Lensing Without Borders. I. A Blind Comparison of the Amplitude of
  Galaxy-Galaxy Lensing Between Independent Imaging Surveys
  Lensing Without Borders is a cross-survey collaboration created to assess the
consistency of galaxy-galaxy lensing signals ($\Delta\Sigma$) across different
data-sets and to carry out end-to-end tests of systematic errors. We perform a
blind comparison of the amplitude of $\Delta\Sigma$ using lens samples from
BOSS and six independent lensing surveys. We find good agreement between
empirically estimated and reported systematic errors which agree to better than
2.3$\sigma$ in four lens bins and three radial ranges. For lenses with $z_{\rm
L}>0.43$ and considering statistical errors, we detect a 3-4$\sigma$
correlation between lensing amplitude and survey depth. This correlation could
arise from the increasing impact at higher redshift of unrecognised galaxy
blends on shear calibration and imperfections in photometric redshift
calibration. At $z_{\rm L}>0.54$ amplitudes may additionally correlate with
foreground stellar density. The amplitude of these trends is within
survey-defined systematic error budgets which are designed to include known
shear and redshift calibration uncertainty. Using a fully empirical and
conservative method, we do not find evidence for large unknown systematics.
Systematic errors greater than 15% (25%) ruled out in three lens bins at 68%
(95%) confidence at $z<0.54$. Differences with respect to predictions based on
clustering are observed to be at the 20-30% level. Our results therefore
suggest that lensing systematics alone are unlikely to fully explain the
"lensing is low" effect at $z<0.54$. This analysis demonstrates the power of
cross-survey comparisons and provides a promising path for identifying and
reducing systematics in future lensing analyses.
