was attributed to time reversal symmetry, but, when CP conservation failed in the decay of the long-lived neutral kaon, an electric dipole moment of one size or another was expected by most particle theories consistent with the kaon results. We have made renewed electric dipole searches when major improvements in sensitivity appeared possible.
V. W. Cohen, R. Nathans, E. Lipworth, H. B. Silsbee and I [Phys. Rev. 177, 1942–1947 (1969)], in a neutron beam experiment at Brookhaven, lowered the neutron electric dipole moment limit to 1 × 10-21 e cm.
W. B. Dress, J. K. Baird and I [Phys. Rev. 170, 1200–1205 (1968) and 179, 1285–1291 (1969)] used a new apparatus at Oak Ridge with the neutrons confined to rectangular pipes to increase the effective neutron intensity. In this way we reduced the upper limit to 5 × 10-23 e cm. W. B. Dress, P. D. Miller and I [Phys. Rev. D&, 3147–3149 (1973)] later modified the apparatus so it could be rotated to allow the beam to pass successively through it in two opposite directions and thereby reduce the uncertainty from the effective magnetic field v × E/c created by the motion of the neutrons through the electric field. This experiment reduced the limit to 1.0 × 10-23 e cm. Continuations of this experiment at the ILL in Grenoble, France, are described in the commentary on Paper 3.3.
