Dark Radiation constraints on minicharged particles in models with a hidden photon (1311.2600v3)

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.

• The paper calculates how MCPs contribute to dark radiation, affecting N_eff and the Universe’s thermal history.
• It uses CMB and BBN data to constrain MCP parameters, ruling out low masses (below 16 MeV) for sizeable minicharge values.
• The study establishes a framework for exploring hidden photon models, guiding future experimental and theoretical research beyond the Standard Model.

Overview of Dark Radiation Constraints on Minicharged Particles in Models with a Hidden Photon

The paper presented by Hendrik Vogel and Javier Redondo explores the cosmological implications of minicharged particles (MCPs) and hidden photons, an extension to the Standard Model with an unbroken hidden U(1) gauge symmetry. The primary objective is to calculate the thermalization and subsequent dark radiation contribution of these particles during the early Universe and its impact on Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) anisotropies as measured by Planck.

Model Framework

The authors consider a hidden sector comprised of minicharged fermions with mass between 100 keV and 10 GeV, alongside massless hidden photons. MCPs acquire a small effective electric charge through kinetic mixing between the hidden photon and the hypercharge boson of the Standard Model, parameterized by ε. The paper explores a range of minicharges from $10^{-11}$ to 1, using a selection of hidden gauge couplings (g') to cover different potential physical scenarios.

Calculations and Constraints

The work provides a detailed thermal history of the Universe involving the hidden sector, accounting for interactions such as fermion pair annihilation and plasmon decay. The significant emphasis is laid on the effective number of relativistic species, $N_{\text{eff}}$, a critical observable from the CMB data. The contribution of MCPs and hidden photons to $N_{\text{eff}}$ is calculated for varying MCP masses and minicharges.

The analysis reveals that MCPs can contribute significantly to dark radiation under certain parameters, leading to deviations in $N_{\text{eff}}$ from the expected value of 3.046, which includes contributions only from active neutrinos in the standard cosmological model. These deviations constrain MCP masses to be above the MeV scale for most values of ε and can reach up to GeV if ε approaches unity.

Another aspect explored is the impact on BBN, where the presence of additional relativistic energy density alters the expansion rate, potentially affecting the relic abundances of light nuclei, particularly He-4. Accurate calculations show consistency with constraints on $Y_p < 0.263$ to rule out MCPs with $m_f < 16$ MeV for minicharges larger than $1.4 \times 10^{-8}$.

Implications and Future Directions

The constraints derived from the data of Planck and BBN significantly disfavour this class of particles with masses less than a GeV unless their minicharge is extremely small, thus narrowing the parameter space for MCPs substantially. This work provides a well-detailed map for future explorations and experimental verifications of hidden sectors and validates the utility of cosmological data in constraining subatomic physics beyond the Standard Model.

As future research develops, experimental bounds on MCPs could improve further, leading to more stringent constraints or potentially detecting such particles, assuming future CMB data continues to refine the precision on $N_{\text{eff}}$. The paper also lays the groundwork for investigating other hidden sector models and their observational signatures, serving as a critical input for exploring physics at the interface of particle physics and cosmology.

This paper not only highlights the contribution of MCPs to the dark radiation but also sets an example of how comprehensive theoretical calculations coupled with precise experimental data can bridge insights into physics beyond the Standard Model and contribute to our understanding of the early Universe.