Light Scalars at the Cosmological Collider

TL;DR

This paper analyzes the self-energies of light scalar fields in de Sitter space, revealing simplifications in the light limit and exploring their impact on the primordial bispectrum as a cosmological collider signal.

Contribution

It provides a detailed calculation of scalar self-energies in de Sitter space for light fields, including UV divergences and their effect on heavy field propagation and cosmological signals.

Findings

Self-energies simplify significantly for light scalar fields.

Light fields can alter the propagation of heavy fields even at weak couplings.

Modifications to the primordial bispectrum can reveal the presence of light scalars.

Abstract

We study the self-energies of weakly interacting scalar fields in de Sitter space with one field much lighter than the Hubble scale. We argue that self-energies drastically simplify in this light limit. We illustrate this in theories with two scalar fields, one heavy and one light, interacting with one another through either cubic or quartic interactions. To regulate infrared divergences, we compute these self-energies in Euclidean de Sitter space and then carefully analytically continue to Lorentzian signature. In particular, we do this for the most general renormalizable theory of two scalar fields with even interactions to leading order in the coupling and the mass of the light field. These self-energies are determined by de Sitter sunset diagrams, whose analytic structure and UV divergences we derive. Even at very weak couplings, the light field can substantially change how the heavy field propagates over long distances. The light field's existence may then be inferred from how it modifies the heavy field's oscillatory contribution to the primordial bispectrum in the squeezed limit, i.e. its cosmological collider signal.