We investigate the impact of non-Gaussian lensing deflections on measurements of the CMB lensing power spectrum. We find that the false assumption of their Gaussianity significantly biases these measurements in current and future experiments at the percent level. The bias is detected by comparing CMB lensing reconstructions from simulated CMB data lensed with Gaussian deflection fields to reconstructions from simulations lensed with fully non-Gaussian deflection fields. The non-Gaussian deflections are produced by ray tracing through snapshots of an N-body simulation and capture both the non-Gaussianity induced by nonlinear structure formation and by multiple correlated deflections. We find that the amplitude of the measured bias can be modeled with analytic expressions for a lensing bispectrum-induced bias derived by Böhm et al. in 2016 when post-Born corrections are included in the lensing bispectrum model. The bias is largest in temperature-based measurements, where it is detected with a significance of $2.84\sigma$ in the power spectrum of reconstructed convergence fields. Cross-correlating the reconstruction with the noiseless input convergence fields results in a $5.21\sigma$ detection. We do not find evidence for the bias in measurements from a combination of polarization fields ($EB,EB$). We argue that this non-Gaussian bias should be even more important for measurements of cross-correlations of CMB lensing with low-redshift tracers of large-scale structure.

• Received 6 June 2018

DOI:https://doi.org/10.1103/PhysRevD.98.123510