Cluster Mass Calibration at High Redshift: HST Weak Lensing Analysis of
  13 Distant Galaxy Clusters from the South Pole Telescope Sunyaev-Zel'dovich
  Survey
  We present an HST/ACS weak gravitational lensing analysis of 13 massive
high-redshift (z_median=0.88) galaxy clusters discovered in the South Pole
Telescope (SPT) Sunyaev-Zel'dovich Survey. This study is part of a larger
campaign that aims to robustly calibrate mass-observable scaling relations over
a wide range in redshift to enable improved cosmological constraints from the
SPT cluster sample. We introduce new strategies to ensure that systematics in
the lensing analysis do not degrade constraints on cluster scaling relations
significantly. First, we efficiently remove cluster members from the source
sample by selecting very blue galaxies in V-I colour. Our estimate of the
source redshift distribution is based on CANDELS data, where we carefully mimic
the source selection criteria of the cluster fields. We apply a statistical
correction for systematic photometric redshift errors as derived from Hubble
Ultra Deep Field data and verified through spatial cross-correlations. We
account for the impact of lensing magnification on the source redshift
distribution, finding that this is particularly relevant for shallower surveys.
Finally, we account for biases in the mass modelling caused by miscentring and
uncertainties in the concentration-mass relation using simulations. In
combination with temperature estimates from Chandra we constrain the
normalisation of the mass-temperature scaling relation ln(E(z) M_500c/10^14
M_sun)=A+1.5 ln(kT/7.2keV) to A=1.81^{+0.24}_{-0.14}(stat.) +/- 0.09(sys.),
consistent with self-similar redshift evolution when compared to lower redshift
samples. Additionally, the lensing data constrain the average concentration of
the clusters to c_200c=5.6^{+3.7}_{-1.8}.
