We report the results of a new experimental search for a permanent electric dipole moment of 199 Hg utilizing a stack of four vapor cells. We find d( 199 Hg) = (0.49±1.29stat ±0.76syst)×10 −29 e cm, and interpret this as a new upper bound, |d( 199 Hg)| < 3.1×10 −29 e cm (95% C.L.). This result improves our previous 199 Hg limit by a factor of 7, and can be used to set new constraints on CP violation in physics beyond the standard model. PACS numbers: 11.30.Er,32.10.Dk,32.80.Xx,24.80.+y The existence of a finite permanent electric dipole moment (EDM) of a particle or atom would violate time reversal symmetry (T ), and would also imply violation of the combined charge conjugation and parity symmetry (CP ) through the CP T theorem [1,2,3]. EDMs are suppressed in the standard model of particle physics (SM), lying many orders of magnitude below current experimental sensitivity. However, it is thought that additional sources of CP violation are needed to account for baryogenesis [4,5], and many theories beyond the SM, such as supersymmetry [6,7], naturally predict EDMs within experimental reach.Experimental searches for EDMs have so far yielded null results. The most precise and significant limits have been set on the EDM of the neutron [8], the electron [9], and the 199 Hg atom [10], leading to tight constraints on supersymmetric extensions of the SM [7]. Here we report the first result of a new mercury experiment, |d( 199 Hg)| < 3.1×10−29 e cm (95% C.L.), which improves our previous limit [10] by a factor of 7 and provides a yet more exacting probe of possible new sources of CP violation.199 Hg has a 1 S 0 electronic ground state and nuclear spin 1/2. An EDM of the ground state atom would point along the nuclear spin axis and arise mainly from CP violation in the nucleus. We measure the nuclear Larmor frequency ν given by hν = |2µB ± 2dE|, where µ and d are the 199 Hg magnetic and electric dipole moments, and B and E are the magnitudes of external magnetic and electric fields aligned parallel (+) or antiparallel (−) with each other. The signature for d = 0 is thus a shift in Larmor frequency when E is reversed relative to B.As shown in Fig. 1, our new apparatus uses a stack of four spin-polarized Hg vapor cells in a common B-field. The middle two cells have oppositely directed E-fields, resulting in EDM-sensitive Larmor shifts of opposite sign; the outer two cells, enclosed by the high voltage (HV) electrodes and thus placed at E = 0, are free of EDM effects and serve to cancel B-field gradient noise and provide checks for spurious HV-correlated B-field shifts.The vapor cells are constructed from high purity fused silica and contain isotopically enriched 199 Hg (92 %) at a density of 4 × 10 13 cm −3 , a paraffin wall coating, and 475 Torr of CO buffer gas. CO efficiently quenches excited state 199 Hg and thus reduces degradation of the wall coating [11]. Spin coherence times T 2 are 100 to 200 sec. A conductive SnO coating on the cell end-caps provides electric field plates separated by 11 mm. The average leakage...
