New constraints on multifield inflation with nonminimal coupling

We study the dynamics and perturbations during inflation and reheating in a multifield model where a second scalar field χ is nonminimally coupled to the scalar curvature (1/2ξRχ²). When ξ is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the χ fluctuation in sub-Hubble modes can be amplified during preheating for large ξ. For negative values of ξ, however, long-wave modes of the χ fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when |ξ| is less than unity. This is because the effective χ mass becomes negative during inflation. We constrain the strength of ξ and the initial χ by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the χ mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.