Weak Lensing Discovery and Tomography of a Cluster at z=0.68

We report the discovery, spectroscopic confirmation, and weak lensing tomography of a massive cluster of galaxies at z=0.68. The first shear-selected cluster from the Deep Lens Survey, a deep optical imaging survey of 28 square degrees, it dominates the weak lensing mass map of its 40' field. At more than twice the redshift of the only previous shear-selected cluster, it demonstrates that the redshift range of shear-selected cluster samples will be competitive with that of X-ray selected samples. Imaging data show an arc which is likely to be a strongly lensed background galaxy. The cluster's line-of-sight velocity dispersion is 980 km/s. A simple singular isothermal sphere model yields dynamical and weak lensing mass estimates (projected within a radius of 71 kpc) of 5.0 +- 2.4 and 5.1 +- 1.5 x 10^{13} solar masses respectively; a rough lower limit from strong lensing is 1.8 x 10^{13} solar masses. The tomography yields a lens redshift consistent with the cluster spectroscopic redshift. Mass maps and tomography of this quality or better will be produced by the Deep Lens Survey over an area a factor of sixty larger.