Preheating after multifield inflation with nonminimal couplings, II:
  Resonance Structure
  This is the second in a series of papers on preheating in inflationary models
comprised of multiple scalar fields coupled nonminimally to gravity. In this
paper, we work in the rigid-spacetime approximation and consider field
trajectories within the single-field attractor, which is a generic feature of
these models. We construct the Floquet charts to find regions of parameter
space in which particle production is efficient for both the adiabatic and
isocurvature modes, and analyze the resonance structure using analytic and
semi-analytic techniques. Particle production in the adiabatic direction is
characterized by the existence of an asymptotic scaling solution at large
values of the nonminimal couplings, $\xi_I \gg 1$, in which the dominant
instability band arises in the long-wavelength limit, for comoving wavenumbers
$k \rightarrow 0$. However, the large-$\xi_I$ regime is not reached until
$\xi_I \geq {\cal O} (100)$. In the intermediate regime, with $\xi_I \sim {\cal
O}(1 - 10)$, the resonance structure depends strongly on wavenumber and
couplings. The resonance structure for isocurvature perturbations is distinct
and more complicated than its adiabatic counterpart. An intermediate regime,
for $\xi_I \sim {\cal O} (1 - 10)$, is again evident. For large values of
$\xi_I$, the Floquet chart consists of densely spaced, nearly parallel
instability bands, suggesting a very efficient preheating behavior. The
increased efficiency arises from features of the nontrivial field-space
manifold in the Einstein frame, which itself arises from the fields' nonminimal
couplings in the Jordan frame, and has no analogue in models with minimal
couplings. Quantitatively, the approach to the large-$\xi_I$ asymptotic
solution for isocurvature modes is slower than in the case of the adiabatic
modes.
