Neutrino Scattering in a Newly Born Neutron Star

We calculate neutrino cross sections from neutral current reactions in the dense matter encountered in the evolution of a newly born neutron star. Effects of composition and of strong interactions in the deleptonization and cooling phases of the evolution are studied. The influence of the possible presence of strangeness-rich hyperons on the neutrino scattering cross sections is explored. Due to the large vector couplings of the Σ and Ξ, |CV | ∼ 2, these particles, if present in protoneutron star matter, give significant contributions to neutrino scattering. In the deleptonization phase, the presence of strangeness leads to large neutrino energies, which results in large enhancements in the cross sections compared to those in matter with nucleons only. In the cooling phase, in which matter is nearly neutrino free, the response of the Σ hyperons to thermal neutrinos is the most significant. Neutrinos couple relatively weakly to the Λ hyperons and, hence, their contributions are significant only at high density. Subject headings: dense matter – stars: neutron – stars: opacities – stars: neutrinos