Introduction

The class of weakly interacting massive particles (WIMPs) is the leading category of particle dark matter candidates: on the one hand, the mechanism of thermal freeze-out of a stable WIMP χ leads to a non-relativistic relic population whose relative matter density can be approximated as Ωχh2 ≈ 3 × 10−27 cm3 s−1/ 〈σAnnv〉, where h is the Hubble constant in units of 100 km s−1 Mpc−1, Ωχ is the ratio of the χ density over the critical density and 〈σAnnv〉 is the thermally averaged χ pair-annihilation cross-section. If the new physics connected to the WIMP is at the electroweak scale, mEW, one can estimate 〈σAnnv〉 ≈ ≈ 10−25 cm3 s−1, giving a thermal leftover χ population that lies in the same ballpark as the dark matter abundance inferred from cosmic microwave background anisotropies, large-scale structure and other astronomical observations [1266]. On the other hand, numerous motivated particle physics extensions to the Standard Model encompass a stable WIMP, including supersymmetry [1184] and universal extra dimensions (UED) [1126], in virtue of unbroken discrete symmetries (R-parity in the case of supersymmetry, and Kaluza–Klein parity in the case of UED).

Since WIMPs were once kept in thermal equilibrium by pair annihilations into Standard Model particles and inverse WIMP pair production processes, even in today's cold Universe, occasionally, WIMPs can pair annihilate, giving rise to energetic, stable ‘ordinary’ Standard Model particles.