Contents

# Contents

## Idea

What are called weakly interacting massive particles or “WIMP”s, for short, are hypothetical fundamental particles/field quanta which have mass but which, besides via gravity, interact only via the weak nuclear force, or via some yet weaker force, but in particular not via electromagnetism.

WIMPs used to be thought of as likely candidates for dark matter, a conclusion suggested by analysis of relic abundancies (see below), but more recently a series of direct detection null results puts that assumption increasingly into question – unless one assumes that the WIMPs interact strictly only via gravity, in which case they would have to be very massive (“WIMPzillas”, CKR 98, CCKR 01).

## Motivation via relic abundancy

In detail, the argument for WIMP dark matter proceeds as follows (recalled e.g. in CCKR 01):

The case for dark, nonbaryonic matter in the universe is today stronger than ever [1]. The observed large-scale structure suggests that dark matter (DM) accounts for at least 30% of the critical mass density of the universe $\rho_C = 3 H_0^2 M_{Pl}^2 / 8 \pi = 1.88 \times 1-^{-29} g cm^{-3}$, where $H_0 = 100 h km sec^{-1} Mpc^{-1}$ is the present Hubble constant and $M_{Pl}$ is the Planck mass.

The most familiar assumption is that dark matter is a thermal relic, i.e., it was initially in chemical equilibrium in the early universe. A particle species, $X$, tracks its equilibrium abundance as long as reactions which keep the species in chemical equilibrium can proceed on a timescale more rapid than the expansion rate of the universe, $H$. When the reaction rate becomes smaller than the expansion rate, the particle species can no longer track its equilibrium value. When this occurs the particle species is said to be “frozen out.”” The more strongly interacting the particle, the longer it stays in local thermal equilibrium and the smaller its eventual freeze-out abundance. Conversely, the more weakly interactingthe particle, the larger its present abundance. If freeze out occurs when the particles $X$ areno nrelativistic, the freeze-out value of the particle number per comoving volume $Y$ is related to the mass of the particle and its annihilation cross section (here characterized by $\sigma_0$) by $Y \propto (1/M_X M_{Pl} \sigma_0)$ where $M_X$ is the mass of the particle $X$. Since the contribution to $\Omega_X = \rho_X/\rho_C$ is proportional to $M_X Y$, the present contribution to $\Omega_X$ from a thermal relic roughly is independentof its mass and depends only upon the annihilation cross section. The cross section that results in $\Omega_X h^2\sim 1$ is of order $10^{-37 } cm^2$, which is of the order the weak scale.