Study of Jet Quenching with $Z+\text{jet}$ Correlations in Pb-Pb and $pp$ Collisions at ${\sqrt{s}}_{NN}=5.02\mathrm{TeV}$

The production of jets in association with $Z$ bosons, reconstructed via the ${\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}$ decay channels, is studied in $pp$ and, for the first time, in Pb-Pb collisions. Both data samples were collected by the CMS experiment at the LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The Pb-Pb collisions were analyzed in the 0%–30% centrality range. The back-to-back azimuthal alignment was studied in both $pp$ and Pb-Pb collisions for $Z$ bosons with transverse momentum $p_{\mathrm{T}}^Z>60\mathrm{GeV}/c$ and a recoiling jet with $p_{\mathrm{T}}^{\text{jet}}>30\mathrm{GeV}/c$. The $p_{\mathrm{T}}$ imbalance $x_{jZ}=p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^Z$, as well as the average number of jet partners per $Z$, $R_{jZ}$, was studied in intervals of $p_{\mathrm{T}}^Z$. The $R_{jZ}$ is found to be smaller in Pb-Pb than in $pp$ collisions, which suggests that in Pb-Pb collisions a larger fraction of partons associated with the $Z$ bosons fall below the $30\mathrm{GeV}/c$ $p_{\mathrm{T}}^{\text{jet}}$ threshold because they lose energy.

Figure 1

Invariant mass distributions of the selected dimuons (top) and dielectrons (bottom), compared to pythia+hydjet $Z(\ell \ell )+\text{jet}$ events. The MC histogram is normalized to the number of events in the data.

Figure 2

Distributions of the azimuthal angle difference $\mathrm{\Delta }{\varphi }_{jZ}$ between the $Z$ boson and the jet (top) and of the transverse momentum ratio $x_{jZ}$ between the jet and the $Z$ boson with $\mathrm{\Delta }{\varphi }_{jZ}>7\pi /8$ (bottom). The distributions are normalized by the number of $Z$ events, $N_Z$. Vertical lines (bands) indicate statistical (systematic) uncertainties.

Figure 3

The mean value of the $x_{jZ}$ distribution (top) and the average number of jet partners per $Z$ boson $R_{jZ}$ (bottom), as a function of $p_{\mathrm{T}}^Z$. Vertical lines (bands) indicate statistical (systematic) uncertainties.

Figure 4

Comparison of the measured $pp$ (top) and Pb-Pb (bottom) $x_{jZ}$ distributions with several theoretical models, smeared by the respective jet energy resolution: JEWEL [26], Hybrid [25], and GLV [27]. The JEWEL error bars represent statistical uncertainties, while the widths of the hybrid bands represent systematic variations. A MadGraph5_amc@nlo calculation [36] is also shown.