Asymmetric Dark Matter and Dark Radiation

TL;DR

The paper investigates asymmetric dark matter (ADM) scenarios in which a light mediator decays into dark radiation (DR), yielding extra relativistic energy density quantified by $\Delta N_{ m eff}$ and altering cosmological evolution. It develops a largely model-independent dark sector parametrization with heavy and light degrees of freedom $g_h$ and $g_\ell$ decoupling at temperature $T_d$, deriving $\Delta N_{ m eff}$ predictions for high and low $T_d$ and providing Planck-era sensitivity forecasts. It also analyzes how DM-DR interactions imprint on the matter power spectrum, deriving constraints on interaction cross sections $Q_0$ and $Q_2$ from current data via CAMB/MCMC and outlining expected Planck improvements, with implications for ADM model-building. Overall, cosmological probes of DR offer a complementary route to reveal DS structure and to test ADM scenarios beyond collider and direct searches.

Abstract

Asymmetric Dark Matter (ADM) models invoke a particle-antiparticle asymmetry, similar to the one observed in the Baryon sector, to account for the Dark Matter (DM) abundance. Both asymmetries are usually generated by the same mechanism and generally related, thus predicting DM masses around 5 GeV in order to obtain the correct density. The main challenge for successful models is to ensure efficient annihilation of the thermally produced symmetric component of such a light DM candidate without violating constraints from collider or direct searches. A common way to overcome this involves a light mediator, into which DM can efficiently annihilate and which subsequently decays into Standard Model particles. Here we explore the scenario where the light mediator decays instead into lighter degrees of freedom in the dark sector that act as radiation in the early Universe. While this assumption makes indirect DM searches challenging, it leads to signals of extra radiation at BBN and CMB. Under certain conditions, precise measurements of the number of relativistic species, such as those expected from the Planck satellite, can provide information on the structure of the dark sector. We also discuss the constraints of the interactions between DM and Dark Radiation from their imprint in the matter power spectrum.

Figures (5)

• Figure 1: The $1 \sigma$ range for the number of heavy degrees of freedom $g_h$ required to heat the light sector in order to account for the presently preferred number of extra effective neutrino species during CMB $\Delta N_{\rm eff} = 0.85 \pm 0.62$ and as a function of the decoupling temperature $T_d$.
• Figure 2: The $1 \sigma$ range for the number of heavy degrees of freedom $g_h$ required to heat the light sector in order to account for a given number of extra effective neutrino species during CMB and as a function of the decoupling temperature $T_d$. The bands correspond to different Planck constraint forecasts as described in the text.
• Figure 3: Upper panel: Matter power spectrum for a $\Lambda$CDM model (thick red curve). The blue (dotted) dotted-dashed lines depict the matter power spectrum for $\Delta N_{\rm eff} =1$ within a (non) interacting scenario with constant cross section and $Q_0=10^{-32}$ cm$^2$ MeV$^{-1}$. The green (long) short dashed lines depict the matter power spectrum for $\Delta N_{\rm eff} =3$ within a (non) interaction scenario. The lower panel shows the analogous but for an interaction cross section $\propto 1/a^2$ and $Q_2=10^{-41}$ cm$^2$ MeV$^{-1}$.
• Figure 4: The left, right upper panels and the lower panel show the 1$\sigma$ and 2$\sigma$ contours in the ($\sigma_8$, $Q_0$), ($\Omega_{\rm {DM}}h^2$, $Q_0$) and (Age, $Q_0$) planes, respectively. The interacting parameter $Q_0$ is in units of $10^{-34}$ cm$^2$ MeV$^{-1}$ and the age of the universe is in Gyrs. The red, blue and green contours denote the three possible interacting scenarios explored here with one, two and three sterile neutrino species in the DR sector.
• Figure 5: The left, right upper panels and the lower panel show the 1$\sigma$ and 2$\sigma$ contours in the ($\sigma_8$, $Q_2$), ($\Omega_{\rm {DM}}h^2$, $Q_2$) and (Age, $Q_2$) planes, respectively. The interacting parameter $Q_2$ is in units of $10^{-43}$ cm$^2$ MeV$^{-1}$ and the age of the universe is in Gyrs. The red, blue and green contours denote the three possible interacting scenarios explored here with one, two and three sterile neutrino species in the DR sector.