Neutrino Trapping in a Collapsing Core and the Conditions for a Neutrino Branch to Become Operative

Whether a fermion species can set a density bound in collapsing matter depends on whether it is trapped with the matter, and neutrinos are the borderline case: they stream freely early in the collapse and are trapped once the core grows dense enough. This paper computes the trapping transition with a standard mean-free-path scaling and evaluates the fractions of both candidate bounds at trapping. Trapping occurs near 1.00e+15 kg per cubic metre, where the mean free path of a 10 MeV neutrino falls to 1.000e+04 m, the size of the core. At that density the core has reached a fraction 9.08e-24 of the neutrino-branch bound of 1.10e+35 g per cubic centimetre and 1.03e-43 of the neutron branch. Trapping therefore changes which species are confined, but it does not bring any branch within reach: the operative bound remains remote on either assignment. Simulated from stated assumptions, not measured.