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 theconsistency of galaxy-galaxy lensing signals (ΔΣ) across differentdata-sets and to carry out end-to-end tests of systematic errors. We perform ablind comparison of the amplitude of ΔΣ using lens samples fromBOSS and six independent lensing surveys. We find good agreement betweenempirically estimated and reported systematic errors which agree to better than2.3σ in four lens bins and three radial ranges. For lenses with zL>0.43 and considering statistical errors, we detect a 3-4σcorrelation between lensing amplitude and survey depth. This correlation couldarise from the increasing impact at higher redshift of unrecognised galaxyblends on shear calibration and imperfections in photometric redshiftcalibration. At zL>0.54 amplitudes may additionally correlate withforeground stellar density. The amplitude of these trends is withinsurvey-defined systematic error budgets which are designed to include knownshear and redshift calibration uncertainty. Using a fully empirical andconservative 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 onclustering are observed to be at the 20-30% level. Our results thereforesuggest that lensing systematics alone are unlikely to fully explain the"lensing is low" effect at z<0.54. This analysis demonstrates the power ofcross-survey comparisons and provides a promising path for identifying andreducing systematics in future lensing analyses.