A Coordinated XMM comma SZE and Optical Survey for Galaxy Cluster Cosmology

In the coming year three new, major experiments are opening a new survey windowto the Universe through the first surveys for galaxy clusters using theSunyaevZeldovich effect. These efforts have been marked as one of the four mostpromising routes to tackle the problem of Dark Energy observationally. To fullyunderstand the results to be obtained we propose a comlementary Xray survey inan area where the three major SZ experiments, SPT, APEX and ACT have focusedtheir efforts into a common test region covering an area of 12.5 deg2 in Xrays.XMMNewton is the only instrument which can effectively provide such a surveyand it would be a pity if the XMM mission would miss this unique chance to helpopening this new window to the Universe. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]