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 ) \langle \sigma v \rangle _ { 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 } = 50 f { GeV } , where f \leq 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 .