Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 4
Processes in a Nuclear
Medium
The interaction rates of neutrinos and axions with nucleons in a nuclear
medium are studied with a focus on neutral-current processes such as
bremsstahlung emission of axions NN → NNa and of neutrino pairs
NN → NNνν as well as neutrino scattering. A severe problem with
the perturbative rate calculations at high density is discussed which has
a strong impact on axion emissivities and the dominant axial-vector
contribution to the neutrino opacities.
4.1 Introduction
New particles which couple to nucleons are emitted from ordinary stars
by analogous processes to those discussed in Chapter 3 for electrons.
For example, axions can be produced by the Compton process γp → pa;
the previous results can be easily adapted to such reactions. Presently
I will focus on processes involving neutrinos or axions that are specific
to a nuclear medium, i.e. to supernova (SN) cores or neutron stars.
The main focus of the literature which deals with microscopic processes
in a nuclear medium was inspired by the problem of late-time
neutron-star cooling. The recent progress of x-ray astronomy has led
to reasonably safe ROSAT identifications of thermal surface emission
from a number of old pulsars (Sect. 2.3). Together with the spin-down
age of these objects one can begin to test neutron-star cooling scenarios,
notably those that involve novel phases of nuclear matter such
as superfluidity, meson condensates, quark matter, etc. (Shapiro and
Teukolsky 1983; Tsuruta 1992).
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