Probing Reheating Cosmology at FORMOSA and FPF: Cosmic Millicharged Background

<!--HTML-->We demonstrate that the searches for dark sector particles can provide probes of reheating scenarios, focusing on the Cosmic Millicharge Background (CmB) produced in the early universe. We discuss two scenarios of millicharge particles (mCPs), with or without kinetic mixing, as they have different theoretical motivations and cosmological signatures. The mCP without an accompanying dark photon can be an indirect test of GUTs and string compactifications, and we discuss its overproduction and CMB constraints with different reheating temperatures as an attempt to identify a region to search for it. The millicharged particle from kinetic mixing also has important constraints regarding different reheating temperatures. In both cases, FORMOSA and FPF provide excellent probes of these rehearing scenarios and can set limits on the maximum temperatures of the SM particles during the reheating process.