First test ofCPinvariance in the decay of positronium

The 3γ decay of spin-aligned triplet positronium (Ps) is used for a test of CP invariance (charge conjugation and parity). This is the first use of the Ps atom to investigate the CP symmetry. The angular correlation (S^⋅k ${\mathrm{^}}_1$)(S^⋅k ${\mathrm{^}}_1$×k ${\mathrm{^}}_2$) was measured, where S is the Ps spin and ‖${\mathbf{k}}_1$‖>‖${\mathbf{k}}_2$‖>‖${\mathbf{k}}_3$‖ are the γ momenta. If this correlation has a nonzero amplitude, CP is not conserved. No amplitude was found at the 1.5% level of uncertainty.