Photopion reactions on deltas preexisting in nuclei

Reactions A(γ, πp) are considered to proceed through the formation of πp pairs on ∆ constituents in nuclei. We develop the nonrelativistic operator of γ∆ → πp process in an arbitrary frame. The calculated cross section of C(γ, πp) reaction is compared to the existing experimental data. The role of isobars as nuclear constituents was a topic of a large body of research over a long time (a comprehensive review can be found in Refs.[1, 2]). There is a convincing evidence that a small admixture of enternally excited nucleons always presents in nuclei. Since these virtual excitations, proceeding during the collisions NN → NN(NN), are more intensive at the large relative momenta of nucleons, the isobar configurations are assumed to be responsible for high momentum components of nuclear wave functions. Therefore, they can manifest themselves in nuclear reactions with large momentum transfers. However, more direct way to demonstrate the existence of isobar constituents in nuclei as well as to gain a better insight into their dynamical properties is to study the so-called isobar knock-out proceses [3]. In this regard, the A(γ, πp) reactions, which can be interpreted merely as absorption of incoming photon on ∆ preformed in a target nucleus followed by its real decay into πp pair, are the most attractive ones. The main advantage of this channel is that it is free of any influence of direct photoproduction mechanism, in contrast to the A(γ, πn) reaction, where the contribution of a preexisting ∆ will be overwhelmed by the ∆ creation on a quasifree proton in γp→ ∆ → πn process. To date, only one exclusive measurement of A(γ, πp) reaction has been reported in Ref.[4], where the results of the first comparative studies of C(γ, πn) and C(γ, πp) channels are presented. While the C(γ, πn) cross section is dominated by the quasifree mechanism, the higher order FSI (Final State Interaction) processes involving also charge exchange are assumed to be the source of the C(γ, πp) reaction yield. The latter assumption is, however, not undoubtedly confirmed by a calculation performed within the Valencia model (VM) [5], which markedly underestimates the data.