Study of Z production in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV in the dimuon and dielectron decay channels

TL;DR

This CMS study measures Z boson production in PbPb and pp collisions at $\sqrt{s_{NN}} = 2.76$ TeV via $Z\to \mu^+\mu^-$ and $Z\to e^+e^-$ channels. By analyzing yields as functions of $p_T$, $y$, and centrality and comparing PbPb results to pp scaled by the nuclear overlap function $T_{AA}$, the work tests binary collision scaling and probes initial-state effects. The results show no significant modification: the combined $R_{AA}$ is consistent with unity in both channels, and kinematic distributions agree with theoretical predictions within uncertainties. This establishes Z bosons as robust baselines for heavy-ion physics and places constraints on isospin and nuclear PDF effects at LHC energies.

Abstract

The production of Z bosons is studied in the dimuon and dielectron decay channels in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV, using data collected by the CMS experiment at the LHC. The PbPb data sample corresponds to an integrated luminosity of about 150 inverse microbarns, while the pp data sample collected in 2013 at the same nucleon-nucleon centre-of-mass energy has an integrated luminosity of 5.4 inverse picobarns. The Z boson yield is measured as a function of rapidity, transverse momentum, and collision centrality. The ratio of PbPb to pp yields, scaled by the number of inelastic nucleon-nucleon collisions, is found to be 1.06 +/- 0.05 (stat) +/- 0.08 (syst) in the dimuon channel and 1.02 +/- 0.08 (stat) +/- 0.15 (syst) in the dielectron channel, for centrality-integrated Z boson production. This binary collision scaling is seen to hold in the entire kinematic region studied, as expected for a colourless probe that is unaffected by the hot and dense QCD medium produced in heavy ion collisions.

Figures (8)

• Figure 1: Dimuon invariant mass spectra for muons with $\lvert \eta^{\mu} \rvert<2.4$ and $p_{\mathrm{T}}\xspace^\mu>20$${\,\text{Ge\spaceV\space/\space}c}$ in PbPb (top left) and in pp (bottom left) collisions and dielectron invariant mass spectra for electrons with $\lvert \eta^\mathrm{e} \rvert < 1.44$ and $p_{\mathrm{T}}\xspace^{\mathrm{e}}>20$${\,\text{Ge\spaceV\space/\space}c}$ in PbPb (top right) and in pp (bottom right) collisions. Full black circles represent opposite-charge lepton pair events and open black squares represent same-charge lepton pair events. Superimposed and normalized to the number of Z boson candidates in data is the MC simulation from pythia$\mathrm{NN}\to{Z}\to {\mu^+}{\mu^-}$ or $\mathrm{e}^+\mathrm{e}^-$, where N is a nucleon from the proper mix of protons and neutrons, embedded in Hydjet simulated events for the PbPb case, and $\mathrm{p}\mathrm{p}\to{Z}\to {\mu^+}{\mu^-}$ or $\mathrm{e}^+\mathrm{e}^-$ for the pp case.
• Figure 2: The measured Z boson production cross section in pp collisions as a function of the Z boson $p_{\mathrm{T}}$ (top) and $y$ (bottom) for the dimuon (left) and the dielectron (right) decay channels. Results are compared with $\mathrm{p}\mathrm{p}\to{Z}\to \ell^{+}\ell^{-}$powheg predictions. Vertical lines (boxes) correspond to statistical (systematic) uncertainties. The theoretical uncertainty of 5% assumed for the powheg reference curve is shown by the grey band.
• Figure 3: Event centrality dependence of the ${Z}\to{\mu^+}{\mu^-}$ (left) and ${Z}\to\mathrm{e}^+\mathrm{e}^-$ (right) yields per MB event in PbPb collisions, divided by the expected average nuclear overlap function, $T_\mathrm{AA}$, which is directly comparable to the $\mathrm{p}\mathrm{p}\to{Z}\to \ell^{+}\ell^{-}$ cross section predicted by the powheg generator displayed as a black dash-dotted line. On the horizontal axis, event centrality is depicted as the average number of participating nucleons, $N_\text{part}$ (see Table \ref{['tab:glauber']}). Vertical lines (boxes) correspond to statistical (systematic) uncertainties. The theoretical uncertainty of 5% assumed for the powheg reference curve is shown by the grey band.
• Figure 4: The measured Z boson yields per MB event in PbPb collisions as a function of the Z boson $p_{\mathrm{T}}$ (top) and $y$ (bottom) for the dimuon (left) and the dielectron (right) decay channels. The yields are compared with $\mathrm{p}\mathrm{p}\to{Z}\to \ell^+\ell^-$powheg predictions scaled by the 0--100% centrality averaged $T_\mathrm{AA}$. The light gray bands in the results vs. $p_{\mathrm{T}}$ represent the theoretical uncertainty of 5% assumed for the powheg reference curve together with the uncertainty of 6.2% due to the $T_\mathrm{AA}$ scaling. The results vs. $y$ are compared to predictions with (green dark band) and without (yellow light band) nuclear modification effects. Vertical lines (boxes) correspond to statistical (systematic) uncertainties.
• Figure 5: The $R_\mathrm{AA}$ distribution for the ${Z}\to \mathrm{e}^+\mathrm{e}^-$ (blue squares) and ${Z}\to{\mu^+}{\mu^-}$ (red circles) events as a function of the Z boson $p_{\mathrm{T}}$ (left), $y$ (right), and $N_\text{part}$ (bottom). For $N_\text{part}$, open points at $N_\text{part} \sim 110$ represent the centrality-integrated $R_\mathrm{AA}$. Points are shifted along the horizontal axis for clarity. The horizontal line at $R_\mathrm{AA}$ = 1 is drawn as a reference. Vertical lines (boxes) correspond to statistical (systematic) uncertainties. The grey bar at $R_\mathrm{AA} = 1$ corresponds to uncertainty in pp luminosity and the green bar corresponds to uncertainty in $T_{AA}$.