Measurement of $Z$ Boson Production in Pb-Pb Collisions at $\sqrt{s_{NN}}\mathbf{=}2.76\mathrm{TeV}$ with the ATLAS Detector

The ATLAS experiment has observed 1995 $Z$ boson candidates in data corresponding to $0.15{\mathrm{nb}}^{-1}$ of integrated luminosity obtained in the 2011 LHC $\mathrm{Pb}+\mathrm{Pb}$ run at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The $Z$ bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event $Z$ boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the $Z$ boson with respect to the event plane is found to be consistent with zero.

Figure 1

The invariant mass distributions of $Z\to ee$ (left) and $Z\to \mu \mu$ (right) candidates, integrated over momentum, rapidity, and centrality. Bars represent the statistical uncertainty. The number of pairs with $66<m_{\ell \ell }<102\mathrm{GeV}$ (marked by the vertical dashed lines) is listed. The simulation is weighted to match the centrality distribution in data and normalized in the region $66<m_{\ell \ell }<102\mathrm{GeV}$.

Figure 2

The corrected per-event rapidity distribution of measured $Z$ bosons. Bars and boxes represent statistical and systematic uncertainties, respectively. The data are compared to the model distribution shown as a band whose width is the normalization uncertainty.

Figure 3

Left: corrected per-event $p_T^Z$ spectra of measured $Z$ bosons in five centrality classes. The data are compared to a Pythia simulation normalized to the NNLO $p+p$ cross section and scaled by $⟨T_{\mathrm{AA}}⟩$, shown as bands. Right: ratios of the data to the model in each centrality class. Bars represent statistical uncertainties, boxes represent systematic uncertainties, and bands represent the normalization uncertainty.

Figure 4

Centrality dependence of $Z$ boson yields divided by $⟨N_{\mathrm{coll}}⟩$. Results for $ee$ (upward pointing triangles) and $\mu \mu$ (downward pointing triangles) channels are shifted left and right, respectively, from their weighted average (diamonds). Bars and boxes represent statistical and systematic uncertainties, respectively. For the combined results, the brackets show the combined uncertainty including the uncertainty on $⟨N_{\mathrm{coll}}⟩$, and the dashed lines show the results of fits, using a constant.

Figure 5

$v_2$ as a function of $|y^Z|$ (left), $p_T^Z$ (center), and $⟨N_{\mathrm{part}}⟩$ (right). Bars and boxes represent statistical and systematic uncertainties, respectively. The dashed lines show the results of constant fits to the $v_2$ values, considering only statistical uncertainties.