Studies of the nuclear stopping power in PbPb collisions at 2.76 TeV with CMS

The energy flow at very high pseudorapidity in PbPb collisions is sensitive to the very low-x components of the nuclear wave-function. The CASTOR calorimeter extends the pseudorapidity coverage of CMS to -6.6, which is only 1.4 units away from the beam rapidity. A comparison of the centrality dependence of forward energy flow to that at lower pseudorapidities can shed light on the gluon saturation at low-x. This problem can also be approached by a direct comparison of PbPb and pp energy flow in the forward region. This analysis is based on data taken in 2010. The energy flow in the pseudorapidity range of -5.2 to -6.6 has been measured for 2.76 TeV PbPb collisions over a wide range of centrality and also for minium bias pp collisions. These data are compared to energy-flow measurements for pseudorapidities between -5.2 and +5.2. The very large angular coverage of the CMS detector allows for a test of limiting fragmentation of energy flow, and for an estimate of nuclear stopping. Finally, these data are compared to predictions of hydrodynamic models and microscopic event generators.