First test of<i>CP</i>invariance in the decay of positronium

Abstract

The 3\ensuremath{\gamma} 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^\ensuremath{\cdot}k${\mathrm{^}}_{1}$)(S^\ensuremath{\cdot}k${\mathrm{^}}_{1}$\ifmmode\times\else\texttimes\fi{}k${\mathrm{^}}_{2}$) was measured, where S is the Ps spin and \ensuremath{\Vert}${\mathbf{k}}_{1}$\ensuremath{\Vert}&gt;\ensuremath{\Vert}${\mathbf{k}}_{2}$\ensuremath{\Vert}&gt;\ensuremath{\Vert}${\mathbf{k}}_{3}$\ensuremath{\Vert} are the \ensuremath{\gamma} momenta. If this correlation has a nonzero amplitude, CP is not conserved. No amplitude was found at the 1.5% level of uncertainty.