Dark Radiation constraints on minicharged particles in models with a hidden photon

TL;DR

The paper tackles the cosmological impact of a hidden sector composed of minicharged fermions and a massless hidden photon, focusing on how their thermalization alters the dark radiation content captured by $N_eff$. By solving coupled SM–dark sector energy-transfer equations across production, decoupling, and other channels, the authors map $N_eff$ for MCP masses from ~100 keV to 10 GeV and minicharges from $10^{-11}$ to 1, for representative hidden gauge couplings. They combine Planck (CMB) constraints with updated BBN helium yields to exclude large regions of the MCP parameter space, while highlighting regions that could still accommodate a detectable dark-radiation signal in current or future data. The work provides precise predictions of dark radiation in strongly and weakly coupled regimes and outlines how upcoming measurements of $N_eff$ could probe GeV-scale MCPs in hidden-sector models.

Abstract

We compute the thermalization of a hidden sector consisting of minicharged fermions (MCPs) and massless hidden photons in the early Universe. The precise measurement of the anisotropies of the cosmic microwave background (CMB) by Planck and the relic abundance of light nuclei produced during big bang nucleosynthesis (BBN) constrain the amount of dark radiation of this hidden sector through the effective number of neutrino species, Neff. This study presents novel and accurate predictions of dark radiation in the strongly and weakly coupled regime for a wide range of model parameters. We give the value of Neff for MCP masses between 100 keV and 10 GeV and minicharges in the range 10^(-11)-1. Our results can be used to constrain MCPs with the current data and they are also a valuable indicator for future experimental searches, should the hint for dark radiation manifest itself in the next release of Planck's data.

Figures (8)

• Figure 1: Summary of constraints on fermionic MCPs in the mass/minicharge plane for $g'=0.1$. The results of this work are: the constraint on $N_\text{eff}$ during BBN (dark blue) and on $N_\text{eff}$ by Planck (light blue). We have also improved the bounds from white dwarves (WD), red giants (RG) and horizontal branch (HB) with respect to the originals by calculating the high mass behavior. The remaining bounds are taken from elsewhere: LHC Jaeckel:2012yz, DM Davidson:1991si, COLL Davidson:1991si, SLAC Prinz:1998ua, OPOS Badertscher:2006fm, TEX Gninenko:2006fi and CMB Dubovsky:2003ynDolgov:2013una (see also appendix \ref{['CMBupdate']}).
• Figure 2: Isocontours of $N_\text{eff}$ at the CMB epoch as a function of the hidden fermion mass $m_f$ and the minicharge $\epsilon$ for a value of the hidden gauge charge $g'=0.1$. Regions denoted with letters are further discussed in the main text. Dark green coloring denotes regions close to the SM value $N_\text{eff} =3$. Light green and yellow regions lie between $3.5-4.5$. Orange and red denote higher values $N_\text{eff} >4.5$. The red dotted line shows the 95% upper exclusion limit $N_\text{eff} =3.84$ (Planck+WP+highL+BAO) by Planck Ade:2013zuv.
• Figure 3: Evolution of the temperature ratios $T_\nu/T_{\gamma}$ (dashed) and $T_{\text{DS}} /T_{\gamma}$ (dot-dashed). The black line at 1 denotes thermalization with the SM. Left: region A ($m_f=0.1 \ \text{GeV}$, $\epsilon=10^{-9}$). Right: region B ($m_f=3.15 \ \text{GeV}$, $\epsilon=3 \times 10^{-7}$).
• Figure 4: Same as in figure\ref{['fig:CMBA']}. Left: region C ($m_f=100 \ \text{MeV}$, $\epsilon=3 \times 10^{-5}$). The dashed and dash-dotted lines lie on top of each other. Right: region D ($m_f=3 \ \text{MeV}$, $\epsilon=10^{-6}$).
• Figure 5: Same as in figure\ref{['fig:CMBA']}. Left: region E ($m_f=0.3 \ \text{MeV}$, $\epsilon=6\times 10^{-9}$). Right: region F ($m_f=31 \ \text{MeV}$, $\epsilon=3 \times 10^{-8}$).