Non-thermal WIMPs as "Dark Radiation" in Light of ATACAMA, SPT, WMAP9 and Planck

TL;DR

The paper addresses hints of extra radiation in the early universe and proposes that a predominantly cold WIMP dark matter population could mimic this signal via a small non-thermal relativistic fraction produced in decays $X'\to DM+\gamma$. It derives a quantitative mapping between the non-thermal energy density and $N_{eff}$, and analyzes cosmological bounds from structure formation, BBN, and the CMB, which require $f \lesssim 0.01$ and $\tau \lesssim 10^4$ s. It then presents four illustrative DM models (spin-0, spin-1, spin-1/2, and a SUSY Bino/Gravitino) that realize $\Delta N_{eff} \sim 1$ only with a large mass hierarchy $M_{X'}/M_{DM} \gtrsim 4\times 10^5$, consistent with the bounds. The work highlights a tightly constrained but plausible alternative to extra neutrino-like radiation and connects to DM mass ranges around 10–100 GeV and related detection prospects.

Abstract

The Planck and WMAP9 satellites, as well as the ATACAMA and South Pole telescopes, have recently presented results on the angular power spectrum of the comic microwave background. Data tentatively point to the existence of an extra radiation component in the early universe. Here, we show that this extra component can be mimicked by ordinary WIMP dark matter particles whose majority is cold, but with a small fraction being non-thermally produced in a relativistic state. We present a few example theories where this scenario is explicitly realized, and explore the relevant parameter space consistent with BBN, CMB and Structure Formation bounds.

Figures (5)

• Figure 1: The parameter space defined by the mother particle $X^\prime$ lifetime ($\tau_{X^\prime}$) and the fraction of relativistically produced DM ($f$) times the mother-to-daughter mass ratio $M_{X^\prime}/M_{DM}$, and constraints from BBN and CMB. The shaded regions show BBN bounds on the non-thermal production of DM via the decay $X^{\prime} \rightarrow DM + \gamma$, corresponding to an excess relativistic degrees of freedom $\Delta N_{eff}=0.2,0.4,0.6,0.8,1$. The green curve represents the CMB bound (regions to the right of the curve are ruled out). We assume $M_{X^{\prime}} \gg M_{DM}$.
• Figure 2: Lower limits on the DM mass $M_{DM}$ for a model in which a heavy scalar particle decays into a spin-1 DM particle plus a photon, for different $\Delta N_{eff}$. The relevant effective operator for this model is given in Eq. (\ref{['eq:spin-0']}). We assume in this figure $f=0.01$ and a lifetime $\tau=10^4\,$s.
• Figure 3: Lower limits on the DM mass for a model in which a heavy spin-1 boson decays into a spin-1 DM particle plus a photon, for different $\Delta N_{eff}$. The relevant operator for this effective model is given in Eq. (\ref{['eq:spin-1']}). We employed $f=0.01$ and a lifetime $\tau=10^4\,$s. Note that the x-axis is multiplied by a factor $10^{-10}$.
• Figure 4: Lower limits on the DM mass for a model in which a heavy fermion decays into a spin-1/2 DM particle plus a photon, for different $\Delta N_{eff}$. The relevant operator for this effective model is given in Eq. (\ref{['Lfermion']}). We employed $f=0.01$ and a lifetime $\tau=10^4\,$s.
• Figure 5: Lower limit on the gravitino mass, for a supersymmetric model where a fraction $f$ of gravitinos are non-thermally produced by the decay of pure Binos. The relevant lifetime is derived from Eq. (\ref{['eqgravitino']}). As in the previous figures, we have used $f=0.01$ and .