  A thermal relic, often referred to as a weakly interacting massive particle
(WIMP), is a particle produced during the early evolution of the Universe whose
present (relic) abundance depends only on its mass and its thermally averaged
annihilation cross section (annihilation rate factor) sigma*v_ann. Late time
WIMP annihilation has the potential to affect the cosmic microwave background
(CMB) power spectrum. Current observational constraints on the absence of such
effects provide bounds on the mass and the annihilation cross section of relic
particles that may, but need not be dark matter candidates. For a WIMP that is
a dark matter candidate, the CMB constraint sets an upper bound to the
annihilation cross section, leading to a lower bound to its mass that depends
on whether or not the WIMP is its own antiparticle. For a self-conjugate WIMP,
m_min = 50f GeV, where f is an electromagnetic energy efficiency factor. For a
non self-conjugate WIMP, the minimum mass is a factor of two larger. For a WIMP
that is a subdominant component of the dark matter density there is no bound on
its mass and the upper bound to its annihilation cross section imposed by the
CMB transforms into a lower bound to its annihilation cross section. These
results are outlined and quantified here using the latest CMB constraints for a
stable, symmetric (equal number of particles and antiparticles), WIMP whose
annihilation is s-wave dominated, and for particles that are, or are not, their
own antiparticle.
