Particle Production During Inflation: Observational Constraints and Signatures

TL;DR

The paper investigates inflationary scenarios where inflaton dynamics trigger production of non-inflaton particles, leading to localized bumps in the primordial power spectrum and potential non-Gaussian signals. It introduces a simple bump-based parametrization of P(k) and confronts it with CMB, galaxy, and weak-lensing data using MCMC, finding no evidence for bursts but allowing sizable features on CMB scales. The authors connect the phenomenology to concrete string-theory and SUSY realizations, notably open string/branes and brane/axion monodromy, which predict either multiple evenly spaced bumps in ln k or arbitrary feature distributions. The work constrains the microphysical coupling g^2 and discusses implications for detectable non-Gaussianity and reheating, illustrating that such microscopically motivated features could be present yet challenging to discern with current data. Overall, it clarifies how inflationary particle production can manifest as localized spectral distortions and how future observations could test these scenarios.

Abstract

In a variety of inflation 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. Particle production during inflation leads to observables in the cosmological fluctuations, such as features in the primordial power spectrum and also nongaussianities. Here we focus on a prototype scenario with inflaton, φ, and iso-inflaton, χ, fields interacting during inflation via the coupling g^2 (φ-φ_0)^2χ^2. Since several previous investigations have hinted at the presence of localized "glitches" in the observed primordial power spectrum, which are inconsistent with the simplest power-law model, it is interesting to determine the extent to which such anomalies can be explained by this simple and well-motivated model. Our prototype scenario predicts a bump-like feature in the primordial power spectrum, rather than an oscillatory "ringing" pattern as has previously been assumed. We discuss the observational constraints on such features. We find that bumps with amplitude as large as 10% of the usual scale invariant fluctuations from inflation are allowed on scales relevant for Cosmic Microwave Background experiments. Our results imply an upper limit on the coupling g^2 which is crucial for assessing the detectability of the nongaussianity produced by inflationary particle production. We also discuss more complicated features that result from superposing multiple instances of particle production. Finally, we point to a number of microscopic realizations of this scenario in string theory and supersymmetry and discuss the implications of our constraints for the popular brane/axion monodromy inflation models.

Figures (6)

• Figure 1: Rescattering diagram.
• Figure 2: The bump-like features generated by IR cascading. We plot the feature power spectrum obtained from fully nonlinear lattice field theory simulations (the red points) and also the result of an analytical calculation (the dashed blue curve) using the formalism described in inprog1. We also superpose the fitting function $\sim k^3 e^{-\pi k^2 / (2 k_\star^2)}$ (the solid black curve) to illustrate the accuracy of this simple formula
• Figure 3: Marginalized posterior likelihood contours for the parameters $A_{\mathrm{IR}}$ and $k_{IR}$ (the magnitude and position of the feature, respectively) in the single-bump model. Black and grey regions correspond to parameter values allowed at 95.4% and 99.7% confidence levels, respectively. At small scales, to the right of the dashed vertical line, our results should be taken with a grain of salt since the nonlinear evolution of the power spectrum may not be modeled correctly in the presence of bump-like distortions.
• Figure 4: 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 5: Marginalized posterior likelihood contours for the parameters $A_{\mathrm{IR}}$ and $\Delta$ (the feature amplitude and spacing, respectively) of the multiple-bump model. Black and grey regions correspond to values allowed at 95.4% and 99.7% confidence levels, respectively.