Multiphoton decays of positronium

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Abstract
We compute the branching ratios for parapositronium and orthopositronium into four and five photons, respectively. We find Γ(p−Ps→4γ)Γ(p−Ps→2γ)=0.274(απ)2≃1.48×10−6 and Γ(o−Ps→5γ)Γ(o−Ps→3γ)=0.177(απ)2≃0.96×10−6. We also describe the energy spectra of photons from these decays.