The axial coupling governs weak nuclear interactions in the dense matter
found in astrophysical phenomena such as core collapse supernovae and
neutron stars.
Weak interaction rates, proportional to the axial coupling constant,
govern the dynamics of proto-neutron star evolution from beginning to
end, since thermal equilibration is achieved through neutrino emission.
A change in the value of the axial coupling constant (
) in a
nuclear medium thus directly affects the observable evolution
following a type-II supernova.
The first step in a complete description of the axial coupling constant is to account for its vacuum value of 1.26, which differs substantially from the value of unity which would arise from purely chiral dynamics. Using the instanton model of the strong interactions, which accounts for spontaneous chiral symmetry breaking at the constituent quark level, we have obtained a result in agreement with experiment [50]. We are now computing the finite-density corrections to this result, which requires detailed modifications of the microscopic physics underlying the interactions between quarks and charged currents.
A nuclear medium necessarily chooses a particular reference frame, meaning
that at finite density Lorentz invariance is broken, and thus nucleons
may couple differently to space-like and time-like axial currents. There is
a large body of experimental evidence which demonstrates that while the
spatial is decreased in the medium, the analogous time-like coupling
grows. We are working on an explanation of these effects in a
chiral effective model of nucleons, and our initial results seem to
reproduce the experimental data. There are, however, corrections terms
which must be computed to verify their irrelevance before a final
conclusion can be reached.