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Photoproduction of Resonances from Compact and Spatially Extended Sources

We discuss the photoproduction of resonances observed in the π+π− system. For the γp→ π+π−p reaction, we consider two production mechanisms: the one-pion exchange known in the literature as the Deck mechanism and collective contribution of all other short-range exchanges which we parametrize in terms of a smooth function of sππ. We have found a very good agreement of the model with experimental mass distributions for partial waves S, P and D. Our calculations are in line with the expectation that while the P and D-wave resonances ρ(770) and f2(1270) are conventional qq̄ states, the S-wave f0(980) resonance is rather a more loosely bound qqq̄q̄ state.