Mixed non-Gaussianity in multiple-DBI inflation

TL;DR

This paper develops a formalism for analyzing mixed non-Gaussianity in multiple-field DBI inflation, deriving analytic expressions for the non-Gaussianity parameters and illustrating scenarios with observable signals.

Contribution

It introduces a Hamilton-Jacobi based approach to compute mixed non-Gaussianity in multiple-field DBI inflation beyond slow-roll, providing analytic formulas for fNL.

Findings

Equilateral non-Gaussianity can be suppressed or enhanced depending on sound speed.

A model with both detectable equilateral and negative local fNL is constructed.

Current bounds are not evaded by the equilateral signal in the first scenario.

Abstract

We study a model of multiple-field DBI inflation leading to mixed form of primordial non-Gaussianity, including equilateral and local bispectrum shapes. We present a general formalism based on the Hamilton-Jacobi approach, allowing us to go beyond slow-roll, combining the three-point function for the fields at Hubble-exit with the non-linear evolution of super-Hubble scales. We are able to obtain analytic results by taking a separable Ansatz for the Hubble rate. We find general expressions for both the equilateral and local type non-Gaussianity parameter fNL. The equilateral non-Gaussianity includes the usual enhancement for small sound speeds, but multiplied by an analytic factor which can lead to a suppression. We illustrate our results with two scenarios. In the first model, previously found to have detectable local non-Gaussianity, we find that the equilateral signal is not sufficiently suppressed to evade current observational bounds. In our second scenario we construct a model which exhibits both a detectable equilateral fNL and a negative local fNL.