Dark Energy Survey Year 1 results: weak lensing mass calibration of redMaPPer galaxy clusters

We constrain the mass--richness scaling relation of redMaPPer galaxy clusters identified in the Dark Energy Survey Year 1 data using weak gravitational lensing. We split clusters into 4×3 bins of richness ? and redshift z for ?=20 and 0.2=z=0.65 and measure the mean masses of these bins using their stacked weak lensing signal. By modeling the scaling relation as ?M 200m |?,z?=M 0 (?/40) F ((1+z)/1.35) G , we constrain the normalization of the scaling relation at the 5.0 per cent level as M 0 =[3.081±0.075(stat)±0.133(sys)]·10 14 M ? at ?=40 and z=0.35 . The richness scaling index is constrained to be F=1.356±0.051 (stat)±0.008 (sys) and the redshift scaling index G=-0.30±0.30 (stat)±0.06 (sys) . These are the tightest measurements of the normalization and richness scaling index made to date. We use a semi-analytic covariance matrix to characterize the statistical errors in the recovered weak lensing profiles. Our analysis accounts for the following sources of systematic error: shear and photometric redshift errors, cluster miscentering, cluster member dilution of the source sample, systematic uncertainties in the modeling of the halo--mass correlation function, halo triaxiality, and projection effects. We discuss prospects for reducing this systematic error budget, which dominates the uncertainty on M 0. Our result is in excellent agreement with, but has significantly smaller uncertainties than, previous measurements in the literature, and augurs well for the power of the DES cluster survey as a tool for precision cosmology and upcoming galaxy surveys such as LSST, Euclid and WFIRST.