Negative spectral index of $f_{NL}$ in the axion-type curvaton model

Abstract

We derive the spectral index of $f_{NL}$ and its running from isocurvature single field and investigate the curvaton models with a negative spectral index of $f_{NL}$ in detail. In particular, a numerical study of the axion-type curvaton model is illustrated, and we find that the spectral index of $f_{NL}$ is negative and its absolute value is maximized around ${\sigma }_{\ast }=\pi f/2$ for the potential $V(\sigma )=m^2{f}^2(1-\cos \frac{\sigma }{f})$. The spectral index of $f_{NL}$ can be $\mathcal{O}(-0.1)$ for the axion-type curvaton model. A convincing detection of a positive $n_{f_{NL}}$ will rule out the axion-type curvaton model. In addition, we also give a general discussion about the detectable parameter space for the curvaton model with a polynomial potential.

Comments: 14 pages, 2 figures; v2, refs added; version accepted for publication in JCAP