UC Santa Cruz

Despite overwhelming evidence of its existence, the exact nature of Dark Matter (DM) remains unknown. Theoretically well-motivated, Weakly Interacting Massive Particles (WIMPs) make strong candidates for fullling the DM role; they provide gravitational

force without participating in the electromagnetic interactions that would make them conventionally visible. Even so, their weak interactions en masse could allow them annihilate or decay into substantial quantities of visible particles, including gamma rays. Continuing observations from the Fermi Large Area Telescope (LAT) provide an opportunity to detect or rule out this possibility. Powerful modern N-body simulations like Via Lactea II make predictions for the distribution and consequent signal of Milky Way (MW) DM substructure for a given WIMP model. Without the fortuitous placement of nearby DM, however, detecting annihilation from a clump would be difficult. Indeed none have yet been conclusively observed. The much larger Host Halo, in whose gravity well the MW resides, constitutes a much more reliable source of possible DM gamma rays. This signal competes with the complex and uncertain background produced by cosmic-ray interactions in the MW medium. A search for DM Host Halo photons including these background systematics gives constraints on the nature of the DM WIMP.