Productive Interactions: heavy particles and non-Gaussianity

TL;DR

This work demonstrates that non-adiabatic production of heavy fields coupled to the inflaton during inflation can imprint observable oscillatory features in both the power spectrum and higher-point correlation functions, even when the heavy masses exceed the typical UV scales. By analyzing two mass-modulation scenarios within axion monodromy-inspired setups, the authors derive detailed templates for power-spectrum and bispectrum shapes, establish controlled parameter windows, and compare amplitudes to resonant non-Gaussianity. A key insight is that UV completion and heavy degrees of freedom can be probed through non-analytic effects in the EFT, with a notable potential for the bispectrum signal to be competitive with power-spectrum oscillations, especially in case (b) where discrete shift-symmetry breaking induces resonant-like features. The results motivate joint analyses of oscillations in P(k) and NG templates, and highlight the importance of EFT considerations, radiative stability, and potential extensions to fermions and string production in inflationary cosmology.

Abstract

We analyze the shape and amplitude of oscillatory features in the primordial power spectrum and non-Gaussianity induced by periodic production of heavy degrees of freedom coupled to the inflaton $φ$. We find that non-adiabatic production of particles can contribute effects which are detectable or constrainable using cosmological data even if their time-dependent masses are always heavier than the scale $\dot φ^{1/2}$, much larger than the Hubble scale. This provides a new role for UV completion, consistent with the criteria from effective field theory for when heavy fields cannot be integrated out. This analysis is motivated in part by the structure of axion monodromy, and leads to an additional oscillatory signature in a subset of its parameter space. At the level of a quantum field theory model that we analyze in detail, the effect arises consistently with radiative stability for an interesting window of couplings up to of order $\lesssim 1$. The amplitude of the bispectrum and higher-point functions can be larger than that for Resonant Non-Gaussianity, and its signal/noise may be comparable to that of the corresponding oscillations in the power spectrum (and even somewhat larger within a controlled regime of parameters). Its shape is distinct from previously analyzed templates, but was partly motivated by the oscillatory equilateral searches performed recently by the {\it Planck} collaboration. We also make some general comments about the challenges involved in making a systematic study of primordial non-Gaussianity.

Figures (5)

• Figure 1: Pictorial representation of our findings: in an inflationary theory with an approximate continuous shift symmetry for the inflaton, only particles that are not much heavier than the Hubble scale $H$ are relevant for the dynamics of the fluctuations. However, as we will see, if the continuous shift symmetry is broken, e.g. to a discrete shift symmetry, heavier particles can become relevant as depicted on the right. In the scenarios studied in this work, the new scale is set by $\dot\phi$. The basic estimate $\exp(-\pi m^2/\dot\phi)\sim 1/\sqrt{N_{\rm modes}}$ suggests observational sensitivity to these massive particles, which we confirm in a detailed analysis.
• Figure 2: Bispectrum shape (a) plotted along the equilateral axis for a range of frequencies. As the frequency increases a plateau develops and the amplitude of the oscillations decreases.
• Figure 3: Overlap of shape (a) for $\gamma=0$ with the equilateral template using the prescription developed in shapes. As the frequency increases the shape approaches the equilateral shape.
• Figure 4: Shape for case (b) plotted along the equilateral axis for a range of frequencies.
• Figure 5: Overlap of shape (b) with the equilateral template using the prescription developed in shapes