CMS-PAS-HIN-24-019

CMS-PAS-HIN-24-019
Energy-energy correlation in Z boson tagged PbPb and pp collisions at $ \sqrt{s_{_\mathrm{NN}}} = $ 5.02 TeV
CMS Collaboration
7 April 2025

Abstract: The first measurement of a Z boson tagged energy-energy correlator (EEC) using charged particle tracks in PbPb collisions at $ \sqrt{s_{_\mathrm{NN}}} = $ 5.02 TeV is presented. This study utilizes PbPb collision data recorded in 2018 with an integrated luminosity of 1.67 $ \pm $ 0.03 nb$ ^{-1} $, as well as pp collision data acquired in 2017 with an integrated luminosity of 301 $ \pm $ 6 pb$ ^{-1} $. The energy-energy correlator of charged hadrons associated with the Z boson, which is predicted to be sensitive to modifications of in-medium parton showers and medium recoils, is measured in bins of event centrality for the first time in PbPb collisions. Significant modification of the EEC is observed in central PbPb collisions compared to pp reference data. The effect disappears in the more peripheral events. The results provide new insight into the jet quenching mechanism and the inner workings of the quark-gluon plasma.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Measured EEC for $pp$ (blue) and $PbPb$ collisions in centrality class 0-30% (orange), 30-50% (red) and 50-90% (purple). The result is shown for two $ p_T $ selections: $ p_{\mathrm{T}}^\text{ch} > $ 2 GeV (top row) and $ p_{\mathrm{T}}^\text{ch} > $ 4 GeV (bottom row). The statistical uncertainties are shown as the error bars, while the total systematic uncertainties, which are partially correlated across $ \Delta R $ bins, are shown as the shaded areas.
png pdf	Figure 2: Comparison of results in the 0-30% centrality class with the JEWEL model (upper row), HYBRID model (middle row), and PYQUEN model (bottom row). The different columns correspond to results obtained with different $ p_{\mathrm{T}}^\text{ch} $ selection on tracks. Markers represent the results for $pp$ and $PbPb$ data, while lines correspond to different settings of the three models. Partial contributions are also shown separately for the JEWEL and HYBRID models, as described in the text. The statistical uncertainties are shown as error bars, while the total systematic uncertainties are shown as the shaded areas.
png pdf	Figure 2-a: Comparison of results in the 0-30% centrality class with the JEWEL model (upper row), HYBRID model (middle row), and PYQUEN model (bottom row). The different columns correspond to results obtained with different $ p_{\mathrm{T}}^\text{ch} $ selection on tracks. Markers represent the results for $pp$ and $PbPb$ data, while lines correspond to different settings of the three models. Partial contributions are also shown separately for the JEWEL and HYBRID models, as described in the text. The statistical uncertainties are shown as error bars, while the total systematic uncertainties are shown as the shaded areas.
png pdf	Figure 2-b: Comparison of results in the 0-30% centrality class with the JEWEL model (upper row), HYBRID model (middle row), and PYQUEN model (bottom row). The different columns correspond to results obtained with different $ p_{\mathrm{T}}^\text{ch} $ selection on tracks. Markers represent the results for $pp$ and $PbPb$ data, while lines correspond to different settings of the three models. Partial contributions are also shown separately for the JEWEL and HYBRID models, as described in the text. The statistical uncertainties are shown as error bars, while the total systematic uncertainties are shown as the shaded areas.
png pdf	Figure 2-c: Comparison of results in the 0-30% centrality class with the JEWEL model (upper row), HYBRID model (middle row), and PYQUEN model (bottom row). The different columns correspond to results obtained with different $ p_{\mathrm{T}}^\text{ch} $ selection on tracks. Markers represent the results for $pp$ and $PbPb$ data, while lines correspond to different settings of the three models. Partial contributions are also shown separately for the JEWEL and HYBRID models, as described in the text. The statistical uncertainties are shown as error bars, while the total systematic uncertainties are shown as the shaded areas.

Summary

In summary, the study presents the first measurement of the energy-energy correlator of charged particles associated with Z bosons in $PbPb$ collisions at a center-of-mass energy of 5.02 TeV. The EEC, which is expected to be sensitive to modifications of in-medium parton showers and medium recoils, is measured as a function of event centrality. Significant differences in the EEC are observed between central PbPb collisions and the pp reference data, while the effect is diminished in more peripheral PbPb events. The results for $ p_{\mathrm{T}}^\text{ch} > $ 4 GeV are dominated by the parton energy loss effect and a reduction in the EEC is observed in central PbPb collisions. Conversely, the results for $ p_{\mathrm{T}}^\text{ch} > $ 2 GeV are more sensitive to low-$ p_{\mathrm{T}} $ hadron production from the QGP, showing a partial recovery of the EEC. By comparing the results with different $ p_{\mathrm{T}}^\text{ch} $ thresholds, the angular scale of the medium response can be mapped out, revealing that the effect extends to large $ \Delta R $. These results provide new insights into the jet quenching mechanism and the properties of the quark-gluon plasma.

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