  We present a unified approach to neutrino processes in nucleon matter based
on Landau's theory of Fermi liquids that includes one- and
two-quasiparticle-quasihole pair states as well as mean-field effects. We show
how rates of neutrino processes involving two nucleons may be calculated in
terms of the collision integral in the Landau transport equation for
quasiparticles. Using a relaxation time approximation, we solve the transport
equation for density and spin-density fluctuations and derive a general form
for the response functions. We apply our approach to neutral-current processes
in neutron matter, where the spin response function is crucial for calculations
of neutrino elastic and inelastic scattering, neutrino-pair bremsstrahlung and
absorption from strongly-interacting nucleons. We calculate the relaxation
rates using modern nuclear interactions and including many-body contributions,
and find that rates of neutrino processes are reduced compared with estimates
based on the one-pion exchange interaction, which is used in current
simulations of core-collapse supernovae.
