  If dark matter (DM) is a weakly interacting massive particle (WIMP) that is a
thermal relic of the early Universe, then its total self-annihilation cross
section is revealed by its present-day mass density. The canonical thermally
averaged cross section for a generic WIMP is usually stated as 3*10^-26
cm^3s^-1, with unspecified uncertainty, and taken to be independent of WIMP
mass. Recent searches for annihilation products of DM annihilation have just
reached the sensitivity to exclude this canonical cross section for 100%
branching ratio to certain final states and small WIMP masses. The ultimate
goal is to probe all kinematically allowed final states as a function of mass
and, if all states are adequately excluded, set a lower limit to the WIMP mass.
Probing the low-mass region is further motivated due to recent hints for a
light WIMP in direct and indirect searches. We revisit the thermal relic
abundance calculation for a generic WIMP and show that the required cross
section can be calculated precisely. It varies significantly with mass at
masses below 10 GeV, reaching a maximum of 5.2*10^-26 cm^3s^-1 at masses around
0.3 GeV, and is 2.2*10^-26 cm^3s^-1 with feeble mass-dependence for masses
above 10 GeV. These results, which differ significantly from the canonical
value and have not been taken into account in searches for annihilation
products from generic WIMPs, have a noticeable impact on the interpretation of
present limits from Fermi-LAT and WMAP+ACT.
