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| CMS-HIN-24-010 ; CERN-EP-2024-345 | ||
| Search for medium effects using jet axis decorrelation in inclusive jets from PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 18 February 2025 | ||
| JHEP 06 (2025) 120 | ||
| Abstract: The jet axis decorrelation in inclusive jets is studied using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The jet axis decorrelation is defined as the angular difference between two definitions of the jet axis. It is obtained by applying two recombination schemes on all the constituents of a given jet reconstructed by the anti-$ k_{\mathrm{T}} $ sequential algorithm with a distance parameter of $ R = $ 0.4. The data set, corresponding to an integrated luminosity of 0.66 nb$^{-1}$, was collected in 2018 with the CMS detector at the CERN LHC. The jet axis decorrelations are examined across collision centrality selections and intervals of jet transverse momentum. A centrality dependent evolution of the measured distributions is observed, with a progressive narrowing seen in more central events. This narrowing could result from medium-induced modification of the internal jet structure or reflect color charge effects in energy loss. This new measurement probes jet substructure in previously unexplored kinematic domains and show great promise for providing new insights on the color charge dependence of energy loss to jet-quenching models. | ||
| Links: e-print arXiv:2502.13020 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The unfolded normalized $ \Delta\text{j} $ distributions are measured across different centrality and $ p_{\mathrm{T}} $ intervals. Each panel displays distributions for four centrality intervals within a specific $ p_{\mathrm{T}} $ bin, with $ p_{\mathrm{T}} $ ranges (indicated in each panel) increasing from left to right. In each panel, the black circles show measurements for 0-10% centrality interval, the red squares for 10-30%, and the blue-up and green-down triangles for 30-50% and 50-80% centrality intervals, respectively. The vertical solid lines represent the statistical uncertainties, while the rectangles and shaded areas represent the correlated and uncorrelated systematic uncertainties, respectively. |
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Figure 2:
The unfolded $ \Delta\text{j} $ distributions (black circles) are compared with simulations from PYTHIA, HERWIG, and a two-component, represented as colored bands, for the 50-80% centrality range across different $ p_{\mathrm{T}} $ intervals. The vertical solid lines represent the statistical uncertainties, while the rectangles and shaded areas represent the correlated and uncorrelated systematic uncertainties, respectively. Only statistical uncertainties are shown for the simulated distributions. |
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Figure 3:
The unfolded $ \Delta\text{j} $ distributions (black circles) are compared with simulations from PYTHIA, HERWIG, and a two-component, represented as colored bands, for the 30-50% centrality range across different $ p_{\mathrm{T}} $ intervals. The vertical solid lines represent the statistical uncertainties, while the rectangles and shaded areas represent the correlated and uncorrelated systematic uncertainties, respectively. Only statistical uncertainties are shown for the simulated distributions. |
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Figure 4:
The unfolded $ \Delta\text{j} $ distributions (black circles) are compared with simulations from PYTHIA, HERWIG, and a two-component, represented as colored bands, for the 10-30% centrality range across different $ p_{\mathrm{T}} $ intervals. The vertical solid lines represent the statistical uncertainties, while the rectangles and shaded areas represent the correlated and uncorrelated systematic uncertainties, respectively. Only statistical uncertainties are shown for the simulated distributions. |
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Figure 5:
The unfolded $ \Delta\text{j} $ distributions (black circles) are compared with simulations from PYTHIA, HERWIG, and a two-component (represented as colored bands), as well as medium q/g and JEWEL predictions (represented as hatched lines) for the 0-10% centrality range across different $ p_{\mathrm{T}} $ intervals. The vertical solid lines represent the statistical uncertainties, while the rectangles and shaded areas represent the correlated and uncorrelated systematic uncertainties, respectively. Only statistical uncertainties are shown for the simulated distributions. |
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Figure A1:
Expected gluon fraction in inclusive $ R = $ 0.4 jet sample in the vacuum (based on PYTHIA8 with tune CP5) and in medium (based on Refs. [51,52]). |
| Tables | |
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Table 1:
Summary of absolute systematic uncertainties, averaged over the $ \Delta\text{j} $ distributions for different centrality intervals. The range of uncertainty values reflects the maximum variation across different $ p_{\mathrm{T}} $ intervals. |
| Summary |
| This paper presents the first measurement of jet axis decorrelation ($ \Delta\text{j} $) between two types of jet axes for inclusive jets from lead-lead (PbPb) collision data collected with the CMS detector at the LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. These measurements are performed using anti-$ k_{\mathrm{T}} $ jets with a radius parameter $ R = $ 0.4 and with pseudorapidity $ |\eta| < $ 1.6 in four centrality intervals and four transverse momentum $ p_{\mathrm{T}} $ intervals ranging from 120 to 300 GeV. Significant modifications of $ \Delta\text{j} $ distributions are observed in central compared to peripheral collisions, with a progressive narrowing of the distributions toward more central events. This narrowing behavior could result from medium-induced modifications of the internal jet structure, although the possibility of a selection bias towards a less-quenched sample should be considered. Such a bias could arise from the predicted color charge dependence of partonic energy loss, with gluon jets expected to lose more energy than quark jets. The measured distributions from peripheral collisions are described by the PYTHIA8 event generator without any jet quenching effects. Progressive narrowing of the $ \Delta\text{j} $ distribution is observed toward more central events for all $ p_{\mathrm{T}} $ intervals studied. Comparisons with phenomenological and simple simulation-based models indicate that the observed narrowing in the $ \Delta\text{j} $ distributions in central PbPb collisions may be largely described by a decrease in the relative gluon jet abundance, except for higher jet $ p_{\mathrm{T}} $ intervals. Deviations at high $ p_{\mathrm{T}} $ suggest that accounting for jet substructure modifications or a more sophisticated implementation of color charge effects in energy loss is needed to fully describe the data. These new measurements explore jet substructure in previously unexplored kinematic domains and show great promise for providing new insights into the color charge dependence of energy loss. |
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