Non-Gaussianity from explicit $U(1)$-breaking interactions

TL;DR

This paper explores how explicit $U(1)$ symmetry-breaking interactions during inflation can produce observable primordial non-Gaussianities in curvature and isocurvature perturbations, with potential signatures detectable by future experiments.

Contribution

It provides a detailed analysis of non-Gaussianity generation from $U(1)$-breaking interactions, including scenarios with curvaton and dark matter, and predicts observable signals in upcoming cosmological measurements.

Findings

Local NG can be suppressed to |f_NL| < 0.1 due to parameter cancellations.

Interactions can enhance isocurvature NG signals, making them potentially observable.

Oscillating correlation signals from heavy radial fields may dominate the bispectrum shape.

Abstract

We investigate primordial non-Gaussianity (NG) arising from the explicit $U(1)$ symmetry-breaking interactions during inflation involving a nearly massless axial component of a complex scalar field $P$. We analyze the induced NG parameter $f_{\mathrm{NL}}$ under scenarios where the axial field functions as either a curvaton or cold dark matter (CDM). In the curvaton framework, there is a conventional contribution to the local NG of $f_{\rm NL} \simeq -O(1)$. Additional positive local NG can result from either the self-interactions of axial field fluctuations, their interactions with a light radial partner, or kinetic mixing with the inflaton via $U(1)$ symmetry-breaking terms. We identify parameter regions where the interactions lead to cancellations, suppressing the overall local NG to $|f^{\rm loc}_{\mathrm{NL}}| \lesssim O(0.1)$, while leaving the trispectrum largely unaffected. In the CDM scenario, these interactions enhance the NG in the isocurvature fluctuations. Moreover, interactions between the axial field and another light scalar, such as a curvaton, can generate $O(1)$ curvature NG signals and significant mixed curvature-isocurvature NGs that are within the reach of future experiments with $\sigma(f^{\rm loc}_{{\rm NL}})\sim1$. We also explore the role of a heavy radial field in generating oscillating correlation signals, noting that such signals can dominate the shape of the mixed adiabatic-isocurvature bispectrum. In certain cases, an oscillatory isocurvature bispectrum signal may be observable in the future, aiding in distinguishing between certain types of the $U(1)$-breaking self-interactions of the axial field.