CMB constraints on the thermal WIMP mass and annihilation cross section

Abstract

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) $⟨\ensuremath{\sigma}v{⟩}_{\text{ann}}$. 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 be, 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, ${m}_{\mathrm{min}}=50f\text{ }\text{ }\mathrm{GeV}$, where $f\ensuremath{\le}1$ 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.