Branching-ratio measurements of multiphoton decays of positronium

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 R₄^C/<3.7×10^-6 (90% C.L.) and a new limit for (p-Ps→5γ) of R₅^C/<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.