Searching for Unparticles with the Cosmic Microwave Background

TL;DR

This paper develops advanced analysis tools to search for unparticle physics signatures in the CMB, finding no evidence but setting constraints and highlighting potential future discovery avenues.

Contribution

It introduces a novel pipeline combining neural networks and optimal estimators to efficiently analyze non-factorizable unparticle templates in CMB data.

Findings

No significant unparticle signals detected in Planck data.

First CMB constraints on orthogonal bispectrum with $f^{ m orth^*}_{ m NL} = -12\pm12$.

Methodology applicable to other non-standard inflationary models.

Abstract

Multi-field models of inflation typically assume that interactions between particles can be treated perturbatively. Strongly-coupled models provide an intriguing alternative and may offer novel inflationary phenomenology. We study the "unparticle" scenario, where the inflaton is weakly mixed with a strongly-coupled sector, specified by a (gapless) conformal field theory. For certain choices of conformal scaling dimension, $\Delta$, the exchange of unparticles leads to distinctive non-Gaussian features in the primordial curvature distribution, including bispectra with enhanced squeezed limits and oscillations close to the equilateral regime. Efficiently analyzing these models using Cosmic Microwave Background (CMB) data is a challenge since the shapes are non-factorizable in momenta and often highly degenerate with single-field self-interactions. Here, we overcome these limitations using a library of tools, including neural-network factorization schemes and optimal CMB estimators. Our pipeline condenses 161 non-separable templates into just 7 factorizable forms, with negligible loss of signal-to-noise. We apply the model to the Planck data, asking two key questions: (1) can we detect unparticles? (2) can we distinguish them from single-field self-interactions? Across $1\leq \Delta\leq 9$, we find a maximal signal-to-noise of $1.2\sigma$, implying no evidence for new physics. We also place the first CMB constraints on the modified consistency-condition-satisfying orthogonal bispectrum with $f^{\rm orth^*}_{\rm NL} = -12\pm12$. While many unparticle models are degenerate with single-field shapes, values of $\Delta$ close to half-integers have very different shapes, offering an intriguing future discovery channel. The methods developed herein can be directly applied to other classes of templates, motivating the exploration of models beyond the standard weakly-coupled paradigm.