Strongly Coupled Sectors in Inflation: Gapped Theories of Unparticles

TL;DR

This paper explores a novel inflationary scenario involving gapped unparticles derived from a compactified five-dimensional conformal field theory, revealing unique correlation features and potential observational signatures.

Contribution

It introduces the concept of gapped unparticles from higher-dimensional theories and analyzes their impact on primordial density perturbations during inflation.

Findings

Gapped unparticles produce oscillatory correlation functions with an envelope influenced by anomalous dimensions.

Distinct signatures allow differentiation between gapped unparticles and heavy scalars in cosmological data.

Universal coupling of gapped unparticles with different effective masses leads to interference effects.

Abstract

We consider a novel scenario for a strongly coupled spectator sector during inflation, that of a higher dimensional conformal field theory with large anomalous dimensions -- ``unparticles'' -- and compactify the extra dimensions. More specifically, we take generalized free fields in five dimensions, where the extra dimension is compactified to a circle. Due to the usual Kaluza-Klein mechanism, the resulting excitations carry properties of both particles and unparticles, so we dub this scenario ``gapped unparticles''. We derive a two-point function of the gapped unparticles by performing dimensional reduction. We then compute, in the collapsed limit, the four-point correlation function of conformally coupled scalars exchanging a gapped unparticle, which are used as seed functions to obtain the correlation function of primordial density perturbations. The phenomenology of the resulting correlators presents some novel features, such as oscillations with an envelope controlled by the anomalous dimension, rather than the usual value of 3/2. Depending on the value of the five-dimensional scaling dimension and effective mass of the gapped unparticles, we find a clear strategy to distinguish gapped unparticles from heavy massive scalars. If we assume the interactions are localized on a brane, gapped unparticles with different effective masses will share a universal coupling, and their exchanges produce an interesting interference pattern.