Positronium (Ps) decay to four- and five-photon final states has been measured. From a sample of 3.73×109 detected events of Ps annihilation, the QED-allowed branching ratios of para-Ps to four photons R4=1.14(33)(21)×10-6 (stat., sys., 68% C.L.), and ortho-Ps to five photons R5=1.67(99)(37)×10-6 (68% C.L.) were measured. No charge-conjugation symmetry violating events were observed, establishing a branching ratio limit for (o-Ps-->4γ) of RC/4<3.7×10-6 (90% C.L.) and a new limit for (p-Ps-->5γ) of RC/5<2.7×10-7 (90% C.L.). The experiment was performed using the Gammasphere array of Compton-suppressed high-purity germanium detectors. Gammasphere's segmentation, high efficiency, and excellent energy resolution allowed event discrimination sufficient to observe the rare decay modes in the presence of substantial backgrounds.
Physical Review A
- Pub Date:
- November 2002
- Positronium muonium muonic atoms and molecules;
- Charge conjugation parity time reversal and other discrete symmetries;
- Positron annihilation;
- Nuclear tests of fundamental interactions and symmetries