Cosmic microwave background constraint on residual annihilations of relic particles - art. no. 023001

Energy injected into the cosmic microwave background at redshifts z less th an or similar to 10(6) will distort its spectrum permanently. In this paper we discuss the distortion caused by annihilations of relic particles. We u se the observational bounds on deviations from a Planck spectrum to constra in a combination of annihilation cross section, mass, and abundance. For pa rticles with an (s-wave) annihilation cross section (sigma \v \ > equivalen t to sigma (0)), the bound is f(m(X)/MeV)(-1)[(sigma (0)/06 x 10(-27) cm(3) s(-1))(Omega (X(X) over bar)h(2))(2)] < 0.2, where m(X) is the particle ma ss, <Omega>(X(X) over bar), is the fraction of the critical density the par ticle and its antiparticle contribute if they survive to the present time, h = H-0/100 km s(-1) Mpc(-1), H-0 is the Hubble constant, and f is the frac tion of the annihilation energy that interacts electromagnetically. We also compute the less stringent limits for p-wave annihilation. We update other bounds on residual annihilations and compare them to our CMB bound.