Stars as Laboratories for Fundamental Physics - MPP Theory Group

152 Chapter 4
(1989) argued on the basis of the nonlinear sigma model that the combination
of parameters α 2 πα a /m 2 N should remain approximately constant.
Mayle et al. (1989) similarly found that this parameter should remain
somewhere in the range 0.3−1.5 of its vacuum value.
The nucleon effective mass m ∗ N deviates substantially from the vacuum
value m N = 939 MeV. This shift has an impact on the kinematics
of reactions and the nucleon phase-space distribution. A calculation of
m ∗ N has to rely on an effective theory which describes the interaction
of nucleons and mesons. A typical result from a self-consistent relativistic
Brueckner calculation including vacuum fluctuations is shown
in Fig. 4.10. For conditions relevant for a SN core, an effective value as
low as m ∗ N/m N = 0.5 is conceivable.
Fig. 4.10. Effective nucleon mass in a nuclear medium according to a selfconsistent
relativistic Brueckner calculation (Horowitz and Serot 1987). Nuclear
density corresponds to about ρ 0 = 3×10 14 g/cm 3 .
4.8 The URCA Processes
Neutral-current weak processes are of prime interest for the topics studied
in this book because they are closely related to “exotic” reactions
involving new particles such as axions. For completeness, however,
it must be mentioned that the charged-current reactions depicted in
Fig. 4.11 dominate the neutrino energy-loss rate of old neutron stars,
and also dominate the ν e opacity in young SN cores. The processes
n → p e ν e (neutron decay) and e p → n ν e are usually called the URCA