Antimatter Signatures of Gravitino Dark Matter Decay

TL;DR

This work assesses gravitino dark matter with broken R-parity as a unifying framework for cosmology and indirect detection. By deriving a two-component source term $Q(E,\vec{r})=\frac{\rho(\vec{r})}{m_{3/2}\tau_{3/2}}\frac{dN}{dE}$ and solving the Galactic diffusion equation, it predicts positron and antiproton fluxes from $\psi_{3/2}$ decays, fixing $m_{3/2}=150$ GeV and $\tau_{3/2}=1.3\times10^{26}$ s to align with the EGRET gamma-ray excess. The analysis yields a robust bump in the positron fraction above $\sim$7 GeV, compatible with HEAT data despite astrophysical uncertainties, but generally predicts antiproton fluxes higher than observations unless propagation is strongly constrained (MIN scenario). The results suggest that decaying DM with gauge-boson final states can produce multi-channel signatures, motivating future measurements by GLAST (Fermi) and PAMELA/AMS-02 to test this scenario.

Abstract

The scenario of gravitino dark matter with broken R-parity naturally reconciles three paradigms that, albeit very well motivated separately, seem to be in mutual conflict: supersymmetric dark matter, thermal leptogenesis and standard Big Bang nucleosynthesis. Interestingly enough, the products of the gravitino decay could be observed, opening the possibility of indirect detection of gravitino dark matter. In this paper, we compute the positron and the antiproton fluxes from gravitino decay. We find that a gravitino with a mass of 150 GeV and a lifetime of 10^26 s could simultaneously explain the EGRET anomaly in the extragalactic diffuse gamma ray background and the HEAT excess in the positron fraction. However, the predicted antiproton flux tends to be too large, although the prediction suffers from large uncertainties and might be compatible with present observations for certain choices of propagation parameters.

Figures (5)

• Figure 1: Interstellar positron flux from the decay of gravitinos with $m_{3/2}\simeq 150\,{\rm GeV}$ and $\tau_{3/2}\simeq 1.3\times 10^{26}\,{\rm s}$. In the left plot we assume the M2 diffusion model (see Table \ref{['tab:param-positron']}) and we study the sensitivity of the positron flux to various halo profiles. On the other hand, in the right plot we assume a NFW halo profile and we study the sensitivity of the positron flux to the diffusion model. We also show for comparison the secondary positron flux from spallation of cosmic rays on the Galactic disk.
• Figure 2: Same as Fig. \ref{['fig:pos-flux']}, but for the positron fraction.
• Figure 3: Same as Fig. \ref{['fig:pos-flux']}, but for the primary antiproton flux at the top of the atmosphere. In the left plot the MIN diffusion model was assumed (see Table \ref{['tab:param-antiproton']}).
• Figure 4: Contributions to the total antiproton flux in the MIN diffusion model.
• Figure 5: Summary of the signatures of gravitino dark matter decay in the extragalactic gamma ray flux (top), the positron fraction (bottom left) and the antiproton flux (bottom right), compared to the EGRET, HEAT and BESS data respectively. In these plots, we have adopted the MIN diffusion model (see Table \ref{['tab:param-antiproton']}), $m_{3/2}\simeq 150\,{\rm GeV}$ and $\tau_{3/2}\simeq 1.3\times 10^{26}\,{\rm s}$.