Neutrino reactions and nuclear shadow

Abstract

We discuss the high-energy spin-isospin collective modes, in the region of the delta resonance and their signature in neutrino reactions. The collective modes arise from the coupled propagation of pions and deltas. In a schematic model there are two such modes, one at low energy, the pion branch, which lies below the free pion and delta branches, the other, the collective delta, branch lies entirely in the time-like region, above the free branches. At large momenta the collective delta branch acquires a strong pionic component. We summarize the status of the detection of the pion branch by hadronic probes. We then turn to the possible detection of the collective branches in neutrino reactions. Here the momentum transfer is space-like, so that the collective delta branch is not accessible in these reactions. It should, however, manifest its existence by a loss of strength, as compared to the free situation, which should produce a shadow effect in the delta region. We make an estimate of the shadow in the schematic model. This shadow arises from the real part of the optical potential and the collective nature of the nuclear response in the delta region, in contradistinction with the usual one which arises from the absorptive part of the potential.