Toward a measurement of the neutron electric dipole moment – n2EDM

Searches for a permanent electric dipole moment (EDM) of fundamental particles or systems are actively pursued worldwide [Kir12]. The discovery of a finite value would be of particular interest since it may reveal new sources of CP violation and therefore physics beyond the standard model (SM). The existence of new CP violation mechanisms is indeed required in cosmology in order to account for the baryogenesis in the early Universe. It is one of the 3 necessary conditions derived by Sakharov [Sak67] to produce some baryon asymmetry. The SM cannot account for the matter-antimatter asymmetry but several extensions offer viable baryogenesis scenarios as, for instance, supersymmetric (SUSY) models. In this context, EDM measurements already bring stringent constraints and the next generation of experiment will either discover a signal or rule out electroweak baryogenesis as the origin of matter-antimatter asymmetry. These searches are complementary to the high energy experiments performed at LHC since they allow to explore the TeV energy scale.

The n2EDM project aims at measuring the neutron EDM with a statistical sensitivity improved by at least a factor 10 with respect to the present best limit, dn < 3 x 10-26 e.cm [Bak06]. Such a goal will be reached with a new highly sensitive spectrometer. The experiment will be running at the Paul Scherrer lnstitute (PSI) next to the newly built high intensity ultra-cold neutron (UCN) source whose commissioning started end of 2010. A statistical sensitivity of 2 x 10-27 e.cm is foreseen after 4 years of data taking (with the current source performances). If the UCN source performances reach their target value, the 10-28 e.cm range will start to be explored

The project is the follow-up of an ongoing experiment at PSI which is based on an upgraded version of the RAL-Sussex-ILL apparatus, which holds the best limit on the neutron EDM [Bak06]. The French contribution to this upgrade was the object of a previous ANR grant (nEDM, ANR-09-BLAN-0046), jointly submitted by LPC Caen and LPSC Grenoble. This experiment is supported by an international collaboration of 12 laboratories. The three French laboratories involved in this proposal (LPC, LPSC and CSNSM) are all working on the current experiment.

The new spectrometer will include two major improvements: a double storage chamber set-up that will allow the simultaneous measurement of the 2 configurations (parallel and antiparallel) of the electric and magnetic fields, and a large dimension magnetic shielding for a better control of the magnetic field environment. The statistical sensitivity gain will be achieved thanks to a larger number of stored UCN (better adaptation between the source and the spectrometer and use of the double storage chamber), a larger electric field intensity and a larger UCN polarization. The survey of systematic effects will be improved by the use of the new shielding and of three atomic magnetometers presenting novel features (vector Cs magnetometers, laser driven Hg magnetometer and He3 magnetometer).

The ANR grant will be used to finance the French contribution to the design and construction of the new n2EDM spectrometer. To take full advantage of the experience acquired so far, the French groups will mostly keep working on the same tasks: UCN detection, guiding field system, spin analysis, coil design, current source and Hg magnetometry. Data taking with the new apparatus will start around 2018.

Finally, it should be mentioned that the n2EDM project has been recently reviewed by the IN2P3 scientific council in October 2013. The scientific council recommendation to fully support this project was endorsed by the IN2P3 direction.