Relic Density of Neutralino Dark Matter in Supergravity Models

We calculate the relic density ($\Omega_{\chi^0_1} h^2$) of neutralino dark matter in supergravity models with radiative electroweak symmetry breaking, for values of the Higgs sector parameter $\tan\beta \equiv v_2/v_1$ between the two infrared fixed points of the top quark Yukawa coupling, $\tan\beta \simeq 1.8$ and $\tan\beta \simeq 56$. For $\tan\beta \alt 10$, the CP-odd Higgs pseudoscalar ($A^0$) and the heavier CP-even Higgs scalar ($H^0$) are much heavier than the lightest neutralino ($\chi^0_1$), and the annihilation cross section of $\chi^0_1$ pairs is significantly enhanced only in the vicinity of the lighter CP-even Higgs boson ($h^0$) and the $Z$ boson poles. For $\tan\beta \agt 40$, $m_A$ and $m_H$ become comparable to $2m_{\chi^0_1}$, and the neutralino annihilation cross section can be significantly enhanced by the poles of the $H^0$ and the $A^0$ as well. For a cosmologically interesting relic density, $0.1 \alt \Omega_{\chi^0_1} h^2 \alt 0.5$, we find that the supergravity parameter space is tightly constrained. The mass spectra for supersymmetric particles are presented for unification mass parameters in the cosmologically interesting region.