CMB lensing reconstruction biases in cross-correlation with large-scale
  structure probes
  The cross-correlation between cosmic microwave background (CMB) gravitational
lensing and large-scale structure tracers will be an important cosmological
probe in the coming years. Quadratic estimators provide a simple and powerful
(if suboptimal) way to reconstruct the CMB lensing potential and are widely
used. For Gaussian fields, the cross-correlation of a quadratic-estimator CMB
lensing reconstruction with a tracer is exactly unbiased if the power spectra
are known and consistent analytic lensing mode response functions are used.
However, the bispectrum induced by non-linear large-scale structure growth and
post-Born lensing can introduce an additional bias term ($N_L^{(3/2)}$) in the
cross-correlation spectrum, similar to the $N_L^{(3/2)}$ bias in the
auto-spectrum demonstrated in recent works. We give analytic flat-sky results
for the cross-correlation bias using approximate models for the post-Born and
large-scale structure cross-bispectra, and compare with N-body simulation
results using ray-tracing techniques. We show that the bias can be at the
5-15\% level in all large-scale structure cross-correlations using small-scale
CMB temperature lensing reconstruction, but is substantially reduced using
polarization-based lensing estimators or simple foreground-projected
temperature estimators. The relative magnitude of these effects is almost three
times higher than in the CMB lensing auto-correlation, but is small enough that
it can be modelled to sufficient precision using simple analytic models. We
show that $N_L^{(3/2)}$ effects in cross-correlation will be detected with high
significance when using data of future surveys and could affect systematic
effects marginalization in cosmic shear measurements mimicking galaxy intrinsic
alignment.
