High Energy Cosmic Rays from the Decay of Gravitino Dark Matter

TL;DR

The paper addresses the origin of dark matter and the EGRET gamma-ray and HEAT positron anomalies by proposing gravitino dark matter that decays through bilinear R-parity violation. It develops the framework for gravitino decays, computes decay spectra with PYTHIA, and propagates the resulting gamma-ray and positron fluxes in the cosmological and Galactic contexts, including realistic backgrounds. The authors show that the observed anomalies can be concurrently explained for ${\tau_{3/2} \sim 10^{26}{-}10^{27}}$ s and ${m_{3/2} \gtrsim 80}$ GeV, with a mild range of RPV couplings ${10^{-11} \lesssim {\kappa_i} \lesssim 10^{-7}}$. They also discuss future testability with GLAST and PAMELA and note implications for LHC phenomenology due to short-lived MSSM-LSPs in this scenario, as well as possibility of extending the idea to other DM candidates.

Abstract

We study gamma ray and positron in high energy cosmic ray from the decay of the gravitino dark matter in the framework of supersymmetric model with R-parity violation. Even though R-parity is violated, the lifetime of the gravitino, which is assumed to be the lightest superparticle, can be longer than the present age of the universe if R-parity violating interactions are weak enough. In such a case, gravitino can be dark matter of the universe and its decay produces high energy cosmic rays. We calculate the fluxes of gamma ray and positron from the decay of the gravitino dark matter and discuss implications of such a scenario to present and future observations. In particular, we show that excesses of the fluxes of gamma ray and positron observed by EGRET and HEAT experiments, respectively, can be simultaneously explained as the cosmic rays from the decay of the gravitino dark matter.

Figures (9)

• Figure 1: Diagrams of gravitino decay.
• Figure 2: Lifetime of gravitino as a function of gravitino mass. Here, we take $\tan \beta=10, \ m_h=115 \ {\rm GeV}, m_{\tilde{B}}=1.5m_{3/2}, \ m_{\tilde{\nu}}=2m_{3/2}$ under large Higgsino-mass limit, and assume GUT relation among gaugino masses.
• Figure 3: Branching ratio for each decay mode. Lines with the indices "$Wl$," "$Z\nu$," "$h\nu$" and "$\gamma\nu$" show $Br(\psi_\mu\rightarrow W^+l^-)+Br(\psi_\mu\rightarrow W^-l^+)$, $Br(\psi_\mu\rightarrow Z\nu) +Br(\psi_\mu\rightarrow Z\bar{\nu})$, $Br(\psi_\mu\rightarrow h\nu) +Br(\psi_\mu\rightarrow h\bar{\nu})$, and $Br(\psi_\mu\rightarrow\gamma\nu) +Br(\psi_\mu\rightarrow\gamma\bar{\nu})$, respectively. (Summation over the generation index is implicit.) Here, we take the MSSM parameters used in Fig. \ref{['fig:tau']}
• Figure 4: Left: Gamma-ray flux from gravitino decay $[E^2 dJ_{\gamma}/dE]_{\rm DM}$("total"). Lines with "$\gamma\nu$," "$Z\nu$," and "$Wl$" are contributions from each decay mode. Right: Positron fraction $[\Phi_{e^+}]_{\rm DM}/([\Phi_{e^+}]_{\rm tot}+[\Phi_{e^-}]_{\rm tot})$. Lines with "$e$," "$\mu$," and "$\tau$" show the results for the case that the gravitino mainly decays to first, second, and third generation leptons, respectively. For both of the figures, we take $m_{3/2}=150~{\rm GeV}$, $\tau_{3/2}=1.0\times 10^{26}~{\rm sec}$, and the MSSM parameters used in Fig. \ref{['fig:tau']}.
• Figure 5: Gamma-ray flux (left figure) and positron fraction (right figure). Here, we take $m_{3/2}=150~{\rm GeV}, \tau_{3/2}=2.2 \times 10^{26}~{\rm sec}$, and MSSM parameters as Fig. \ref{['fig:tau']}.