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| CMS-HIN-24-008 ; CERN-EP-2025-112 | ||
| Jet peak shapes based on two-particle angular correlations in lead-lead collisions at $ \sqrt{{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 15 March 2026 | ||
| Submitted to Physics Letters B | ||
| Abstract: The longitudinal invariance of jet-induced peaks in two-particle correlation functions from relativistic lead-lead collisions is experimentally explored. The data were collected at a center-of-mass energy per nucleon pair of 5.02 TeV in 2018 using the CMS detector. The dataset corresponds to an integrated luminosity of 0.607$ \text{nb}^{-1}$. Long- and short-range correlations are studied through two-dimensional distributions of the separations in pseudorapidity and azimuth between particles in an event. Jets manifest as a well-defined peak at small angular separations, and the shape of this peak provides insight into jet medium interactions. This Letter examines the evolution of the jet peak shape, focusing on the dependence of its width and longitudinal asymmetry on the transverse momentum, collision centrality, and pseudorapidity of the associated charged particles. The jet-peak distributions of lower transverse momentum particles broaden in both pseudorapidity and azimuth with increasing collision overlap, with the broadening in pseudorapidity being more pronounced. The longitudinal asymmetry of the peaks is also found to increase as the average pseudorapidity increases. These results are compared to proton-proton collision data that were obtained at the same nucleon-nucleon collision center-of-mass energy with an integrated luminosity of 252$ \text{nb}^{-1}$. | ||
| Links: e-print arXiv:2603.14385 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Average signal (left), mixed-event (middle), and pair-acceptance corrected per-trigger associated yield (right) distributions for 0--10% centrality events, with 4 $ < p_\text{T,trig} < $ 8 GeV and 2 $ < p_\text{T,asso} < $ 3 GeV, in PbPb collision data at 5.02 TeV. |
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Figure 2:
(Left) One-dimensional $ \Delta\eta $ projection and (right) One-dimensional $ \Delta\phi $ projection. |
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Figure 2-a:
(Left) One-dimensional $ \Delta\eta $ projection and (right) One-dimensional $ \Delta\phi $ projection. |
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Figure 2-b:
(Left) One-dimensional $ \Delta\eta $ projection and (right) One-dimensional $ \Delta\phi $ projection. |
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Figure 3:
Two-dimensional correlation distributions in the 0--10% centrality interval for PbPb (left) and pp (right) collisions are shown, with 1.5 $ < |\eta_\text{trig}| < $ 2, 12 $ < p_\text{T,trig} < $ 16 GeV, and 2 $ < p_\text{T,asso} < $ 3 GeV. |
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Figure 3-a:
Two-dimensional correlation distributions in the 0--10% centrality interval for PbPb (left) and pp (right) collisions are shown, with 1.5 $ < |\eta_\text{trig}| < $ 2, 12 $ < p_\text{T,trig} < $ 16 GeV, and 2 $ < p_\text{T,asso} < $ 3 GeV. |
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Figure 3-b:
Two-dimensional correlation distributions in the 0--10% centrality interval for PbPb (left) and pp (right) collisions are shown, with 1.5 $ < |\eta_\text{trig}| < $ 2, 12 $ < p_\text{T,trig} < $ 16 GeV, and 2 $ < p_\text{T,asso} < $ 3 GeV. |
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Figure 4:
One-dimensional $ \Delta\eta $ projection from the long-range background-subtracted averaged 2D correlation, combining 1.5 $ < \eta_\text{trig} < $ 2 (original) and $ -2 < \eta_\text{trig} < - $ 1.5 (mirror). Three vertical lines correspond to a $ \Delta\eta $ value of --0.7, 0 and 0.7 from left to right. The dashed line indicates the zero level on the y-axis. |
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Figure 5:
Centrality dependence of the longitudinal $ \sigma_{\Delta\eta} $ (left panel) and transverse $ \sigma_{\Delta\phi} $ (right panel) values are presented in different $ p_{\mathrm{T}} $ ranges for PbPb collisions and pp collisions (rightmost points in each panel). The same symbols are used for the same $ p_{\text{T,asso}} $ selections, with open markers representing high $ p_{\text{T,trig}} $ and closed markers for low $ p_{\text{T,trig}} $. The statistical uncertainties of the data points are smaller than the marker size, and rectangular boxes indicate the systematic uncertainties. Expected values from HYDJET 1.9 are represented by bands: hatched for low $ p_{\text{T,trig}} $ and solid-filled for high $ p_{\text{T,trig}} $. |
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Figure 6:
Associated yield ratios between $ \Delta\eta > $ 0 and $ \Delta\eta < $ 0 within the range of 0 $ < |\Delta\eta| < $ 0.7 as a function of centrality in different $ p_\text{T,asso} $ and $ \eta_\text{trig} $ ranges for PbPb and pp collisions (rightmost bin) are presented in the above panel. The above panel consists of four plots, each corresponding to a different value of $ \eta_\text{trig} $. Each plot consists of three colored markers representing different $ p_\text{T,asso} $ values (1.5 $ < p_\text{T,asso} < $ 2 GeV, 2 $ < p_\text{T,asso} < $ 3 GeV, 3 $ < p_\text{T,asso} < $ 4 GeV), while $ p_\text{T,trig} $ is fixed (12 $ < p_\text{T,trig} < $ 16 GeV). The vertical bars indicate statistical uncertainties, while the rectangular open boxes represent systematic uncertainties. |
| Tables | |
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Table 1:
Summary of the absolute systematic uncertainties for $ \sigma_{\Delta\eta} $, $ \sigma_{\Delta\phi} $, and the associated yield ratio. The leftmost column lists all systematic sources, while the other columns show the range of systematic uncertainties across all $ p_{\mathrm{T}} $, $ \eta $ and centrality ranges in minimum-bias PbPb collisions. All values are scaled by a factor of $ 10^{3} $. |
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Table 2:
Summary of the absolute systematic uncertainties for $ \sigma_{\Delta\eta} $, $ \sigma_{\Delta\phi} $, and the associated yield ratio. The leftmost column lists all systematic sources, while the other columns show the range of systematic uncertainties across all $ p_{\mathrm{T}} $ and $ \eta $ ranges in minimum-bias pp collisions. All values are scaled by a factor of $ 10^{3} $. |
| Summary |
| The centrality and pseudorapidity ($ \eta $) dependencies of jet peak shapes have been explored using two-particle correlations. Lead-lead (PbPb) collision data at a center-of-mass energy per nucleon pair of 5.02 TeV were recorded using the CMS detector at the CERN LHC. These minimum-bias data correspond to an integrated luminosity of 0.607$ \text{nb}^{-1}$. Proton-proton (pp) collision data at the same nucleon-nucleon collision energy and with an integrated luminosity of 252$ \text{nb}^{-1}$ are also analyzed to provide a reference. The separations in pseudorapidity ($ \Delta\eta $) and azimuth ($ \Delta\phi $) of all particles within specific transverse momentum ($ p_{\mathrm{T}} $) ranges are used to form two-dimensional correlation distributions that are averaged over all events. Jets result in a peak shaped structure corresponding to particle pairs with small angular separation. Jet peak shapes, characterized by associated widths in $ \Delta\eta $ and $ \Delta\phi $, are presented as functions of centrality. The jet peak asymmetry as a function of $ \eta $ is also studied. For PbPb collisions, both the longitudinal ($ \sigma_{\Delta\eta} $) and transverse ($ \sigma_{\Delta\phi} $) widths of the near-side jet peak exhibit a centrality-dependent broadening compared to the pp reference. The effect is more pronounced in $ \Delta\eta $, indicating a stronger modification of the correlation structure along the longitudinal direction. At higher particle transverse momenta, the centrality dependence of both widths becomes weaker, approaching the pp baseline within uncertainties. Overall, the stronger longitudinal than transverse broadening may reflect the influence of longitudinal expansion or partonic energy loss mechanisms in the medium. The asymmetry of the jet peak distribution in $ \Delta\eta $ is studied through the ratio of yields on the positive and negative sides of the jet peak. This ratio is evaluated in distinct $ |\eta_\text{trig}| $ ranges and for various energy thresholds of the higher-$ p_{\mathrm{T}} $ particle in each pair. The ratio is found to increase with the average $ |\eta_\text{trig}| $ value. For a given $ |\eta_\text{trig}| $ range, the ratio measured in peripheral events is found similar to that in pp collisions, but increases significantly towards more central collisions. These results suggest that the jet properties are modified through in medium interactions in a longitudinally expanding QGP medium. |
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