Neutralino relic density in minimal supergravity with co-annihilations

TL;DR

The paper computes the neutralino relic density in the mSUGRA framework by including all $2\rightarrow2$ annihilations and co-annihilations, with relativistic thermal averaging implemented via the Gondolo–Gelmini formalism. Sparticle spectra are generated with ISAJET and matrix elements with CompHEP, and the relic density is obtained by solving the Boltzmann equation with a three-dimensional Monte Carlo integration to handle resonances and co-annihilation channels. The study finds that co-annihilations matter mainly at parameter-space edges, while at large $\tan\beta$ broad $A/H$ Higgs resonances create wide, low-$\Omega h^2$ corridors across substantial regions of parameter space, yielding $0.1<\Omega_{\tilde Z_1} h^2<0.3$ with relatively little fine-tuning in region (iv). The results are then contrasted with collider reach projections (LEP2, Tevatron, LHC, NLC), illustrating how cosmologically favored regions intersect with current and future experimental capabilities and highlighting the practical significance for guiding SUSY searches at high-energy colliders.

Abstract

We evaluate the relic density of neutralinos in the minimal supergravity (mSUGRA) model. All 2-->2 neutralino annihilation diagrams, as well as all processes 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 tan(beta), annihilation through very 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. Finally, we compare our results against the reach of various e+e- and hadron colliders for supersymmetric matter.