Stars as Laboratories for Fundamental Physics - MPP Theory Group

118 Chapter 4
From the perspective of particle physics, however, a more interesting
nuclear environment is a young neutron star for the first few seconds
after the progenitor collapsed. The nuclear medium here is so hot that
it is essentially nondegenerate, and neutrinos are trapped. Therefore,
the production of even more weakly interacting particles such as axions
or right-handed neutrinos can compete with neutrino energy transfer
which is essentially a diffusion process. For a quantitative understanding
of the emissivities of the new particles, but also for the conventional
transport of energy and lepton number by neutrinos, a knowledge of
the microscopic interaction rates is needed.
The neutrino opacities that went into standard SN collapse and explosion
calculations as well as the particle emissivities that went into
the derivation of, say, axion bounds from SN 1987A were based on the
assumption that the hot nuclear medium can be treated as an ideal
Boltzmann gas of free particles, except for degeneracy effects that are
easy to include. It turns out, however, that the approximations made
are internally inconsistent, notably for the dominant processes which
involve couplings to the nucleon spin (axial-vector current interactions).
In order for the “naive” neutral-current neutrino opacities to be correct
one needs to assume that the nucleon spins do not fluctuate too fast on
a time scale set by the temperature. A naive perturbative calculation,
however, yields a spin-fluctuation rate which is much larger than this
limit. This large rate went into the axion emissivities. Therefore, the
existing studies of SN axion bounds are based on microscopic interaction
rates which simultaneously make use of the opposite limits of a
spin-fluctuation rate very small and very large compared with T .
Beyond the ideal-gas approximation there is virtually no literature
on the microscopic interaction rates in a hot nuclear medium, presumably
because of the historical focus on old neutron stars, and presumably
because degenerate nuclear matter is more reminiscent of actual
nuclei. Thus there is a dearth of reliable microscopic input physics for
conventional studies of SN evolution, and for variations involving novel
particle-physics ideas.
Most of this chapter focusses on the dominant axial-vector current
interactions of neutrinos and axions with nucleons in a dense and hot
medium. Most of the material is based on a series of papers which
I have co-authored (Raffelt and Seckel 1991, 1995; Keil, Janka, and
Raffelt 1995; Keil et al. 1995) and as such does not represent a community
consensus. On the other hand, I am not aware of a significant
controversy. Rather, it appears that very little serious interest has
been taken in the difficult question of weakly interacting particles in-