Large non-Gaussianities with Intermediate Shapes from Quasi-Single Field Inflation

TL;DR

The paper analyzes slow-roll multifield inflation with a turning trajectory and a massive isocurvature field of order the Hubble parameter, showing that non-Gaussianities can be amplified and carry intermediate shapes between equilateral and local due to transfer from isocurvature to curvature modes. Using the in-in formalism and a transfer vertex, the authors compute the bispectrum and reveal a one-parameter family of shapes controlled by the isocurvature mass via $\nu = \sqrt{9/4 - m^2/H^2}$. They provide a shape ansatz based on a Neumann function to enable data analysis and discuss perturbativity constraints that limit the amplitude $f_{NL}^{int}$ to be large but not arbitrarily so, with potential for observational detection of these intermediate shapes.

Abstract

We study the slow-roll inflation models, where the inflaton slow-rolls along a trajectory whose orthogonal directions are lifted by potentials with masses of order the Hubble parameter. In these models large non-Gaussianities can be generated through the transformation from the isocurvature modes to the curvature mode, once the inflaton trajectory turns. We find large bispectra with one-parameter family of novel shapes, interpolating between the equilateral and local shape. According to the in-in formalism, the shapes of these non-Gaussianities are different from a simple projection from the isocurvature non-Gaussian correlation functions.

Figures (3)

• Figure 1: Feynman diagrams for the transfer vertex (a), and the corrections from the isocurvature mode to the power spectrum (b) and bispectrum (c).
• Figure 2: The numerical coefficient $s(\nu)$ in the squeezed limit.
• Figure 3: Shapes of bispectra with intermediate form: 1) quasi-equilateral ($\nu=0.2$), 2) quasi-local ($\nu=1$). The amplitudes are normalized by a factor of $(p_1p_2p_3)^2$ to be dimensionless.