Combined local and equilateral non-Gaussianities from multifield DBI inflation

TL;DR

This paper explores multifield DBI inflation, showing how angular brane fluctuations produce observable local and equilateral non-Gaussianities, with a unique trispectrum signature linking the two.

Contribution

It demonstrates that multifield DBI inflation naturally generates combined local and equilateral non-Gaussianities with a distinctive trispectrum relation, applicable to any brane trajectory.

Findings

Non-Gaussianities include both local and equilateral shapes.

The trispectrum has a momentum-dependent component proportional to $f_{NL}^{loc} f_{NL}^{eq}$.

The relation between trispectrum and bispectrum shapes is universal in multifield DBI models.

Abstract

We study multifield aspects of Dirac-Born-Infeld (DBI) inflation. More specifically, we consider an inflationary phase driven by the radial motion of a D-brane in a conical throat and determine how the D-brane fluctuations in the angular directions can be converted into curvature perturbations when the tachyonic instability arises at the end of inflation. The simultaneous presence of multiple fields and non-standard kinetic terms gives both local and equilateral shapes for non-Gaussianities in the bispectrum. We also study the trispectrum, pointing out that it acquires a particular momentum dependent component whose amplitude is given by $f_{NL}^{loc} f_{NL}^{eq}$. We show that this relation is valid in every multifield DBI model, in particular for any brane trajectory, and thus constitutes an interesting observational signature of such scenarios.