Towards An Accurate Calculation of the Neutralino Relic Density

We compute the neutralino relic density in the minimal supersymmetric standard model by using exact expressions for the neutralino annihilation cross section into all tree-level final states, including all contributions and interference terms. We find that several final states may give comparable contributions to the relic density, which illustrates the importance of performing a complete calculation. We compare the exact results with those of the usual expansion method and demonstrate a sizeable discrepancy (of more than 10%) over a significant range of the neutralino mass of up to several tens of GeV which is caused by the presence of resonances and new final-state thresholds. We perform several related checks and comparisons. In particular, we find that the often employed approximate iterative procedure of computing the neutralino freeze-out temperature gives generally very accurate results, except when the expansion method is used near resonances and thresholds.