The Role of the Curvaton Post-Planck

Abstract

The expected improvements in the precision of inflationary physics observables including the scalar spectral index $n_{s}$ and the tensor-to-scalar ratio $r$ will reveal more than just the viability of a particular model of inflation. In the presence of a curvaton field $\chi$, supposedly dead models of inflation can be resurrected as these observables are affected by curvaton perturbations. For currently successful models, improved constraints will enable us to constrain the properties of extra decaying scalar degrees of freedom produced during inflation. In this work, we demonstrate these diverse uses of a curvaton field with the most recent constraints on ($n_{s},r$) and two exemplary inflation models, the Starobinsky model, and a model of new inflation. Our analysis invokes three free parameters: the curvaton mass $m_{\chi}$, its decay rate $\Gamma_{\chi}$ the reheating temperature $T_{\rm RH}$ produced by inflaton decays. We systematically analyze possible post-inflationary era scenarios of a curvaton field. By projecting the most recent CMB data on ($n_{s},r$) into this parameter space, we can either set constraints on the curvaton parameters from successful models of inflation (so that the success is not spoiled) or determine the parameters which are able to save a model for which $n_{s}$ is predicted to be below the experimental data. We emphasize that the initial value of $\langle \chi^2 \rangle \propto H^4/m_\chi^2$ produced during inflation is determined from a stochastic approach and thus not a free parameter in our analysis. We also investigate the production of local non-Gaussianity $f_{NL}^{(\rm loc)}$ and apply current CMB constraints to the parameter space. Intriguingly, we find that a large value of $f_{NL}^{(\rm loc)}$ of $\mathcal{O}(1)$ can be produced for both of the two representative inflation models.