¹²⁹Xe comagnetometer for neutron EDM experiment

The neutron electric dipole moment (nEDM) has been providing strict constraints on theories underlying the CP violation that predict larger nEDM than the standard model. In order to exceed the present upper limit on the nEDM of 3 × 10⁻²⁶ ecm measured at Grenoble, several groups are now developing new methods of nEDM measurements to reduce systematic errors as well as new ultracold neutron (UCN) sources to achieve higher statistics. The systematic error at the Grenoble experiment was dominated by the the geometric phase effect (GPE) for the ¹⁹⁹Hg comagnetometer. We have proposed a ¹²⁹Xe comagnetometer which can possibly reduce the GPE to 4 × 10⁻²⁸ ecm because of a buffer gas effect. Spin polarized ¹²⁹Xe nuclei are used to monitor the static magnetic field in an nEDM cell where both spin polarized UCN and ¹²⁹Xe are confined. Highly spin polarized ¹²⁹Xe produced by means of the spin exchange optical pumping (SEOP) technique is accumulated in a cold trap, and then is introduced into the nEDM cell. The spin precession of ¹²⁹Xe nuclei will be observed using two-photon excitation of Xe atoms. We have achieved 70% of ¹²⁹Xe polarization in the SEOP cell and are now developing the cold trap. The progress of our development will be presented.