We discuss new strict upper limits on the 7-odd, />-even electron-electron, electron-nucleon, and nucleon-nucleon interactions which follow from the limits on the electron and neutron electric dipole moments. We also consider searches for T-odd, P-even interactions with atomic radiative transitions in positronium (electron-electron) and other atoms (electron-nucleon).

The vacuum decay rate A. T of orthopositronium (1'S&) formed in a gas has been measured to be A, T=7.0514+0.0014 ps '. Measurements of kT in four di6'erent gases, all in agreement, are averaged to obtain this result. As systematic tests, two entirely separate digital timing systems are simultaneously used throughout the experiment; the cross section for collisional quenching of the long-lived 2 S& excited state is determined; A, T is redetermined in two of the gases (N2 and Ne) using only high-gas-density measurements; and the collisional quenching rate of water vapor, the major residual gas contaminant, is directly measured. The final value of A, T from this gas experiment, which represents a factor of 4 improvement in accuracy over previous measurements, is 9.4 experimental standard deviations above the theoretical value.

Parity nonconservation is observed in the 6 2 P t /2~7 2 Pt/2 transition in thallium. Absorption of circularly polarized 293-nm photons by 6 jp t^ atoms in an E field results in polarization of the 7 2 P\/2 state through interference of Stark El amplitudes with Ml and paritynonconserving El amplitudes 3fR and S p . Detection of this polarization yields the circular dichroism 6^ + (5.2±2.4)xl0" 3 , which agrees in sign and magnitude with theoretical estimates based on the Weinberg-Salam model.We report preliminary observations of parity nonconservation (PNC) in the &P l / 2 -r l 2 P 1 / 2 transition (292.7 nm) in atomic thallium (see Fig. 1). The transition is forbidden Ml with measured amplitude m = (-2.1 ±0.3)xlO" 5 \eti/2m e c\. 1 If parity is not conserved, the 6 2 Py 2 an( * ^2P\ji states are admixed with 2 Sj 2 states. The transition amplitude then contains an additional El component S p , and circular dichroism exists, defined bywhere a± are the cross sections for absorption of 293-nm photons with ± helicity, respectively,, Theoretical estimates of 6 based on the Weinberg-Salem (W-S) model 2 yield 3 ' 4 ^theor =2lm(5 /)f theor)/^expt = (+2.3±0.9)xl0" 3 (2)for sin 2 0w = O.25, where 0^ is the Weinberg angle. The uncertainty in 6 theor arises from the uncertainties in Sftexpt (~ 15%) and S Pttheot (~ 25%). The aim of this experiment is to measure 6. Investigations of this type were first suggested by Bouchiat and Bouchiat, 5 and their experiment on Cs is in progress. 6 Also, optical-rotation experiments on bismuth have been reported (but with contradictory results), 7 " 9 while PNC in highenergy electron scattering, consistent with the W-S model, has been observed. 10 The simplest way to measure 6* would be to illuminate Tl vapor in a field-free region with circularly polarized 293-nm light and observe the helicity dependence of the decay fluorescence (e.g., at 535 nm; see Fig. 1). Unfortunately this is impractical because of background effects. Instead, using a technique first suggested by Bouchiat and Bouchiat, 5 we apply an external field E which Stark mixes 2 P x / 2 states with 2 S x / 2 and 2 D 3 / 2 stateSo The transition intensity, proportional to E 2 , is thereby increased above the background; moreover interference between the Stark transition amplitudes and the 3ft and S p amplitudes polarizes the 7 2 P 1 / 2 state, permitting measurement of 9H and 6 0 Let the 293-nm photon beam be alongx, and choose E =Ey [see Fig. 1(b)]. Ignoring terms of order [STC* 8 P ] 2 , we find the r l 2 P l / 2 polarization 8*S FIG. 1. (a) Low-lying energy levels of Tl (not to scale), (b) Coordinate system, orientation of photon beams, and electric field direction, (c) Schematic diagram indicating production and analysis of 7 2 P 1//S polarization in the 0-1 transition. The rates at which the F=l 9 m F = -l, 0, and +1 levels are populated in the 7 2 P i/2 states are proportional to i(p+WL*S p ) 2 , JOKT^) 2 , and J(0 -9R±^) 2 , respectively. The polarization is analyzed by circularly polarized 2.18-/im radiation (7 2 P 1 ...

The 2 3 5'i-* 2 3 / > i and 2 Z S\-+ 2 3 Po transition frequencies (vi and vo, respectively) in positronium have been measured for the first time and an improved measurement of the 2 3 S'i-• 2 3 P2 transition frequency (v 2 ) has been made. The results v 0 = 18 504.1 ± 10.0 ± 1.7 MHz, vi = 13 001.3 ± 3.9 ± 0.9 MHz, and V2 =8619.6 ± 2.7 ± 0.9 MHz are in reasonable agreement with the predictions of quantum electrodynamics which are, to order a 3 ft (ft is the frequency equivalent of the rydberg); vo(a 3 ) = 18496.1 MHz, v\(a 3 ) = 13 010.9 MHz, and V2(a 3 ) =8625.2 MHz. More precise measurements are planned to test the, as yet uncalculated, order a 4 ft corrections.