  We present a stacked weak-lensing analysis of an approximately mass-selected
sample of 50 galaxy clusters at 0.15<z<0.3, based on observations with
Suprime-Cam on the Subaru Telescope. We develop a new method for selecting
lensed background galaxies from which we estimate that our sample of red
background galaxies suffers just 1% contamination. We detect the stacked
tangential shear signal from the full sample of 50 clusters, based on this red
sample of background galaxies, at a total signal-to-noise ratio of S/N=32.7.
The Navarro-Frenk-White model is an excellent fit to the data, yielding sub-10%
statistical precision on mass and concentration:
Mvir=7.19^{+0.53}_{-0.50}\times10^{14}h^{-1}Msol, cvir=5.41^{+0.49}_{-0.45}
(c_{200}=4.22^{+0.40}_{-0.36}). Tests of a range of possible systematic errors,
including shear calibration and stacking-related issues, indicate that they are
sub-dominant to the statistical errors. The concentration parameter obtained
from stacking our approximately mass-selected cluster sample is broadly in line
with theoretical predictions. Moreover, the uncertainty on our measurement is
comparable with the differences between the different predictions in the
literature. Overall our results highlight the potential for stacked
weak-lensing methods to probe the mean mass density profile of cluster-scale
dark matter halos with upcoming surveys, including Hyper-Suprime-Cam, Dark
Energy Survey, and KIDS.
