The ATLAS experiment has observed 1995 $Z$ boson candidates in 0.15 nb$^{ -1}$ of integrated luminosity obtained in the 2011 LHC Pb+Pb run at $\sqrt{ s_{ NN}}$ 2.76 TeV. The $Z$ bosons are reconstructed via di-electron and di-muon decay channels. The results from both channels are consistent with each other and are combined. The background is less than 3% of the selected sample. Within the statistical and systematic uncertainties, the per-event $Z$ boson yield integrated over rapidity $|y^{ Z}|<2.5$ is proportional to the number of binary collisions estimated by the Glauber model. The elliptic flow coefficient of the azimuthal distribution of the $Z$ boson with respect to the event plane is consistent with zero.
Categories: nucl-ex hep-ex