Non-Gaussianity from Axionic Curvaton

Abstract: We study non-Gaussianity of density perturbations generated by an axionic curvaton, focusing on the case that the curvaton sits near the hilltop of the potential during inflation. Such hilltop curvatons can generate a red-tilted density perturbation spectrum without invoking large-field inflation. We show that, even when the curvaton dominates the Universe, the non-Gaussianity parameter fNL is positive and mildly increases towards the hilltop of the curvaton potential, and that fNL = O(10) is a general and robust prediction of such hilltop axionic curvatons. In particular, we find that the non-Gaussianity parameter is bounded as fNL <~ 30 - 40 for a range of the scalar spectral index, ns = 0.94 - 0.99, and that fNL = 20 - 40 is realized for the curvaton mass m_\sigma = 10 - 10^6 GeV and the decay constant f = 10^{12} - 10^{17} GeV. One of the plausible candidates for the axionic curvaton is an imaginary component of a modulus field with mass of order 10 - 100 TeV and decay constant of 10^{16} - 10^{17} GeV. We also discuss extreme cases where the curvaton drives a second inflation and find that fNL is typically smaller compared to non-inflating cases.