Relic density of neutralinos in minimal supergravity

We evaluate the relic density of neutralinos in the minimal supergravity (mSUGRA) model. All 2 → 2 neutralino annihilation diagrams, as well as all initial states involving sleptons, charginos, neutralinos, and third generation squarks are included. Relativistic thermal averaging of the velocity times cross sections is performed. We find that co-annihilation effects are only important on the edges of the model parameter space, where some amount of fine-tuning is necessary to obtain a reasonable relic density. Alternatively, at high tanβ, annihilation through the broad Higgs resonances gives rise to an acceptable neutralino relic density over broad regions of parameter space where little or no fine-tuning is needed.