Gravitino Dark Matter and Cosmological Constraints

Abstract

The gravitino is a promising candidate for cold dark matter. We study cosmological constraints on scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP). We obtain new results for the hadronic nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into the gravitino, the associated lepton, and a quark-antiquark pair. The bounds from the observed dark matter density are refined by taking into account gravitinos from both late NLSP decays and thermal scattering in the early Universe. We examine the present free-streaming velocity of gravitino dark matter and the limits from observations and simulations of cosmic structures. Assuming that the NLSP sleptons freeze out with a thermal abundance before their decay, we derive new bounds on the slepton and gravitino masses. The implications of the constraints for cosmology and collider phenomenology are discussed and the potential insights from future experiments are outlined. We propose a set of benchmark scenarios with gravitino dark matter and long-lived charged NLSP sleptons and describe prospects for the Large Hadron Collider and the International Linear Collider.

Figures (20)

• Figure 1: The lifetime of the stau NLSP as a function of its mass $m_{\tilde{\tau}}$ for values of the gravitino mass $m_{\widetilde{G}}$ ranging from 10 MeV up to 1 TeV.
• Figure 2: The electromagnetic energy release from the 2-body decay of a single stau NLSP as a function of $m_{\tilde{\tau}}$ for $m_{\widetilde{G}} \lesssim 1~\mathrm{GeV}$ and $m_{\widetilde{G}} = 100~\mathrm{GeV}$, $300~\mathrm{GeV}$, and $1~\mathrm{TeV}$. The solid, dashed, and dotted lines are obtained with $\epsilon_{\mathrm{em}}({\widetilde{\tau}}\to\tau\,{\widetilde{G}}) = E_{\tau}$, $0.5\,E_{\tau}$, and $0.3\,E_{\tau}$, respectively.
• Figure 3: The 4-body NLSP decay ${\widetilde{\tau}}_{\mathrm R} \to \tau {\widetilde{G}} \mathrm{q}\bar{\mathrm{q}}$
• Figure 4: The branching ratio of the 4-body decay ${\widetilde{\tau}}_{\mathrm{R}} \to \tau {\widetilde{G}} \mathrm{q} \bar{\mathrm{q}}$ as a function of $m_{\tilde{\tau}}$ for $m_{\widetilde{G}} \lesssim 1~\mathrm{GeV}$ (left) and $m_{\widetilde{G}}=100~\mathrm{GeV}$ (right). The solid lines give the full results including $\gamma^*$--$Z^*$ interference. The dashed and dotted lines show the contributions from pure $\gamma^*$ and pure $Z^*$ exchange, respectively. The curves are obtained with $m_{\mathrm{q}\bar{\mathrm{q}}}^{\mathrm{cut}} = 2~\mathrm{GeV}$ and a bino mass of $m_{\tilde{B}} = 1.1\,m_{\tilde{\tau}}$. For comparison, the previous estimates of the branching ratio inferred from a real Z boson (i.e., in the zero width approximation) alone Feng:2004zu coincide basically with the dotted lines.
• Figure 5: The branching ratio of the 4-body decay ${\widetilde{\tau}}_{\mathrm{R}} \to \tau {\widetilde{G}} \mathrm{q} \bar{\mathrm{q}}$ as a function of $m_{\tilde{\tau}}$ for $m_{\widetilde{G}} \lesssim 1~\mathrm{GeV}$ and $m_{\widetilde{G}} = 0.1$, $0.2$, $0.3$, $0.5$, $0.7$, and $1~\mathrm{TeV}$ (from the left to the right). The solid lines present the full results and the dotted lines the contributions from pure $Z^*$ exchange for comparison with the previous estimates. The curves are obtained with $m_{\mathrm{q}\bar{\mathrm{q}}}^{\mathrm{cut}} = 2~\mathrm{GeV}$ and a bino mass of $m_{\tilde{B}} = 1.1\,m_{\tilde{\tau}}$.