Calculations of Neutralino-Stau Coannihilation Channels and the Cosmologically Relevant Region of MSSM Parameter Space

TL;DR

The paper tackles how neutralino-stau coannihilation (and related slepton coannihilations) alters the cosmological relic density of a neutralino LSP within the CMSSM. It develops a framework that adds coannihilation channels to the standard relic-density calculation through an effective cross section $\langle \sigma_{\rm eff} v_{\rm rel}\rangle$, and provides analytic expressions for many relevant processes. The key result is that including coannihilations relaxes the cosmological upper bound on the LSP mass from $\sim 200$ GeV to $\sim 600$ GeV in the CMSSM, with a qualitatively similar effect in the general MSSM, thereby expanding the viable SUSY dark-matter parameter space and influencing LHC/LEP constraints. This work thus links detailed particle-physics cross sections to cosmological constraints and has practical implications for dark-matter searches and SUSY phenomenology.

Abstract

Assuming that the lightest supersymmetric particle (LSP) is the lightest neutralino, we present a detailed exploration of neutralino-stau coannihilation channels, including analytical expressions and numerical results. We also include neutralino coannihilations with the selectron and smuon. We evaluate the implications of coannihilations for the cosmological relic density of the LSP, which is assumed to be stable, in the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which the soft supersymmetry-breaking parameters are universal at the supergravity GUT scale. We evaluate the changes due to coannihilations in the region of the MSSM parameter space that is consistent with the cosmological upper limit on the relic LSP density. In particular, we find that the upper limit on the mass of the neutralino is increased from about 200 GeV to about 600 GeV in the CMSSM, and estimate a qualitatively similar increase for gauginos in the general MSSM.

Figures (8)

• Figure 1: The separate contributions to the cross sections $\hat{\sigma}\equiv a+{1\over2}b x$ for $x=T/m_{\widetilde{\chi}}=1/23$ and $m_0=120{\rm \, Ge V}$, as functions of $m_{1\!/2}$: a)$\tilde{\tau}\tilde{\tau}^*$, b)$\tilde{\tau}{\widetilde{\chi}}$, and c) other interactions. For comparison, the thick dotted line is the ${\widetilde{\chi}} {\widetilde{\chi}}$ cross section.
• Figure 2: As in Fig. \ref{['fig:ss']}, but for the choice $m_0=200$ GeV.
• Figure 3: As in Fig. \ref{['fig:ss']}, but for the choice $\tan\beta=10$.
• Figure 4: As in Fig. \ref{['fig:ss']}, but for the choice $\tan\beta=10$, and $A_0=-3m_{1\!/2}$.
• Figure 5: The separate contributions to the cross sections $\hat{\sigma}_{\rm eff}$ for $x=T/m_{\widetilde{\chi}}=1/23$, as functions of $\Delta M\equiv(m_{\tilde{\tau}_R}- m_{\widetilde{\chi}})/m_{\widetilde{\chi}}$, with a) $(m_{1\!/2}, \tan \beta) = (500$ GeV$, 3)$, b) =$(500$ GeV$,10)$, c) =$(300$ GeV$,3)$, and d) =$(1000$ GeV$,3)$.