Lead


Lay summary
We are working in an international collaboration on an experiment aiming at an improved sensitivity to the neutron electric dipole moment (nEDM). The EDM for an elementary particle is forbidden by both Parity and Time reversal symmetries. The starting point is the reasonable presumption that an improvement of the experimental sensitivity to below a level of 10e-27 e cm can be achieved using in-vacuum techniques: note that the in-vacuum technique was used by the Sussex--RAL--ILL collaboration to set the present limit of dn <63x10e-27 e cm, and that the measurement is claimed to be at its statistical limit. A long-term ultimate sensitivity in the 10e-28 e cm region is interesting since several extensions to the Standard Model (Left--Right symmetric models, and Supersymmetry) predict a value for the nEDM in that range.Central to this work is the measurement and control of the magnetic field used to precess the magnetic moments of an ensemble of neutrons.The Ramsey method of separated oscillatory fields is used to measure the perturbation of the spin precession in a magnetic field caused by an additional electric field. The measurement then compares aligned fields with anti-aligned fields to isolate the electric field effect. The stability of the magnetic field between the two measurements is paramount to the success and sensitivity of the method.The Fribourg atomic physics group (FRAP) will undertake the task of maintaining the stability of the magnetic field to the precision levels necessary for the experiment. Our approach will be to place an array of laser--pumped Cs magnetometers into the vacuum apparatus and use the resulting measurements to both actively stabilize the magnetic field, and to correct the Ramsey oscillating--field phase for rapid magnetic field fluctuations. The magnetometry data collected during the operation of the existing nEDM apparatus will be used to optimize the design of a future apparatus for the conditions at PSI. The neutron data collected at the same time should already serve to increase the sensitivity to any nEDM, with the immediate short term goal of dn < few x 10e-26 e cm being comparable to the existing world limit.