Performance of an active nuclear spin maser with double-cell geometry

A permanent electric dipole moment (EDM) violates $T$-invariance and, through the \textit{CPT} theorem, its magnitude~sets limits on \textit{CP}-violation phases beyond the Standard Model. We aim to search for an EDM in $^{129}$Xe beyond the present upper limit at the level of 10$^{-28} e$cm. We use an active nuclear spin maser in order to achieve a precession frequency precision of 1 nHz in an applied electric field of 10 kV/cm. A co-magnetometer using $^{3}$He and a double-cell geometry have been incorporated into our setup in order to reduce systematic uncertainties. The Rb polarization causes shifts in the frequencies for $^{129}$Xe and $^{3}$He precession. The frequency shift for $^{129}$Xe cannot be canceled out by this type of co-magnetometer because the $^{129}$Xe-Rb coupling differs significantly from that of $^{3}$He-Rb one. We found that in the double-cell geometry, the dominant contribution to the frequency shift from the Rb polarization occurs in the pumping cell We attempt to clarify the mechanism and to reduce the frequency shift.