SuperWIMP Gravitino Dark Matter from Slepton and Sneutrino Decays

TL;DR

This work investigates gravitino SWIMP dark matter produced in late decays of slepton or sneutrino NLSPs, focusing on the hadronic energy release from three-body decays that proceed via $Z$, $W$, or virtual photons. The authors compute hadronic branching fractions, apply BBN and CMB constraints on both EM and hadronic energy injection, and map viable regions in the $(m_{\tilde{G}}, \delta m)$ plane. They find that hadronic constraints are a major limiting factor, but substantial parameter space remains where the gravitino can constitute all non-baryonic dark matter, with potential implications for the Li-7 problem and collider signatures of long-lived NLSPs. The study also discusses how the results extend to other gravitationally interacting SWIMPs and the prospects for distinguishing scenarios with R-parity violation or extra dimensions.

Abstract

Dark matter may be composed of superWIMPs, superweakly-interacting massive particles produced in the late decays of other particles. We focus on the case of gravitinos produced in the late decays of sleptons or sneutrinos and assume they are produced in sufficient numbers to constitute all of non-baryonic dark matter. At leading order, these late decays are two-body and the accompanying energy is electromagnetic. For natural weak-scale parameters, these decays have been shown to satisfy bounds from Big Bang nucleosynthesis and the cosmic microwave background. However, sleptons and sneutrinos may also decay to three-body final states, producing hadronic energy, which is subject to even more stringent nucleosynthesis bounds. We determine the three-body branching fractions and the resulting hadronic energy release. We find that superWIMP gravitino dark matter is viable and determine the gravitino and slepton/sneutrino masses preferred by this solution to the dark matter problem. In passing, we note that hadronic constraints disfavor the possibility of superWIMPs produced by neutralino decays unless the neutralino is photino-like.

Figures (6)

• Figure 1: Summary of the leading constraints from D and $^4$He on late time EM and hadronic energy injection into the early Universe. Constraints from $^3$He and $^6$Li may also be important. (See text.)
• Figure 2: (a) Lifetime (in seconds) for $\tilde{l}_{L,R}$, $\tilde{\nu}$ NLSPs and (b) EM energy release (in GeV) for the $\tilde{\tau}_R$ NLSP as functions of gravitino mass $m_{\tilde{G}}$ and mass difference $\delta m = m_{\tilde{l}} - m_{\tilde{G}} - m_Z$. For (b), we take $\epsilon_{\text{E}M}=0.5 E_{\text{total}}$, $B_{\text{EM}}=1$, and $\Omega_{\text{SWIMP}} = 0.23$.
• Figure 3: Feynman diagrams for slepton decays $\tilde{l}\to l' V \tilde{G}$ leading to hadronic energy. $\chi$ in diagram (c) is a neutralino or chargino. When kinematically accessible, $V$ is a $Z$ or $W$ boson. For small $\Delta m < m_Z, m_W$, the leading hadronic decay is $\tilde{l}\to l \gamma^* \tilde{G}$, followed by $\gamma^* \to q \bar{q}$.
• Figure 4: Hadronic branching fractions for (left) $\tilde{\tau}_R$ NLSPs and (right) $\tilde{\nu}$ NLSPs as functions of the NLSP decay lifetime. Along each curve, the NLSP mass is fixed at the value indicated and the gravitino mass varies. The neutralino/chargino parameters are chosen to be $M_1=2 m_{\text{NLSP}}$, $M_2=\mu=4 m_{\text{NLSP}}$, and $\tan\beta=10$. For the $\tilde{\tau}_R$ case, the dashed line gives the hadronic branching fraction resulting from $\tilde{\tau}_R \to \tau \tilde{G}$, where the tau decays to a meson, and the meson interacts hadronically before decaying. We take the meson rest lifetime to be $3\times 10^{-8}~\text{s}$.
• Figure 5: Hadronic energy releases $\xi_{\text{had}} \equiv \epsilon_{\text{had}} B_{\text{had}} Y_{\text{NLSP}}$ as a function of decay lifetime $\tau_{\text{NLSP}}$ for (left) $\tilde{\tau}_R$ NLSPs and (right) $\tilde{\nu}$ NLSPs. Along each curve, $m_{\tilde{G}}$ is held fixed and $\Delta m \equiv m_{\text{NLSP}} - m_{\tilde{G}}$ varies. Particular values of $\Delta m$ are marked by the symbols indicated. The gravitino mass is $m_{\tilde{G}}=10, 10^2, 10^3, 10^4$ and $10^5$ GeV for the curves ordered from left to right by their value at $\Delta m = 100~\text{GeV}$. Constraints from BBN Kawasaki:2004yh, with and without ${}^6$Li/H, are also shown; regions above the curves are disfavored. We have assumed $\Omega_{\text{SWIMP}} = 0.23$ and $\epsilon_{\text{had}}=\frac{1}{3} (m_{\text{NLSP}}-m_{\tilde{G}})$.