Pre-launch estimates for GLAST sensitivity to Dark Matter annihilation signals

TL;DR

This paper assesses GLAST-LAT's capability to detect dark matter via indirect gamma-ray signals from WIMP annihilation, integrating LAT response with particle-physics models and DM density profiles. It develops a unified likelihood-based framework to predict sensitivity across the Galactic Center, Galactic halo, satellites, and extragalactic contexts, including line signals and, for UED, electron channels. The results indicate GLAST could probe cross sections near the thermal relic value over a broad mass range (tens to hundreds of GeV) and test line signatures, with substantial dependence on DM density profiles and diffuse-background modeling. The conclusions stress the importance of halo structure uncertainties and backgrounds, while highlighting the potential for GLAST to image DM distribution in the Milky Way if a signal is present.

Abstract

We investigate the sensitivity of the Gamma-ray Large Area Space Telescope (GLAST) to indirectly detect weakly interacting massive particles (WIMPs) through the $γ$-ray signal that their pair annihilation produces. WIMPs are among the favorite candidates to explain the compelling evidence that about 80% of the mass in the Universe is non-baryonic dark matter (DM). They are serendipitously motivated by various extensions of the standard model of particle physics such as Supersymmetry and Universal Extra Dimensions (UED). With its unprecedented sensitivity and its very large energy range (20 MeV to more than 300 GeV) the main instrument on board the GLAST satellite, the Large Area Telescope (LAT), will open a new window of discovery. As our estimates show, the LAT will be able to detect an indirect DM signature for a large class of WIMP models given a cuspy profile for the DM distribution. Using the current state of the art Monte Carlo and event reconstruction software developed within the LAT collaboration, we present preliminary sensitivity studies for several possible sources inside and outside the Galaxy. We also discuss the potential of the LAT to detect UED via the electron/positron channel. Diffuse background modeling and other background issues that will be important in setting limits or seeing a signal are presented.

Figures (29)

• Figure 1: A diagrammatic flow of how gamma rays are produced by annihilation of dark matter and elements of the analysis chain used by the GLAST collaboration to detect them. The double question mark in the simulation chain indicates high uncertainty in the models of dark matter density and the new particle theories discussed in the paper. The single question mark over the cosmic ray propagation and interaction models indicates lesser, although significant, uncertainty in those models that generate backgrounds to the potential dark matter gamma ray signal. In this paper GALPROP (sec:bckgd) is used to estimate those backgrounds. In the next step, $\gamma$-ray detection is simulated using standard detector simulation packages (GEANT 4). Finally,these simulated LAT events are treated by various analysis software programs (event reconstruction and statistical analysis) to generate the results presented in this work. The same procedure is applied to the smoking gun signal of $\chi \chi \rightarrow \gamma \gamma$, except that in this case hadronization does not have to be taken into account.
• Figure 2: The simulated LAT exposure for 5 years of all-sky scan. The effect of turning off the LAT while in the SAA is included in the exposure. This exposure is calculated for a photon energy of 100 GeV. The plot is in Galactic coordinates with the values of exposure shown on the grey (coloured in coloured versions of the paper) bar in units of cm$^2\,$sec.
• Figure 3: $\gamma$-ray spectrum of the inner Galaxy ($300^\circ < l < 30^\circ$,$|b| < 5$) derived from the "conventional" model (see text for more details). Dotted: contribution from $\pi^0$ decay, dashed: contribution from inverse Compton scattering, dash-dotted: contribution from bremsstrahlung, solid: extragalactic background, bold solid: total flux. Also shown are the data points from EGRET (dark bars, red in colored versions) and COMPTEL (light bars, green in colored versions). Figure taken from Strong:2007nh.
• Figure 4: $\gamma$-ray spectrum of the inner Galaxy ($300^\circ < l < 30^\circ$,$|b| < 5$) derived from the "optimized" model (see text for more details). Dotted: contribution from $\pi^0$ decay, dashed: contribution from inverse Compton scattering, dash-dotted: contribution from bremsstrahlung, solid: extragalactic background, bold solid: total flux. Also shown are the data points from EGRET (dark bars, red in colored versions) and COMPTEL (light bars, green in colored versions). Figure taken from Strong:2007nh.
• Figure 5: Cross-sections $<\sigma v>$ ($v \rightarrow 0)$ versus the WIMP mass $m_{WIMP}$ for the $b \bar{b}$annihilation channel. Left panel shows the result for $3 \sigma$ significance, right panel shows the result for $5 \sigma$ significance for 5 years of GLAST operation, The upper part of the plots corresponds to regions which are already excluded by the EGRET data around the GC and the lower part corresponds to regions not detectable by GLAST. The "detectable by GLAST region" corresponds to models detectable by GLAST for both "conventional" and "optimized" astrophysical background. The shaded region represents models which can be detected only under the assumption of "conventional" Galactic diffuse background. See text for more details.