On Features and Nongaussianity from Inflationary Particle Production

TL;DR

The paper develops and validates an analytical framework for inflationary particle production that occurs when the inflaton couples to an iso-inflaton field, triggering a burst of particle production at $\phi=\phi_0$ and driving infrared cascading. This mechanism generates a localized bump in the inflaton power spectrum and an unusual, localized non-Gaussian signature in the bispectrum, which the authors quantify using nonlinear lattice simulations and a Renormalized Green-function approach that includes cosmic expansion and metric perturbations. The key contributions are (i) a precise computation of the power spectrum and bispectrum from rescattering, (ii) a robust characterization of the non-Gaussian PDF via moments and an Edgeworth expansion, and (iii) a gauge-consistent cosmological perturbation theory showing that curvature perturbations inherit the same localized features with a clear translation between $\mathcal{P}_φ$ and $\mathcal{P}_ζ$. The results indicate that inflationary particle production could yield observable signatures in upcoming CMB and LSS data, offering a direct probe of inflaton couplings beyond the potential.

Abstract

Interactions between the inflaton and any additional fields can lead to isolated bursts of particle production during inflation (for example from parametric resonance or a phase transition). Inflationary particle production leaves localized features in the spectrum and bispectrum of the observable cosmological fluctuations, via the Infra-Red (IR) cascading mechanism. We focus on a simple prototype interaction g^2 (φ-φ_0)^2χ^2 between the inflaton, φ, and iso-inflaton, χ; extending previous work on this model in two directions. First, we quantify the magnitude of the produced nongaussianity by extracting the moments of the probability distribution function from lattice field theory simulations. We argue that the bispectrum feature from particle production might be observable for reasonable values of the coupling, g^2. Second, we develop a detailed analytical theory of particle production and IR cascading during inflation, which is in excellent agreement with numerical simulations. Our formalism improves significantly on previous approaches by consistently incorporating both the expansion of the universe and also metric perturbations. We use this new formalism to estimate the shape of the bispectrum from particle production, showing this to be distinguishable from other mechanisms that predict large nongaussianity.

Figures (5)

• Figure 1: rescattering diagram.
• Figure 2: The PDF of the inflaton fluctuations generated by rescattering and IR cascading, at a series of different values of the scale factor, $a$. The dotted black curve shows a Gaussian fit at late times and we have normalized the scale factor so that $a=1$ at the moment when particle production occurs. For illustration, we have chosen $g^2=0.1$ and a standard chaotic inflation potential $V(\phi) = m^2\phi^2/2$.
• Figure 3: The PDF of the total curvature fluctuation, $\zeta$, at late times (well after all relevant modes have crossed the horizon and frozen). The solid black curve is the exact result from our HLattice simulations and the dotted red curve is a gaussian fit. We have also plotted the leading correction to the gaussian result in the Edgeworth expansion, given explicitly by equation (24). For illustration, we have chosen $g^2=0.1$ and a standard chaotic inflation potential $V(\phi) = m^2\phi^2/2$.
• 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 late-time evolution. For each time step we plot the analytical result (the solid line) and the data points obtained using lattice field theory simulations (diamonds). The agreement between these two independent results is evident. For illustration, we have set $\mu^2=0$.
• Figure 5: The shape function $S(k,k x_2, k x_3)$, defined by (\ref{['S']}), as a function of the dimensionless quantities $x_2,x_3$ which parametrize the shape of the triangle. The upper left panel corresponds to $k = e^{-1}k_{\mathrm{bump}}$, the upper right panel is $k=k_{\mathrm{bump}}$, the lower left panel is $k=e^{+1} k_{\mathrm{bump}}$ and the lower right panel is $k=e^{+2}k_{\mathrm{bump}}$. In the IR ($k \leq k_{\mathrm{bump}}$) the shape of the bispectrum is similar to the equilateral shape, however, there is also some support on flattened triangles near $k \sim e^{+1} k_{\mathrm{bump}}$. At larger values of $k$ the shape is unlike any other template proposed in the literature. For illustration we have chosen $\mu^2=0$.