Polarized triplet positronium, formed in the ground state with a polarized slow-e+ beam, is used for an experimental test of CPT (charge-conjugation-parity-time-reversal} invariance. The angular A A A correlation 8 ki Xk2 was measured (where 8 is the positronium spin angular momentum and I k, I & I k2 I & I k3 I are the moments of the three decay y rays}. It is expected to have zero amplitude if CPTis conserved. No effect was found in the experiment at the 2.3% level of uncertainty in the coeScient of the angular correlation. Information gained from a study of the systematic e8'ects, along with straightforward improvements in the apparatus, would allow a 0.1% measurement. Other possible angular correlations using polarized and aligned positronium, including tests of CP invariance, are discussed.

We report a new cooling scheme used to actively collimate an atomic beam of 85 Rb with onedimensional optical molasses. The atoms' transverse velocity is damped by a standing wave of circularly polarized light in a magnetic field of -20 /iT to 2 cm/s (rms), well below the Doppler velocity v D -(lhy/20M) U2~-\0 cm/s for one-dimensional optical molasses. The minimum measured velocity is limited only by the experimental geometry. We have developed a model in which the mixing by the field and optical pumping combine to form a cycle that cools below the Doppler limit, in agreement with our measurements.

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.

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