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| CMS-PAS-HIN-25-017 | ||
| Measurement of the jet quenching effect with Z + jet events in pp and PbPb collisions at 5.36 TeV | ||
| CMS Collaboration | ||
| 2026-07-14 | ||
| Abstract: This note presents the first unfolded measurement of the transverse momentum imbalance between a Z boson and its recoiling leading jet in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.36 TeV. The Z bosons are reconstructed through their dimuon decays. The data samples were collected by the CMS experiment at the LHC, corresponding to integrated luminosities of 1.64 $ \mathrm{nb}^{-1} $ and 479 $ \mathrm{pb}^{-1} $ for PbPb and pp collisions, respectively. The momentum imbalance, $ x_{Zj} = p_\mathrm{T}^\mathrm{jet}/p_\mathrm{T}^\mathrm{Z} $, is measured for Z bosons with transverse momentum $ p_\mathrm{T}^\mathrm{Z} > $ 40 GeV and the leading recoiling jet with $ p_\mathrm{T}^\mathrm{jet} > $ 30 GeV, $ |\eta^\mathrm{jet}| < $ 2.1 and $ \Delta\phi_{Zj} > 7\pi/ $ 8. The per-Z-boson jet yield distributions $ (1/N_{Z})(\mathrm{d}N/\mathrm{d}x_{Zj}) $, measured as a function of $ x_{Zj} $, are corrected for detector effects via an unfolding procedure and reported at the particle level. A significant modification of the unfolded $ x_{Zj} $ distribution is observed in the 30% most central PbPb collisions compared to the pp reference data, due to partonic energy loss (jet quenching) in the quark-gluon plasma. Comparisons with the Hybrid model reveal that while the model captures the suppression of balanced Z+jet, it overestimates the energy loss for highly quenched jets and exhibits insensitivity to medium wake and elastic scattering effects at small jet radii. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
png pdf |
Figure 1:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in 0--30% central PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The simulated signal corresponds to an unquenched baseline; therefore, the observed suppression of the reconstructed jet $ p_\mathrm{T} $ in data relative to the simulation is a consequence of jet quenching in the medium. The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 1-a:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in 0--30% central PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The simulated signal corresponds to an unquenched baseline; therefore, the observed suppression of the reconstructed jet $ p_\mathrm{T} $ in data relative to the simulation is a consequence of jet quenching in the medium. The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 1-b:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in 0--30% central PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The simulated signal corresponds to an unquenched baseline; therefore, the observed suppression of the reconstructed jet $ p_\mathrm{T} $ in data relative to the simulation is a consequence of jet quenching in the medium. The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 2:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in pp collisions at $ \sqrt{s} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 2-a:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in pp collisions at $ \sqrt{s} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 2-b:
Detector-level distributions of the transverse momentum for inclusive Z boson candidates (left) and reconstructed jets (right) in pp collisions at $ \sqrt{s} = $ 5.36 TeV. The data (points) are compared to the sum of the simulated signal and background processes (histograms). The lower panels show the corresponding data-to-MC ratios. Vertical bars represent the statistical uncertainties. |
png pdf |
Figure 3:
Unfolded distribution of $ x_{Zj} $, normalized by the number of Z events, for 0--30% central PbPb collisions and pp reference data at 5.36 TeV. Vertical bars represent statistical uncertainties, while the shaded areas represent the total systematic uncertainties. |
png pdf |
Figure 4:
Comparisons of the $ x_{Zj} $ distributions normalized by the number of Z bosons. Left: Unfolded PbPb data compared to the four quenched Hybrid model scenarios. Right: Unfolded pp data compared to the Hybrid vacuum and MADGRAPH+PYTHIA reference predictions. The lower panels show the corresponding ratios of the MC calculations and data. The bands around the data represent the total experimental uncertainties, whereas the vertical bars represent the statistical uncertainties. The bands around the theory predictions represent the statistical uncertainties of the prediction. |
png pdf |
Figure 4-a:
Comparisons of the $ x_{Zj} $ distributions normalized by the number of Z bosons. Left: Unfolded PbPb data compared to the four quenched Hybrid model scenarios. Right: Unfolded pp data compared to the Hybrid vacuum and MADGRAPH+PYTHIA reference predictions. The lower panels show the corresponding ratios of the MC calculations and data. The bands around the data represent the total experimental uncertainties, whereas the vertical bars represent the statistical uncertainties. The bands around the theory predictions represent the statistical uncertainties of the prediction. |
png pdf |
Figure 4-b:
Comparisons of the $ x_{Zj} $ distributions normalized by the number of Z bosons. Left: Unfolded PbPb data compared to the four quenched Hybrid model scenarios. Right: Unfolded pp data compared to the Hybrid vacuum and MADGRAPH+PYTHIA reference predictions. The lower panels show the corresponding ratios of the MC calculations and data. The bands around the data represent the total experimental uncertainties, whereas the vertical bars represent the statistical uncertainties. The bands around the theory predictions represent the statistical uncertainties of the prediction. |
png pdf |
Figure 5:
Ratio of the unfolded $ x_{Zj} $ distributions (PbPb/pp) compared to the corresponding medium/vacuum ratios predicted by the four scenarios of the Hybrid model. The bands around the data points represent the total experimental uncertainties, whereas the vertical bars represent the statistical uncertainties. The uncertainties in the PbPb-to-pp ratio have been obtained assuming the PbPb and pp measurements are uncorrelated. The bands around the theory predictions represent the statistical uncertainties of the prediction. |
| Tables | |
png pdf |
Table 1:
Relative systematic uncertainties (%) for the unfolded $ x_{Zj} $ distribution in PbPb and pp collisions. The ranges reflect the variations across the measured $ x_{Zj} $ bins. The total systematic uncertainty is calculated as the quadratic sum of the individual components. Values smaller than 1% are denoted as $ < $ 1. |
| Summary |
| In summary, we presented the measurement of the transverse momentum imbalance between a Z boson and its recoiling leading jet, $ x_{Zj} = p_\mathrm{T}^\mathrm{jet} / p_\mathrm{T}^\mathrm{Z} $, for PbPb and pp collisions at 5.36 TeV. Data were collected by the CMS experiment, corresponding to integrated luminosities of 1.64 $ {nb}^{-1} $ and 479 $ {pb}^{-1} $, respectively. A significant modification of the jet momentum balance is observed in central (0--30%) PbPb collisions compared to the pp reference. In particular, for central PbPb events the yield decreases monotonically with increasing $ x_{Zj} $ over the measured range, in strong contrast to the sharply peaked distributions in pp events. Furthermore, the unfolded data exhibit a strong suppression of highly balanced jet pairs and an overall shift of the distribution towards lower $ x_{Zj} $ values. A decoupled shape-normalization statistical test confirms that this modification is significant at the 5.0 standard deviation level against the null hypothesis of no medium modification. The measurements are compared to predictions from the Hybrid coupling model, which reproduces the suppression of balanced jets in PbPb collisions, but fails to describe the migration of jets toward low $ x_{Zj} $. The tension with the predictions from the Hybrid model at low $ x_{Zj} $, which is also observed in analogous $ \gamma $+jet measurements, suggests an over-quenching effect within the model's energy loss implementation near the kinematic threshold. Additionally, the comparison reveals that the inclusive $ x_{Zj} $ observable using small-radius jets ($ R= $ 0.2) is largely insensitive to specific microscopic interactions, such as Moli\`ere scatterings and the medium wake. These high-precision measurements provide essential constraints on theoretical models of parton-medium interactions and suggest the need for an improved treatment of the wake, also for small-radius jets, and in general the inclusion of next-to-leading-order effects, which can affect jet multiplicity. Future studies, including the exploration of larger jet radii and jet substructure observables, will be instrumental in further disentangling macroscopic bulk energy loss from microscopic quasiparticle interactions within the quark-gluon plasma. |
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