Angular-correlation test of CPT in polarized positronium

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 correlation S^⋅k^₁×k^₂ was measured (where S is the positronium spin angular momentum and ‖k₁‖>‖k₂‖>‖k₃‖ are the momenta of the three decay γ rays). It is expected to have zero amplitude if CPT is conserved. No effect was found in the experiment at the 2.3% level of uncertainty in the coefficient of the angular correlation. Information gained from a study of the systematic effects, 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.