Quasi-Single Field Inflation and Non-Gaussianities

TL;DR

The paper studies quasi-single field inflation with massive isocurvature modes coupled to the inflaton via a turning trajectory. It shows that this setup yields large bispectra with shapes that interpolate between local and equilateral. Trispectra can also be very large, with $t_{NL}$ potentially much larger than $f_{NL}^2$. Methodologically, it uses the in-in formalism and transfer vertices, introducing a mixed representation to facilitate calculations. The work highlights connections between single-field and multi-field inflation and suggests observable signatures.

Abstract

In quasi-single field inflation models, massive isocurvature modes, that are coupled to the inflaton and have mass of order the Hubble parameter, can have nontrivial impacts on density perturbations, especially non-Gaussianities. We study a simple example of quasi-single field inflation in terms of turning inflaton trajectory. Large bispectra with a one-parameter family of novel shapes arise, lying between the well-known local and equilateral shape. The trispectra can also be very large and its magnitude tNL can be much larger than fNL squared.

Figures (3)

• Figure 1: This figure illustrates a model of quasi-single field inflation in terms of turning trajectory. The $\theta$ direction is the inflationary direction, with a slow-roll potential. The $\sigma$ direction denotes the isocurvature direction, which typically has mass of order $H$.
• Figure 2: The relationship between single field inflation, quasi-single field inflation and multi-field inflation. Here we take inflation evolving two fields as an example. Quasi-single field inflation fills the gap between single field inflation and multiple field inflation.
• Figure :