Neutrino absorption cross sections in supernova environment

We study charged-current neutrino cross sections on neutronrich nuclei in the mass A ∼ 60 region. Special attention is paid to environmental effects, i.e. finite temperature and density, on the cross sections. As these effects are largest for small neutrino energies, it is sufficient to study only the Gamow-Teller (GT) contributions to the cross sections. The relevant GT strength distributions are derived from large-scale shell model calculations. We find that the low-energy cross sections are enhanced at finite temperatures. However , for (ν e , e −) reactions Pauli blocking of the electrons in the final state makes the cross sections for low-energy neutrinos much smaller than for the competing inelastic scattering on electrons at moderate and large densities. Absorption cross sections for low-energy antineutrinos are strongly enhanced at finite temperatures. 1 It has long been recognized that neutrino-induced reactions play a crucial role during the collapse stage of a type II supernova [1]. In particular, elastic neutrino scattering off nuclei reduces the neutrino mean-free path and leads eventually to 'neutrino trapping' at densities in excess of a few 10 11 g/cm 3. Subsequently the neutrinos in the core are thermalized by inelastic neutrino-electron scattering. Obviously collapse simulations require a detailed description of neutrino transport which is nowadays achieved within Boltzmann neutrino transport calculations and should in principle include all potentially important neutrino reactions [2,3]. Haxton pointed out that charged-and neutral-current reactions on nuclei should be included in these simulations [4]. Interpreting 56 Fe as a representative nucleus the relevant neutrino-nucleus cross sections have been calculated in [5] based on the allowed and first-forbidden ground-state response and compared to other weak-interaction processes under core-contraction conditions. Neutrino-induced reactions are usually dominated by capture to the giant resonances. As a consequence allowed (Fermi and Gamow-Teller) transitions determine the neutrino cross sections at low energies, while first-forbidden transitions cannot be neglected for neutrino energies in excess of say 20 MeV. We also note that for a nucleus with negative Q-value, like 56 Fe, a minimal neutrino energy is required. Consequently the cross section drops sharply at low energies. However, for neutronrich nuclei, as typically encountered during the late core contraction stage, the Q-value is positive allowing (ν e , e −) reactions for all neutrino energies. On the contrast, (¯ ν e , e +) reactions have to overcome the reaction Q-value. The picture described here corresponds to neutrino reactions on the nuclear …