Inflationary Particle Production and Implications for WIMP Substructure

TL;DR

Inflationary resonant particle production can imprint localised features in the primordial power spectrum, boosting small-scale density peaks that seed prompt cusps and enhance dark matter annihilation signals. The authors connect this early-universe mechanism to present-day gamma-ray constraints by modelling cusp formation, computing the boosted J-factor, and analyzing 15 years of Fermi-LAT data on the Virgo cluster with three benchmark inflaton–spectator couplings $g$ and feature scales $k_*$. They find that for canonical WIMP cross sections $\langle \sigma v \rangle \simeq 3 \times 10^{-26}\,\mathrm{cm}^3\mathrm{s}^{-1}$, the resulting gamma-ray flux excludes $m_χ \gtrsim \mathrm{O}(3)\ \mathrm{TeV}$ across the explored $g$–$k_*$ ranges. This work links early-universe inflationary dynamics to indirect detection and demonstrates that gamma-ray observations can place meaningful limits on inflationary couplings; future gamma-ray measurements will tighten these bounds.

Abstract

We explore the observational consequences of resonant particle production during inflation, focusing on its impact on dark matter annihilation signals today. A transient burst of particle production generates localised features in the primordial power spectrum, enhancing the formation of compact small-scale dark matter structures known as prompt cusps. If dark matter consists of thermal WIMPs, the resulting small-scale structures substantially boost annihilation rates, leaving potentially detectable imprints in gamma-ray observations. Using 15 years of Fermi-LAT data targeting the Virgo cluster, we derive upper limits on the thermally averaged annihilation cross section $\langle σv \rangle$, connecting inflationary particle production in the early universe with present-day observations constraining dark matter annihilation.

Figures (2)

• Figure 1: Dimensionless matter power spectrum $\Delta^2_m(k)$ as a function of comoving wavenumber $k$, for three benchmark scenarios of resonant particle production during inflation. Each curve corresponds to a different coupling $g$ and characteristic scale $k_\star$, illustrating how stronger couplings lead to larger enhancements and sharper features. The free-streaming cutoff for a dark matter particle with mass $m_\chi = 100\,\mathrm{GeV}$ and $T_{\rm kd}= 1 \, \rm{GeV}$ is shown at $k_{\mathrm{fs}} = 8.3 \times 10^6\,\mathrm{Mpc}^{-1}$, suppressing power at the smallest scales.
• Figure 2: Upper limits on the dark matter annihilation cross section $\langle \sigma v \rangle$ into $b\bar{b}$ from the Virgo cluster for three benchmark scenarios of resonant particle production (RPP) during inflation. Each panel compares the standard $\Lambda$CDM prediction (dashed) with a modified spectrum including a bump (solid), showing the impact of small-scale power enhancement.