Measurement of the branching ratio for the π0-meson decay into a photon and a positronium atom

The ratio of the number of ultrarelativistic positronium atoms (A2e) to the photon number in the momentum interval 800–2000 MeV/c has been measured in pC interactions at Ep=70 GeV and at an angle of 8.4° in the laboratory frame. The branching ratio ϱπ=(1.84±0.29)×10-9 for π0 decay into a photon and A2e has been obtained. The parameter ϱπ is determined by the conversion probability ϱA of a time-like photon into A2e in radiative decays of elementary particles.