Measurement of the pseudorapidity and centrality dependence of the very forward energy density in PbPb collisions at sqrt s=2.76 ATeV

The energy density, $\mathrm{d} E/\mathrm{d} \eta$, in PbPb collisions at $2.76$ A TeV nucleus-nucleus center-of-mass energy is measured as a function of pseudorapidity, $\eta$, and collision centrality using the CMS detector at the LHC. The very large pseudorapidity coverage of this measurement is used as a powerful constraint for various hadronic interaction models. The CASTOR calorimeter extends the $\eta$ coverage of CMS up to $|\eta|=6.6$, which is only 1.4 units away from the beam rapidity for $\sqrt{s_{_\text{NN}}} = 2.76$ TeV. This is comparable to the most forward measurements at RHIC. A comparison of the centrality dependence of the very forward energy density to that at smaller pseudorapidities is presented and compared to event generator predictions of models for nuclear collisions. The very wide pseudorapidity range of the presented measurement is unique at this center-of-mass energy and the average energy weighted pseudorapidity can be estimated, which is a useful observable to discriminate between model predictions.