Photoproduction of neutral pion pairs from the proton

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 $\mu$b at $E_\gamma = 740$ MeV. Dalitz plots of $m^2(\pi^0,\pi^0)$ versus $m^2(p,\pi^0)$ for seven bins of incident photon energy have been analysed. For $E_\gamma >610$ MeV, a strong contribution of a sequential decay is observed with the $\Delta(1232)$-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the $D_{13}(1520)$- and also the $P_{11}(1440)$-resonance