Constraints on Annihilating Dark Matter from Gamma-Ray Background-Galaxy Shape Correlations: Model-independent Null Results and Moderate Template-based Signals

We revisit the cross-correlation between the unresolved gamma-ray background and galaxy shapes to constrain the annihilation cross section of particle dark matter. Our analysis uses gamma-ray photons from 14 years of observations with the Fermi Large Area Telescope (LAT), together with galaxy shape catalogs from the Dark Energy Survey Year 3 (DES Y3) and the Dark Energy Camera All Data Everywhere (DECADE) project, enabling us to probe cosmological large-scale signals over a common sky area of about 12, 000, deg squared shared by the gamma-ray and galaxy data sets. In order to better access signals from large-scale structure, we employ a Fourier-space estimator for the cross-correlation in contrast to the previous DES Y3 analysis. We find that our measurements are consistent with a null detection in a model-independent chi squared test, while template-based analyses yield signals at the about 3 sigma level. Our null results exclude an enhanced annihilation cross section for wino-like dark matter with a mass of 2-3 TeV under a modest substructure boost factor of about 30 in Milky Way-sized halos. For larger boost factors of about 100, the constraints become significantly stronger and exclude the canonical thermal annihilation cross section sigma v = 3 times 10-26, cm 3/s for a 7-40 GeV dark matter particle annihilating into b or tau+ tau-. The template-based analysis favors a power-law gamma-ray energy dependence of the cross-correlation, but also indicates deviations from that expected based on the mean intensity of the unresolved gamma-ray background around 100 GeV. We further consider decaying dark matter scenarios and derive 2 sigma lower limits on the particle lifetime of about 10 26-10 27, s, depending on the decay channel.