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| CMS-PAS-HIN-24-008 | ||
| Jet shapes based on two-particle angular correlations in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 8 October 2024 | ||
| Abstract: In ultrarelativistic heavy ion collisions, long- and short-range correlations can be studied by forming two-dimensional histograms of the separations in pseudorapidity and azimuth of each particle in an event with every other particle in that event, with the two particles forming a pair selected in specified transverse-momentum ranges. Averaged over many events, jets result in a well defined peak structure centered at zero separation in pseudorapidity and azimuth. This note explores the evolution of the two-particle jet peak shape with the pair transverse momentum ranges, the collision centrality, and the pseudorapidity of the jet peak. Lead-lead collision results at a center-of-mass energy per nucleon pair of 5.02 TeV are presented. The data were obtained using the CMS detector and correspond to an integrated luminosity of 0.607 nb$^{-1}$. Proton-proton collision data at the same nucleon-nucleon collision energy and corresponding to 252 nb$^{-1}$ are also presented to provide a vacuum reference. The results are discussed in terms of the boost invariance of the two-particle correlation jet peak shape. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Signal distribution (left), mixed event distribution (middle), correlation function (right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions data at 5.02 TeV. |
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Figure 2:
2D correlations + 2D Fit functions (upper panel), 2D correlations (lower panel: left), 2D Fit function (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions data at 5.02 TeV. |
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Figure 2-a:
2D correlations + 2D Fit functions (upper panel), 2D correlations (lower panel: left), 2D Fit function (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions data at 5.02 TeV. |
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Figure 2-b:
2D correlations + 2D Fit functions (upper panel), 2D correlations (lower panel: left), 2D Fit function (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions data at 5.02 TeV. |
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Figure 2-c:
2D correlations + 2D Fit functions (upper panel), 2D correlations (lower panel: left), 2D Fit function (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions data at 5.02 TeV. |
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Figure 3:
2D Fit functions (upper panel), Flow background fit (lower panel: left), Near-side 2D peak fit (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions. |
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Figure 3-a:
2D Fit functions (upper panel), Flow background fit (lower panel: left), Near-side 2D peak fit (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions. |
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Figure 3-b:
2D Fit functions (upper panel), Flow background fit (lower panel: left), Near-side 2D peak fit (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions. |
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Figure 3-c:
2D Fit functions (upper panel), Flow background fit (lower panel: left), Near-side 2D peak fit (lower panel: right) for 0-10% centrality, 3.0 $ < p_\mathrm{T,trig} < $ 4.0 GeV and 2.0 $ < p_\mathrm{T,asso} < $ 3.0 GeV in PbPb collisions. |
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Figure 4:
1D $ \Delta\eta $(left) and $ \Delta\phi $(right) projection fit. |
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Figure 4-a:
1D $ \Delta\eta $(left) and $ \Delta\phi $(right) projection fit. |
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Figure 4-b:
1D $ \Delta\eta $(left) and $ \Delta\phi $(right) projection fit. |
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Figure 5:
1D fit of $ \Delta\eta $ including one generalized Gaussian (blue) and a standard Gaussian (green) in the range $ |\Delta\eta| < $ 4.0. |
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Figure 6:
2D correlation of 1.5 $ < |\eta_\mathrm{trig}| < $ 2.0. In upper panel: the left plot (first) corresponds to $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5, the right plot (second) is the mirror image of the first. In lower panel: the left plot (third) is for 1.5 $ < \eta_\mathrm{trig} < $ 2.0, and the right plot (fourth) is the average of the second and the third plots. |
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Figure 6-a:
2D correlation of 1.5 $ < |\eta_\mathrm{trig}| < $ 2.0. In upper panel: the left plot (first) corresponds to $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5, the right plot (second) is the mirror image of the first. In lower panel: the left plot (third) is for 1.5 $ < \eta_\mathrm{trig} < $ 2.0, and the right plot (fourth) is the average of the second and the third plots. |
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Figure 6-b:
2D correlation of 1.5 $ < |\eta_\mathrm{trig}| < $ 2.0. In upper panel: the left plot (first) corresponds to $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5, the right plot (second) is the mirror image of the first. In lower panel: the left plot (third) is for 1.5 $ < \eta_\mathrm{trig} < $ 2.0, and the right plot (fourth) is the average of the second and the third plots. |
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Figure 6-c:
2D correlation of 1.5 $ < |\eta_\mathrm{trig}| < $ 2.0. In upper panel: the left plot (first) corresponds to $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5, the right plot (second) is the mirror image of the first. In lower panel: the left plot (third) is for 1.5 $ < \eta_\mathrm{trig} < $ 2.0, and the right plot (fourth) is the average of the second and the third plots. |
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Figure 6-d:
2D correlation of 1.5 $ < |\eta_\mathrm{trig}| < $ 2.0. In upper panel: the left plot (first) corresponds to $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5, the right plot (second) is the mirror image of the first. In lower panel: the left plot (third) is for 1.5 $ < \eta_\mathrm{trig} < $ 2.0, and the right plot (fourth) is the average of the second and the third plots. |
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Figure 7:
1D $ \Delta\eta $ projection from the averaged 2D correlation, combining 1.5 $ < \eta_\mathrm{trig} < $ 2.0 (original) and $ -$2.0 $ < \eta_\mathrm{trig} < - $1.5 (mirror). |
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Figure 8:
Longitudinal width $ \sigma_{\Delta\eta} $ (left panel) and transverse width $ \sigma_{\Delta\phi} $ (right panel) as a function of centrality in different $ p_{\mathrm{T}} $ ranges for PbPb collisions and pp collisions (rightmost points in each panel). The statistical uncertainties of the data points are smaller than the marker size, and rectangular boxes indicate the systematic uncertainties. The dashed lines represent the expectation from the HYDJET 1.9 Monte Carlo event generator. |
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Figure 8-a:
Longitudinal width $ \sigma_{\Delta\eta} $ (left panel) and transverse width $ \sigma_{\Delta\phi} $ (right panel) as a function of centrality in different $ p_{\mathrm{T}} $ ranges for PbPb collisions and pp collisions (rightmost points in each panel). The statistical uncertainties of the data points are smaller than the marker size, and rectangular boxes indicate the systematic uncertainties. The dashed lines represent the expectation from the HYDJET 1.9 Monte Carlo event generator. |
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Figure 8-b:
Longitudinal width $ \sigma_{\Delta\eta} $ (left panel) and transverse width $ \sigma_{\Delta\phi} $ (right panel) as a function of centrality in different $ p_{\mathrm{T}} $ ranges for PbPb collisions and pp collisions (rightmost points in each panel). The statistical uncertainties of the data points are smaller than the marker size, and rectangular boxes indicate the systematic uncertainties. The dashed lines represent the expectation from the HYDJET 1.9 Monte Carlo event generator. |
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Figure 9:
Associated yield ratios between $ \Delta\eta > $ 0.0 and $ \Delta\eta < $ 0.0 within the range of 0.0 $ < |\Delta\eta| < $ 0.7 as a function of centrality in different $ p_{\mathrm{T}} $ and $ \eta_\mathrm{trig} $ ranges for PbPb collisions and pp collisions (rightmost bin) are presented in the upper two panels. Each panel (upper and lower) consists of four plots, each corresponding to a different value of $ \eta_\mathrm{trig} $. In the top panel, each plot consists of three colored markers representing different $ p_\mathrm{T,trig} $ values (4 $ < p_\mathrm{T,trig} < $ 8, 8 $ < p_\mathrm{T,trig} < $ 12, 12 $ < p_\mathrm{T,trig} < $ 16 GeV), while $ p_\mathrm{T,asso} $ is fixed (1.5 $ < p_\mathrm{T,asso} < $ 2 GeV). In the bottom panel, each plot consists of three colored markers representing different $ p_\mathrm{T,asso} $ values (1.5 $ < p_\mathrm{T,asso} < $ 2, 2 $ < p_\mathrm{T,asso} < $ 3, 3 $ < p_\mathrm{T,asso} < $ 4 GeV), while $ p_\mathrm{T,trig} $ is fixed (12 $ < p_\mathrm{T,trig} < $ 16 GeV). The rectangular open boxes represent the systematic uncertainties, while the vertical bars indicate the statistical uncertainties. |
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Figure 10:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 10-a:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 10-b:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 10-c:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 10-d:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 10-e:
Cross-check of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak with ALICE results [34]. |
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Figure 11:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
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Figure 11-a:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
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Figure 11-b:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
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Figure 11-c:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
png pdf |
Figure 11-d:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
png pdf |
Figure 11-e:
Comparisons of longitudinal width ($ \sigma_{\Delta\eta} $) and transverse width ($ \sigma_{\Delta\phi} $) of the near side peak between CMS $ \eta < $ 0.8 and CMS $ \eta < $ 2.4 (sideband fit). |
| Tables | |
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Table 1:
Summary of the absolute systematic uncertainties for $ \sigma_{\Delta\eta} $, $ \sigma_{\Delta\phi} $, and the associated yield ratio. The leftmost column lists all systematic sources, while the other columns show the range of systematic uncertainties across all centrality bins in PbPb and pp MB. |
| Summary |
| The centrality and pseudorapidity dependencies of jet peak shapes are explored using two-particle correlations. Lead-lead collision data at a center-of-mass energy per nucleon pair of 5.02 TeV were obtained using the CMS detector. These minimum-bias data correspond to an integrated luminosity of 0.607 nb$^{-1}$. Proton-proton collision data at the same nucleon-nucleon collisions energy are also shown to provide a vacuum reference. Particles detected in one transverse-momentum range are correlated with all particles in an event within a second range. The separations in pseudorapidity and azimuth of particles within each two-particle pair are then averaged over all events. Jets result in a peak shaped structure corresponding to pairs where there is a minimal separation of particles. The widths in pseudorapidity and azimuth of the jet peak shape are presented as functions of centrality and pseudorapidity. The skewness of the jet peak shape as a function of pseudorapidity is also explored by taking ratios of yields on the two sides of the peak. For PbPb collisions, the widths of the jet peak azimuth distributions are found to be nearly independent of collision centrality, and are comparable to those in the pp reference distribution. Similarly, when one of the particles in each PbPb collision pair has a high transverse momentum, with $ p_{\mathrm{T}} > $ 4 GeV, the jet peak pseudorapidity distributions also show little dependence on centrality and remain consistent with the pp reference distribution. However, when the high-$ p_{\mathrm{T}} $ particle in each PbPb pair is restricted to a relatively low momentum range of 3 $ < p_{\mathrm{T}} < $ 4 GeV, an enhancement is found in the pseudorapidity width that increases as collisions become more central. In this case, the pseudorapidity width only agrees with the pp reference for the most peripheral events. These results might be explained either by the influence of longitudinal hydrodynamic flow on the pseudorapidity width or by the progenitor parton losing energy while passing through the medium. The skewness of the jet peak distribution in the particle pseudorapidity difference is studied by taking the ratio of yields on either side of the jet peaks. This ratio is determined in different, non-overlapping pseudorapidity ranges for the higher $ p_{\mathrm{T}} $ particle in each pair. The observed ratios increase as the average pseudorapidity increases. For a given pseudorapidity, the ratios tend to remain relatively constant as a function of centrality, and typically higher than the pp reference except for the most peripheral PbPb events. These results might again reflect the influence of longitudinal hydrodynamic flow. |
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