Circumstellar Medium of Supernovae as New Probes for Feebly Interacting Particles.

We propose a novel strategy to probe feebly interacting particles (FIPs) by exploiting the dense, confined circumstellar medium (CSM) surrounding core-collapse supernovae. FIPs produced in the protoneutron star can deposit substantial visible energy into the CSM via decay prior to the shock breakout from the progenitor star. This energy injection heats and ionizes the CSM, establishing an FIP-induced photosphere that generates distinctive precursor blackbody emission. Using early-time observations of supernova 2023ixf, we translate the nondetection of excessive precursor luminosity into stringent new constraints on MeV-scale dark photons as an exemplary model. Our results significantly extend existing core-collapse supernova bounds and exclude previously unexplored regions of parameter space. We further demonstrate that the FIP-induced dust sublimation offers key diagnostics for future Galactic supernovae, opening a new avenue to explore the dark sector.