Superweakly Interacting Massive Particle Solutions to Small Scale Structure Problems

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 super-WIMP dark matter, produced with large velocity in late decays at times ${10}^5–{10}^8\mathrm{s}$. As analyzed by Kaplinghat in a more general setting, we find that super-WIMPs 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.

Figure 1

Preferred regions (shaded) of phase space density $Q$ and free-streaming length ${\lambda }_{\mathrm{FS}}$ in the $(m_{\mathrm{SWIMP}},\Delta m)$ plane, where $\Delta m\equiv m_{\mathrm{NLSP}}-m_{\mathrm{SWIMP}}$, for gravitino super-WIMP with a sneutrino NLSP. The regions under both bands are disfavored. In the regions above both bands, super-WIMP dark matter becomes similar to CDM; representative values of $Q$ and ${\lambda }_{\mathrm{FS}}$ are shown. Contours of typical lifetimes ${\tau }_{\tilde{\nu }}$ are also shown. We have assumed ${\Omega }_{\mathrm{SWIMP}}{h}^2=0.11$.

Figure 2

Same as in Fig. 1, but for gravitino super-WIMP with a photino NLSP.