Light from darkness: history of a hot dark sector

TL;DR

This paper investigates a hot hidden sector's role in early universe expansion, focusing on dark matter freeze-out, reheating, and experimental constraints within a dark QED framework, extending previous models to larger DM masses.

Contribution

It provides a detailed analysis of dark sector reheating, explores the parameter space for dark matter and dark photon, and extends the viable mass range up to 10^{11} GeV.

Findings

Current experiments can probe relativistic reheating scenarios.

New bounds on the temperature ratio T'/T are established.

The viable dark matter mass range extends to ~10^{11} GeV.

Abstract

We study a scenario in which the expansion of the Early Universe is driven by a hot hidden sector (HS) with an initial temperature $T'$ that is significantly higher than that of the visible sector (VS), $T' \gg T$. The latter is assumed to be made of Standard Model (SM) particles and our main focus is on the possibility that dark matter (DM) is part of the dominant HS and that its abundance is set by secluded freeze-out. In particular, we study the subsequent evolution and fate of the DM companion particle after freeze-out all the way toward reheating of the VS. To make this scenario more concrete, we work within dark QED, a framework in which the DM is a Dirac fermion and its companion, a massive dark photon; coupling between the SM and HS is through kinetic mixing. We provide a detailed and comprehensive numerical and analytical analysis of the different regimes of reheating of the VS. Extending and complementing the work of Coy et al on the``Domain of thermal dark matter candidates", we use our results to explore the viable parameter space of both the DM matter particle and its companion, here the dark photon. We show that current and future fixed target experiments can probe scenarios along which the expansion was driven by relativistic DM photons, a scenario dubbed relativistic reheating. We also set new bounds on the maximal temperature ratio $T'/T$ and argue for an extension of the domain toward very large DM masses, $m_{\rm dm} \sim 10^{11}$ GeV. These are possible assuming that DM annihilation is bounded by unitarity and that reheating of the VS occurs just before big bang nucleosynthesis. We also discuss some possible implications for (and constraints on) baryogenesis, including simple leptogenesis mechanisms, and how they may set additional constraints on the domain of DM candidates.