Reheating the FCC: Probing Early Matter Domination with Long-Lived Particles

Abstract

We study a GeV-scale Higgs-portal scalar $\varphi$ that can dominate the early Universe and later decay into Standard Model states, ending an early matter-dominated era. Ordinary cosmic reheating is the minimal example of this scenario. The small Higgs mixing makes $\varphi$ a long-lived particle (LLP), linking the reheating temperature $T_{\text{rh}}$ to displaced-decay signatures at colliders. Including finite-temperature suppression of the decay width, we map FCC-hh LLP sensitivity onto the $[m_{\varphi },T_{\text{rh}}]$ plane. We find that FCC-hh displaced searches could probe GeV-scale transition temperatures, up to the electroweak scale within the broken-phase Higgs-portal description.

Comments: 22 pages, 12 figures