The role of Delta(1700) excitation and rho production in double pion photoproduction

Recent information on invariant mass distributions of the gammap --> pi (+)pi (0)n reaction, where previous theoretical models had shown deficiencies, have made more evident the need for new mechanisms, so far neglected or inaccurately included. We have updated a previous model to include new necessary mechanisms. We find that the production of the rho meson, and the Delta (1700) excitation, through interference with the dominant terms, are important mechanisms that solve the puzzle of the gammap --> pi (+)pi (0)n reaction without spoiling the early agreement with the gammap --> pi (+)pi (-)p and gammap --> pi (0)pi (0)p reactions