Constraining Inflationary Particle Production with CMB Polarization

Abstract

Following Philcox et al. (2025), we investigate a scenario with a massive partner to the inflaton ($O(100)$ times the inflationary Hubble scale), in which particles are produced during a narrow time period, leaving characteristic hot- or cold-spots in the cosmic microwave background (CMB). Using tools developed for thermal Sunyaev-Zel'dovich cluster-finding, we search component-separated Planck PR4 $E$-mode maps for these hotspots, and compare to analogous results in $T$. Our analysis pipeline is validated on simulated observations and gives unbiased constraints for sufficiently large and bright hotspots. At Planck sensitivities, the temperature data are more sensitive to small hotspots, but for sufficiently large hotspots the polarization data are more sensitive. We improve upon earlier work by building a full Poissonian likelihood for the hotspot abundance. We find no strong evidence for primordial hotspots and thereby place novel bounds on the couplings between the inflaton and massive scalars during inflation, probing physics at energies many orders of magnitude above any feasible terrestrial collider. The bounds derived from our new likelihood improve upon those of Philcox et al. (2025) by more than an order of magnitude for sufficiently light particles ($M_0\lesssim100H_I$). We also forecast the inferred bounds on inflationary physics for a search using Atacama Cosmology Telescope (ACT) data, and from an optimistic cosmic-variance-limited experiment (CV), for which $E$-mode data provide stronger constraints than $T$ on nearly all scales. ACT should improve on the Planck constraints by $\gtrsim10\%$, nearing the CV limit allowed by its sky coverage. Finally, we compare the constraining power of localized searches to that of a power spectrum analysis, and demonstrate that for sufficiently few produced particles the localized search performed herein is dominant.