Systematic study of projection biases in weak lensing analysis*

We present a systematic study of projection biases in the weak lensing analysis of the first year of data from the Dark Energy Survey (DES) experiment. In the analysis, we used a ΛCDM cosmology model and the three two-point correlation functions: (i) the cosmic shear correlation function, (ii) the galaxy angular correlation function (galaxy clustering), and (iii) the galaxy-shear cross-correlation function (galaxy-galaxy lensing). We show that these biases are a consequence of projecting or marginalizing, over parameters like h, Ω_b, n_s, and Ω_νh² that have wide likelihoods with respect to their priors (poorly constrained) and correlated with the parameters of interest like Ω_m, σ₈, and S₈. Covering the relevant parameter space, we show that the projection biases are a function of where the true values of the poorly constrained parameters lie with respect to the parameter priors. We also show that the one-dimensional credible intervals are consistently inflated and discuss how careful study of these biases could transform the WL analysis credible intervals into confidence intervals with an equivalent gain of up to three times the data available to the experiment.