Recent Results from 2π0 Photoproduction off the Proton

The reaction γ p → π0π0 p has been measured using the TAPS BaF2 calorimeter at the tagged photon facility of the Mainz Microtron accelerator in the beam energy range from threshold up to 820 MeV. Close to threshold, chiral perturbation theory (ChPT) predicts that this channel is significantly enhanced compared to double pion final states with charged pions. The strength is attributed dominantly to pion loops in the 2π0 channel a finding that opens new prospects for the test of ChPT. Our measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data are also in agreement with a calculation in the unitary chiral approach. In the second resonance region, a recent model interpretation of new GRAAL data claimed a dominance of the P11(1440) → σN reaction process. We present very accurate invariant mass distributions of π0π0 and π0 p systems, which are in contrast to the σN intermediate state and which show a dominance of the ∆π intermediate state.