Understanding Projection Effects in Dark Energy Survey Cluster Selection

Clusters of galaxies in optical imaging surveys like the DES have thestatistical power to be the single most constraining probe of dark energy.However, realizing this potential relies on our ability to control forsystematics, the largest remaining of which are the effects of projection oncluster selection and mass determinations. We propose XMM observations of sixDES clusters where existing spectroscopy indicates a significant contaminationfrom projection/correlated structure resulting in high velocity dispersions fortheir richness. These data, along with existing data for an additional fiveclusters, will allow us to understand the origin of this population (trulymassive clusters, low mass clusters, unvirialized projections) and their contribution to the DES cluster catalog. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]