Dark Matter, Light Stops and Electroweak Baryogenesis

TL;DR

The paper investigates neutralino dark matter in the MSSM in the presence of a light stop, motivated by electroweak baryogenesis and current cosmological data. It identifies three distinct mechanisms—stop co-annihilation, Higgs-mediated annihilation, and Z-boson–mediated annihilation—that yield the observed relic density and examines collider and direct-detection tests under CP-conserving assumptions. It also maps out high-energy soft SUSY-breaking boundary conditions that can produce a viable low-energy spectrum, showing that a consistent EWBG–DM scenario is reachable with specific relations among gaugino masses, scalar masses, and trilinear couplings. The results emphasize the complementary roles of Tevatron/LHC searches and direct-detection experiments, with future linear colliders offering precision tests of the light stop and neutralino properties.

Abstract

We examine the neutralino relic density in the presence of a light top squark, such as the one required for the realization of the electroweak baryogenesis mechanism, within the minimal supersymmetric standard model. We show that there are three clearly distinguishable regions of parameter space, where the relic density is consistent with WMAP and other cosmological data. These regions are characterized by annihilation cross sections mediated by either light Higgs bosons, Z bosons, or by the co-annihilation with the lightest stop. Tevatron collider experiments can test the presence of the light stop in most of the parameter space. In the co-annihilation region, however, the mass difference between the light stop and the lightest neutralino varies between 15 and 30 GeV, presenting an interesting challenge for stop searches at hadron colliders. We present the prospects for direct detection of dark matter, which provides a complementary way of testing this scenario. We also derive the required structure of the high energy soft supersymmetry breaking mass parameters where the neutralino is a dark matter candidate and the stop spectrum is consistent with electroweak baryogenesis and the present bounds on the lightest Higgs mass.

Figures (8)

• Figure 1: Regions in the $\mu$--$M_1$ parameter space at which an acceptable value of cold dark matter develops. The green bands show the region where the neutralino relic density is consistent with the WMAP data. The black contours indicate cross section values for neutralino-proton scattering. Neutralino and stop mass contours are also shown. Here we set $\tan\beta=10$, $m_{\tilde{L}_2}=m_{\tilde{E}_2}=250{\rm~GeV}$, and the CP-odd Higgs mass has been chosen to be equal to 500 GeV.
• Figure 2: Same as Figure \ref{['fig:mssm1']}, except the CP-odd Higgs mass has been chosen to be equal to 300 GeV.
• Figure 3: Same as Figure \ref{['fig:mssm1']}, except the CP-odd Higgs mass has been chosen to be equal to 200 GeV.
• Figure 4: Same as Figure \ref{['fig:mssm2']}, except for $\tan\beta = 50$ and $m_{\tilde{L}_2}=m_{\tilde{E}_2}=1.1{\rm~TeV}$.
• Figure 5: Same as Figure \ref{['fig:mssm3']}, except for $m_{\tilde{U}_3}=-(35$ GeV$)^2$ and $m_{\tilde{Q}_3}=1.25$ TeV.