Multifield Inflation after Planck: Isocurvature Modes from Nonminimal
  Couplings
  Recent measurements by the {\it Planck} experiment of the power spectrum of
temperature anisotropies in the cosmic microwave background radiation (CMB)
reveal a deficit of power in low multipoles compared to the predictions from
best-fit $\Lambda$CDM cosmology. The low-$\ell$ anomaly may be explained by the
presence of primordial isocurvature perturbations in addition to the usual
adiabatic spectrum, and hence may provide the first robust evidence that
early-universe inflation involved more than one scalar field. In this paper we
explore the production of isocurvature perturbations in nonminimally coupled
two-field inflation. We find that this class of models readily produces enough
power in the isocurvature modes to account for the \emph{Planck} low-$\ell$
anomaly, while also providing excellent agreement with the other {\it Planck}
results.
