Double Pion Production Reactions

We report on reactions producing two pions induced by real and virtual photons or nucleons. The role of different resonances in these reactions is emphasized. Novel results on coherent 2π photoproduction in nuclei are also reported. 1 Double pion photoproduction on the nucleon. The γN → π+π−N reaction is attracting attentions of both theoretical and experimental groups and is bound to play a significant role in photonuclear reactions much as the γN → πN played in the past. Apart from the work of the Valencia group which I will report here, there is work by other groups. A simplified model containing many of the important features of the reaction was worked out in [1] and improved in [2], [3]. The model of [3] contains more mechanisms than the one of [2], presently under revision [4]. On the other hand, the model of [2] incorporates an approximate unitarization prescription which allows one to go to higher energies with the model. In ref. [5] a simplified model is also used incorporating, however, some ρ decay channels. This work has been revised in [6] in view that some mass distribution was in disagreement with the data, and a new parametrization is offered, which relies on a range parameter for the ρ of the order of 200MeV , which would not accommodate easily other known facts of phenomenology as the isovector πN s-wave amplitude. The model of [3] contains parameters determined solely from γ and π couplings to nucleons and resonances plus known properties of resonance decay