Improved Limit on the Permanent Electric Dipole Moment ofHg199

We report the results of a new experimental search for a permanent electric dipole moment of ${}^{199}\mathrm{Hg}$ utilizing a stack of four vapor cells. We find $d({}^{199}\mathrm{Hg})=(0.49\pm {1.29}_{\mathrm{stat}}\pm {0.76}_{\mathrm{syst}})\times {10}^{-29}e\mathrm{cm}$, and interpret this as a new upper bound, $|d({}^{199}\mathrm{Hg})|<3.1\times {10}^{-29}e\mathrm{cm}$ (95% C.L.). This result improves our previous ${}^{199}\mathrm{Hg}$ limit by a factor of 7, and can be used to set new constraints on $CP$ violation in physics beyond the standard model.