  We present DLSCL J0916.2+2951 (z=0.53), a newly discovered major cluster
merger in which the collisional cluster gas has become dissociated from the
collisionless galaxies and dark matter. We identified the cluster using optical
and weak lensing observations as part of the Deep Lens Survey. Our follow-up
observations with Keck, Subaru, Hubble Space Telescope, and Chandra show that
the cluster is a dissociative merger and constrain the dark matter
self-interaction cross-section {\sigma}_{DM}m_{DM}^{-1}\leq7 cm^2g^{-1}. The
system is observed at least 0.7\pm0.2 Gyr since first pass-through, thus
providing a picture of cluster mergers 2-5 times further progressed than
similar systems observed to date. This improved temporal leverage has
implications for our understanding of merging clusters and their impact on
galaxy evolution.
