Cross-section measurement of two-photon in-flight annihilation of positrons at p s=20 MeV with the PADME detector

A measurement of the inclusive cross section of in-flight electron-positron annihilation to photons, e+e- -> gamma gamma, is presented using the PADME detector at the Laboratori Nazionali di Frascati. A beam of 430 MeV positrons, corresponding to a center-of-mass energy of 20 MeV, strikes a thin diamond target. The two photons produced in the interaction are detected by an electromagnetic calorimeter made of BGO crystals. The measurement is the first based on direct detection of the photon pair and one of the most precise for positron energies below 1 GeV. It represents an intermediate step in the ultimate PADME goal of searching for dark sector particles and mediators weakly coupled to photons and electrons, with masses ranging from 1 to 20 MeV. The final value, sigma e+e-->gamma gamma = (1.977 +/- 0.018(stat) +/- 0.119(syst)) mb, agrees with next-to-leading-order QED predictions within the 6% experimental uncertainty.