SuperWIMP Dark Matter Signals from the Early Universe

TL;DR

The paper introduces superWIMP dark matter, consisting of particles like gravitinos or KK gravitons produced in late decays of WIMPs, with decays at times $\tau \sim 10^{5}-10^{8}\ \text{s}$ that naturally yield the observed relic density $\Omega_{\text{DM}} \simeq 0.23$. Although superWIMPs interact only gravitationally, their decays inject energy into the early universe, leaving imprints on Big Bang Nucleosynthesis and the Cosmic Microwave Background that can be searched for and may even address the $^7$Li underabundance without upsetting D and CMB baryometry. The work provides decay-width formulas for neutralino and slepton WIMPs, shows that a favored Li7-destruction region exists around $\tau \sim 3\times10^{6}\ \text{s}$ and $\zeta_{\text{EM}} \sim 10^{-9}\ \text{GeV}$, and connects cosmological signatures to collider phenomenology, including metastable charged tracks in some scenarios. It also identifies CMB spectral distortions and future BBN measurements as tests of the scenario and calls for more hadronic-BBN studies for late decays; overall, superWIMPs offer a testable bridge between weak-scale physics and dark matter through early-universe signatures.

Abstract

Cold dark matter may be made of superweakly-interacting massive particles, superWIMPs, that naturally inherit the desired relic density from late decays of metastable WIMPs. Well-motivated examples are weak-scale gravitinos in supergravity and Kaluza-Klein gravitons from extra dimensions. These particles are impossible to detect in all dark matter experiments. We find, however, that superWIMP dark matter may be discovered through cosmological signatures from the early universe. In particular, superWIMP dark matter has observable consequences for Big Bang nucleosynthesis and the cosmic microwave background (CMB), and may explain the observed underabundance of 7Li without upsetting the concordance between deuterium and CMB baryometers. We discuss implications for future probes of CMB black body distortions and collider searches for new particles. In the course of this study, we also present a model-independent analysis of entropy production from late-decaying particles in light of WMAP data.

Figures (5)

• Figure 1: Predicted values of WIMP lifetime $\tau$ and electromagnetic energy release $\zeta_{\text{EM}} \equiv \varepsilon_{\text{EM}} Y_{\text{WIMP}}$ in the $\tilde{B}$ (left) and $\tilde{\tau}$ (right) WIMP scenarios for $m_{\text{SWIMP}} = 1~\text{GeV}$, $10~\text{GeV}$, …, $100~\text{TeV}$ (top to bottom) and $\Delta m \equiv m_{\text{WIMP}} - m_{\text{SWIMP}} = 1~\text{TeV}$, $100~\text{GeV}$, …, $100~\text{MeV}$ (left to right). For the $\tilde{\tau}$ WIMP scenario, we assume $\varepsilon_{\text{EM}} = \frac{1}{2} E_{\tau}$.
• Figure 2: The grid gives predicted values of WIMP lifetime $\tau$ and electromagnetic energy release $\zeta_{\text{EM}} \equiv \varepsilon_{\text{EM}} Y_{\text{WIMP}}$ in the $\tilde{B}$ (left) and $\tilde{\tau}$ (right) WIMP scenarios for $m_{\text{SWIMP}} = 100~\text{GeV}$, $300~\text{GeV}$, $500~\text{GeV}$, $1~\text{TeV}$, and $3~\text{TeV}$ (top to bottom) and $\Delta m \equiv m_{\text{WIMP}} - m_{\text{SWIMP}} = 600~\text{GeV}$, $400~\text{GeV}$, $200~\text{GeV}$, and $100~\text{GeV}$ (left to right). For the $\tilde{\tau}$ WIMP scenario, we assume $\varepsilon_{\text{EM}} = \frac{1}{2} E_{\tau}$. The analysis of BBN constraints by Cyburt, Ellis, Fields, and Olive Cyburt:2002uv excludes the shaded regions. The best fit region with $(\tau, \zeta_{\text{EM}}) \sim (3 \times 10^6~\text{s}, 10^{-9}~\text{GeV})$, where $^7$Li is reduced to observed levels by late decays of WIMPs to superWIMPs, is given by the circle.
• Figure 3: Contours of fractional entropy production $\Delta S/S_i$ from late decays in the $(\tau, \zeta_{\text{EM}})$ plane. Regions predicted by the superWIMP dark matter scenario and BBN excluded and best fit regions are given as in Fig. \ref{['fig:bbn']}.
• Figure 4: Contours of $\mu$, parameterizing the distortion of the CMB from a Planckian spectrum, in the $(\tau, \zeta_{\text{EM}})$ plane. Regions predicted by the superWIMP dark matter scenario, and BBN excluded and best fit regions are given as in Fig. \ref{['fig:bbn']}.
• Figure 5: Contours of constant $\tau$ (dashed, red) and constant $\zeta_{\text{EM}} = \varepsilon_{\text{EM}} Y_{\text{WIMP}}$ (solid, blue) in the $(m_{\text{SWIMP}}, \Delta m)$ plane in the $\tilde{B}$ (left) and $\tilde{\tau}$ (right) WIMP scenarios. The regions with BBN preferred values $(\tau, \zeta_{\text{EM}}) \sim (3 \times 10^6~\text{s}, 10^{-9}~\text{GeV})$ are given by the circles. For the $\tilde{\tau}$ WIMP scenario, the solid circle is favored if $\varepsilon_{\text{EM}} = \frac{1}{2} E_{\tau}$; the dashed circles are favored if $\varepsilon_{\text{EM}} = \frac{1}{3} E_{\tau}$ or $\varepsilon_{\text{EM}} = E_{\tau}$.