Searching for WIMP signals with galaxies - gamma ray cross correlations: optimal weights in the angular power spectrum

Although one of the two namesakes of the ΛCDM cosmological model, the hypothesis of cold dark matter existence still chiefly relies on its gravitational effects, whilst both direct and indirect detection via non-gravitational signatures have not yet been achieved. Weakly interacting massive particles (WIMP) are a candidate cold relic with a mass from 0.1 GeV to several TeV: they might then annihilate or decay in γ photons and contribute to the unresolved gamma-ray background (UGRB) detected by experiments such as Fermi – LAT. Even if dominated by an isotropic shot-noise component, a degree of anisotropy was detected in the past already in the autocorrelation angular power spectrum. The subsequently detected UGRB-galaxies angular power spectrum cross-correlation further enhances such anisotropy, showing a link between the UGRB and the large scale structure of the Universe (LSS), and allows a better understanding of its composition: some classes of astrophysical objects, like Active Galactic Nuclei, are the most likely sources, but dark matter contributions are not excluded; at low redshifts, the astrophysical contribution should even be subdominant compared to WIMP annihilation or decay signatures, as shown in previous works. Within this general framework, we present a weighting scheme of the galaxy tracer distribution which proved effective in enhancing the anisotropic contribution of other shot-noise-dominated LSS tracers, such as cosmic rays and gravitational waves, and assess its efficiency in terms of signal-to-noise ratio and constraining power on the WIMP mass and its annihilation or decay cross-sections.