Constraining Inflationary Particle Production with CMB Polarization

Inflation generates correlated temperature ($T$) and polarization ($E$-mode) fluctuations in the cosmic microwave background (CMB), which differ only by their transfer functions. Following Philcox et al. (2025), we investigate a scenario with a massive partner to the inflaton ($\mathcal{O}(100)$ times the inflationary Hubble scale), in which particles are produced during a narrow time period, leaving characteristic hot- or cold-spots in the CMB. Using tools developed for thermal Sunyaev-Zel'dovich cluster-finding, we search component-separated $\textit{Planck}$ 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. We find no strong evidence for primordial hotspots and thereby place novel bounds on the marginal and relevant coupling between the inflaton and massive scalars during inflation, probing physics at energies many orders of magnitude above any feasible terrestrial collider. Due to the large noise in $\textit{Planck}$ polarization, the temperature data dominate constraints for small hotspots, but for sufficiently large hotspots, our bounds improve on those of Philcox et al. (2025) by $\sim 25\%$. 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 $\textit{Planck}$ constraints by $\gtrsim25\%$, nearing the CV limit on intermediate scales. 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.