Medium modification of the charged-current neutrino opacity and its implications

Abstract

Previous work on neutrino emission from proto-neutron stars which employed full solutions of the Boltzmann equation showed that the average energies of emitted electron neutrinos and antineutrinos are closer to one another than predicted by older, more approximate work. This in turn implied that the neutrino driven wind is proton rich during its entire life, precluding r-process nucleosynthesis and the synthesis of Sr, Y, and Zr. This work relied on charged current neutrino interaction rates that are appropriate for a free nucleon gas. Here, it is shown in detail that the inclusion of the nucleon potential energies and collisional broadening of the response significantly alters this conclusion. Iso-vector interactions, which give rise to the nuclear symmetry energy, produce a difference between the neutron and proton single-particle energies ∆U = Un − Up and alter the kinematics of the charged current reactions. In neutron-rich matter, and for a given neutrino/antineutrino energy, the rate for νe + n → e − + p is enhanced whileνe + p → n + e + is suppressed because the Q value for these reactions is altered by ±∆U , respectively. In the neutrino decoupling region, collisional broadening acts to enhance both νe andνe cross-sections and RPA corrections decrease the νe cross-section and increase theνe cross-section, but mean field shifts have a larger effect. Therefore, electron neutrinos decouple at lower temperature than when the nucleons are assumed to be free and have lower average energies. The change is large enough to allow for a reasonable period of time when the neutrino driven wind is predicted to be neutron rich. It is also shown that the electron fraction in the wind is influenced by the nuclear symmetry energy.

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…Neutrino-driven winds in regular CCSNe, but this site is contradicted by observational constraints and present simulations result at most in a weak r-process [40,59]. 2.…”

“…If the neutrino flux is sufficient to have an effect (scaling with 1/r 2 ), and the total luminosities are comparable for neutrinos and anti-neutrinos, only conditions with E av,ν − E av,ν > 4(m n −m p )c 2 lead to Y e < 0.5. Present calculations [1,40,59] therefore exclude strong r-process conditions in CCSNe and the interaction with neutrinos leads even to proton-rich conditions. The latter favors improvements in the Fe-group composition of elements like Sc, Ti, Co, as well as for 64 Ge (decaying to 64 Zn!…”

“…Mn comes in form of its only stable isotope 55 Mn, and is the decay product of 55 Co, produced in incomplete and complete Si-burning under optimal conditions with Y e = Z/A = 0.491. In alpha-rich freeze-out of explosive Si-burning, determined by the entropy S ∝ T 3 /ρ of explosive burning layers with values of T 9 and ρ 8 exceeding T 3 9 /ρ 8 > 180, 55 Co is moved to 59 Cu (→ 59 Co). The inner zones of M Ch -models experience only moderate entropies and such lower Y e 's are attained via electron capture, due to degenerate electrons with high Fermi energy.…”

Neutrino-driven wind simulations and nucleosynthesis of heavy elements

“…Moreover, the latest investigations of charged-current neutrino opacity [32,33,34] indicate that these cross sections can significantly change due to in medium modifications [35]. In the early supernova explosion phase, mean field effects have a minor effect on the neutrino spectra because neutrinospheres are not at very high densities.…”

“…As suggested in Refs. [32,33,34], the consideration of in medium modifications could change the electron fraction towards more neutron-rich conditions (Y e ≈ 0.45). However, the exact Y e value depends on medium correlations and the equation of state, both of which are still uncertain.…”