Cross-correlation of weak lensing and gamma rays: implications for the
  nature of dark matter
  We measure the cross-correlation between Fermi-LAT gamma-ray photons and over
1000 deg$^2$ of weak lensing data from the Canada-France-Hawaii Telescope
Lensing Survey (CFHTLenS), the Red Cluster Sequence Lensing Survey (RCSLenS),
and the Kilo Degree Survey (KiDS). We present the first measurement of
tomographic weak lensing cross-correlations and the first application of
spectral binning to cross-correlations between gamma rays and weak lensing. The
measurements are performed using an angular power spectrum estimator while the
covariance is estimated using an analytical prescription. We verify the
accuracy of our covariance estimate by comparing it to two internal covariance
estimators. Based on the non-detection of a cross-correlation signal, we derive
constraints on weakly interacting massive particle (WIMP) dark matter. We
compute exclusion limits on the dark matter annihilation cross-section
$\langle\sigma_\rm{ann} v \rangle$, decay rate $\Gamma_\rm{dec}$, and particle
mass $m_\rm{DM}$. We find that in the absence of a cross-correlation signal,
tomography does not significantly improve the constraining power of the
analysis. Assuming a strong contribution to the gamma-ray flux due to
small-scale clustering of dark matter and accounting for known astrophysical
sources of gamma rays, we exclude the thermal relic cross-section for particle
masses of $m_\rm{DM}\lesssim 20$ GeV.
