SuperWIMP Solutions to Small Scale Structure Problems

Abstract

Collisionless, cold dark matter in the form of weakly-interacting massive particles (WIMPs) is well-motivated in particle physics, naturally yields the observed relic density, and successfully explains structure formation on large scales. On small scales, however, it predicts too much power, leading to cuspy halos, dense cores, and large numbers of subhalos, in apparent conflict with observations. We consider superWIMP dark matter, produced with large velocity in late decays at times 10^5 s - 10^8 s. As analyzed by Kaplinghat in a more general setting, we find that superWIMPs have sufficiently large free-streaming lengths and low phase space densities to help resolve small scale structure problems while preserving all of the above-mentioned WIMP virtues.

Figures (2)

• Figure 1: Preferred regions (shaded) of phase space density $Q$ and free-streaming length $\lambda_{\text{FS}}$ in the $(m_{\text{SWIMP}}, \Delta m)$ plane, where $\Delta m \equiv m_{\text{NLSP}} - m_{\text{SWIMP}}$, for gravitino superWIMPs with a sneutrino NLSP. The regions under both bands are disfavored. In the regions above both bands, superWIMP dark matter becomes similar to CDM; representative values of $Q$ and $\lambda_{\text{FS}}$ are shown. Contours of typical lifetimes $\tau_{\tilde{\nu}}$ are also shown. We have assumed $\Omega_{\text{SWIMP}}h^2=0.11$.
• Figure 2: Same as in Fig. \ref{['sneu']}, but for gravitino superWIMPs with a photino NLSP.