Nongaussianity from Particle Production During Inflation

TL;DR

The paper introduces a robust mechanism for generating cosmological perturbations during inflation via non-inflaton particle production and IR cascading, quantified by a simple prototype coupling $g^2/2(\phi-\phi_0)^2\chi^2$. It develops both nonlinear lattice simulations and an analytic framework to show that IR cascading dominates the observable signatures, producing a bump-like feature in the power spectrum and a distinctive, uncorrelated non-Gaussian signature in the bispectrum. The work connects these signatures to concrete particle-physics models, including open-string inflation and brane monodromy, and provides observational constraints from CMB, LSS, and weak lensing data, showing compatibility with current bounds for reasonable couplings while allowing potentially detectable NG in future missions. Overall, the study highlights inflationary particle production as a window into high-energy microphysics and emphasizes its non-decoupled imprint on cosmological observables with distinctive shapes and localization in momentum space.

Abstract

In a variety of models the motion of the inflaton may trigger the production of some non-inflaton particles during inflation, for example via parametric resonance or a phase transition. Such models have attracted interest recently for a variety of reasons, including the possibility of slowing the motion of the inflaton on a steep potential. In this review we show that interactions between the produced particles and the inflaton condensate can lead to a qualitatively new mechanism for generating cosmological fluctuations from inflation. We illustrate this effect using a simple prototype model g^2 (φ-φ_0)^2χ^2 for the interaction between the inflaton, φ, and iso-inflaton, χ. Such interactions are quite natural in a variety of inflation models from supersymmetry and string theory. Using both lattice field theory simulations and analytical calculations, we study the quantum production of χparticles and their subsequent rescatterings off the condensate φ(t), which generates bremsstrahlung radiation of light inflaton fluctuations δφ. This mechanism leads to observable features in the primordial power spectrum. We derive observational constraints on such features and discuss their implications for popular models of inflation, including brane/axion monodromy. Inflationary particle production also leads to a very novel kind of nongaussian signature which may be observable in future missions. We argue that this mechanism provides a simple and well-motivated option to generate large nongaussianity, without fine-tuning the inflationary trajectory or appealing to re-summation of an infinite series of high dimension operators.

Figures (15)

• Figure 1: Rescattering diagram.
• Figure 2: $|\dot\phi|/(M_pm)$ plotted against $m t$ for $g^2=0.1$ (where $m=V_{,\phi\phi}$ is the effective inflaton mass). Time $t=0$ corresponds to the moment when $\phi=\phi_0$ and $\chi$-particles are produced copiously. The solid red line is the lattice field theory result taking into account the full dynamics of rescattering and IR cascading while the dashed blue line is the result of a mean field theory treatment which ignores rescattering sasaki. The dot-dashed black line is the inflationary trajectory in the absence of particle creation.
• Figure 3: The top panel shows a sample bump in the power spectrum with amplitude $A_{\mathrm{IR}} = 2.5\times 10^{-10}$ which corresponds to a coupling $g^2 \sim 0.01$. The feature is located at $k_{\mathrm{IR}} = 0.01 \, \mathrm{Mpc}^{-1}$. This example represents a distortion of $\mathcal{O}(10\%)$ as compared to the usual vacuum fluctuations and is consistent with the data at $2\sigma$. The bottom panel shows the CMB angular TT power spectrum for this example, illustrating that the distortion shows up mostly in the first peak.
• Figure 4: The power spectrum of inflaton modes induced by rescattering (normalized to the usual vacuum fluctuations) as a function of $\ln(k/k_\star)$, plotted for three representative time steps in the evolution, showing the cascading of power into the IR. For each time step we plot the analytical result (the solid line) and the data points obtained using lattice field theory simulations (diamonds). The time steps correspond to the following values of the scale factor: $a = 1.03, 1.04, 2.20$ (where $a = 1$ at the moment when $\phi = \phi_0$). By this time the amplitude of fluctuations is saturated due to the expansion of the universe. The vertical lines show the range of scales from our lattice simulation.
• Figure 5: The dependence of the power spectrum $P_\phi$ on the coupling $g^2$. The three curves correspond to $P_\phi$ for $g^2=0.01,0.1,1$, evaluated at a fixed value of the scale factor, $a = 2.20$. We see that even for small values of $g^2$ the inflaton modes induced by rescattering constitute a significant fraction of the usual vacuum fluctuations after only a single $e$-folding.