Wigner-Eckart Theorem and the False EDM of ¹⁹⁹Hg

In neutron electric dipole moment experiments, 199Hg is used as a comagnetometer. The comagnetometer suffers from a false EDM arising in leading order from a gradient dBz/dz in the magnetic field. Our work concerns higher-order multipole corrections to the false EDM of 199Hg. We show that for spherical traps, all higher-order multipole are identically zero. We further show that for the usual cylindrical traps used in EDM experiments, selection of quasi-spherical dimensions for the trap can reduce the higher-order contributions. The results are another indication that trap geometry is an important consideration for experiments desiring to control this systematic effect.