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 in accurately included. We have updated a previous model to include new necess ary mechanisms. We find that the production of the rho meson, and the Delta (1700) excitation, through interference with the dominant terms, are impor tant mechanisms that solve the puzzle of the gammap --> pi (+)pi (0)n react ion without spoiling the early agreement with the gammap --> pi (+)pi (-)p and gammap --> pi (0)pi (0)p reactions. (C) 2001 Elsevier Science B.V. All rights reserved.