0 20 60 55 v 1 2 4 Ju n 20 02 The σ meson in a nuclear medium through two pion photoproduction

The σ meson in a nuclear medium through two pion photoproduction We present theoretical results for (γ, π 0 π 0) and (γ, π ± π 0) production on nucleons and nuclei in the kinematical region where the scalar isoscalar ππ amplitude is influenced by the σ pole. The final state interaction of the pions modified by the nuclear medium produces a spectacular shift of strength of the π 0 π 0 invariant mass distribution induced by the moving of the σ pole to lower masses and widths as the nuclear density increases. In the last years there has been an intense theoretical and experimental debate about the nature of the σ meson, especially the modification of its properties in nuclear matter. Both the interpretation as a qq state or as a ππ resonance, seem to agree in a decrease of the σ mass as the nuclear density increases. For instance, in the theoretical work of [3], the ππ amplitude in the σ channel showed a shift of strength towards low ππ invariant masses at finite nuclear density. In the present work, our purpose is to find the possible experimental signature of this moving of the ππ pole in the (γ, π 0 π 0) reaction in nuclei. This reaction is much better suited than the (π, ππ) one to investigate the modification of the ππ in nuclear matter because the photons are not distorted by the nucleus and one can test higher densities. For the model of the elementary (γ, ππ) reaction we follow [2] which considers the coupling of the photons to mesons, nucleons, and the resonances ∆(1232), N * (1440), N * (1520) and ∆(1700). This model relies upon tree level diagrams. Final state interaction of the πN system is accounted for by means of the explicit use of resonances with their widths. However, since we do not include explicitly the σ resonance, the final state interaction of the two pions has to be implemented to generate it. The γN → N π 0 π 0 amplitude can be decomposed in a part which has in the final state the combination of pions in isospin I=0 and another part where the pions are in I=2.