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 $\chi$ is nonminimally coupled to the scalar curvature $\left(\frac{1}{2}\xi R{\chi }^2\right).\mathrm{}$ When $\xi$ is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the $\chi$ fluctuation in sub-Hubble modes can be amplified during preheating for large $\xi .$ For negative values of $\xi ,$ however, long-wave modes of the $\chi$ fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when $|\xi |$ is less than unity. This is because the effective $\chi$ mass becomes negative during inflation. We constrain the strength of $\xi$ and the initial $\chi$ by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the $\chi$ mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.