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| CMS-PAS-HIN-16-020 | ||
| Jet properties in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| February 2017 | ||
| Abstract: Modifications to high transverse momentum ($p_{\rm T}$) jets in the quark gluon plasma (QGP) are studied in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV via correlations between jets with $p_{\rm T} > $ 120 GeV and charged particles with $p_{\rm T} > $ 0.7 GeV. This measurement aims to extend results of the same correlations at $\sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV, which observed significant modification of jet track correlations out to large distances from the jet axis in central PbPb events. This analysis measures correlations in relative pseudorapidity ($\Delta \eta$), relative azimuth ($\Delta \phi$), and relative distance from the jet axis $\Delta r = \sqrt{\Delta\eta^{2}+\Delta\phi^{2}}$. Results are presented as a function of PbPb collision centrality and track $p_{\rm T}$, providing a differential picture of jet modifications in the medium. Measurements include charged particle densities as a function of distance from jet axis, total charged-particle yields as a function of track-$p_{\rm T}$ (fragmentation functions), and the jet momentum density profile (''jet shape''). This analysis uses data collected with the CMS detector at collision energy $\sqrt{\rm s_{NN}} = $ 5.02 TeV in 2015, corresponding to integrated luminosities of 404 $\mu\mathrm{b}^{-1}$ for PbPb and 27.4 $\mathrm{pb}^{-1}$ for pp. | ||
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These preliminary results are superseded in this paper, JHEP 05 (2018) 006. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} > $ 120 GeV as a function of $\Delta \eta $ (projected over $|\Delta \phi | < $ 1), shown differentially for all $p_{\rm T}^{\rm trk}$ bins for pp, peripheral PbPb, and central PbPb data. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$-inclusive points show total systematic uncertainties. |
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Figure 2:
Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} > $ 120 GeV as a function of $\Delta \phi $ (projected over $|\Delta \eta | < $ 1), shown differentially for all $p_{\rm T}^{\rm trk}$ bins for pp, peripheral PbPb, and central PbPb data. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$-inclusive points show total systematic uncertainties. |
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Figure 3:
Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} > $ 120 GeV as a function of $\Delta r$, shown differentially for all $p_{\rm T}^{\rm trk}$ bins. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$-inclusive points show total systematic uncertainties. |
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Figure 4:
Top row: integrated yields of charged particle yields correlated to jets with $p_{\rm T} > $ 120 GeV as a function of $p_{\rm T}^{\rm trk}$ bins for PbPb data, compared to pp reference. Bottom row: integrated excess yield, PbPb minus pp. New measurements of excess yields at 5.02 TeV are compared to those measured at 2.76 TeV in CMS study [12]. |
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Figure 5:
Top row: Transverse momentum profile of jets ${\rm P}(\Delta r)$ in pp and PbPb, shown differentially in $p_{\rm T}^{\rm trk}$. Middle row: jet shapes $\rho (\Delta r)$ (normalized to unity over $\Delta r < $ 1) in PbPb and pp. Bottom row: jet shape ratio $\rho (\Delta r)_{\rm PbPb} / \rho (\Delta r)_{\rm pp}$. Hatched lines on $p_{\rm T}^{\rm trk}$-inclusive points show total systematic uncertainties. |
| Tables | |
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Table 1:
Systematic uncertainties in the measurement of the jet track correlations in PbPb and pp collisions. The numbers presented in this table summarize typical range of systematic uncertainty as a function of collision centrality. The upper limits of the cited values correspond to uncertainties at lowest $p_{\rm T}^{\rm trk}$, and uncertainties decrease with rising $p_{\rm T}^{\rm trk}$. |
| Summary |
| This analysis studies properties of high-$p_{\rm T}$ jets in heavy ion collisions, using LHC Run-2 data collected by the CMS detector at collision energy $\sqrt{\rm s_{NN}} = $ 5.02 TeV in 2015. These studies are based on correlations between jets with $p_{\rm T} > $ 120 GeV, and charged-particle tracks with 0.7 $ < p_{\rm T}^{\rm trk} < $ 300 GeV, analyzed differentially in $p_{\rm T}^{\rm trk}$ and collision centrality. Charged-particle correlations are presented as a function of angular distance from the jet, individually $\Delta\eta$ and $\Delta\phi$. In these studies, a low-$p_{\rm T}$ excess (tracks with $p_{\rm T}^{\rm trk} < $ 3 GeV) extending to large angles $\Delta\eta = $ 1 and $\Delta\phi = $ 1 is found in central PbPb collisions relative to pp reference. In addition to separate studies in $\Delta\eta$ and $\Delta\phi$, charged-particle densities are also measured as a function of distance from the jet axis $\Delta r$. To summarize these measurements, correlations are integrated over $\Delta r < $ 1 to obtain a measurement of the total number of particles in each $p_{\rm T}^{\rm trk}$ class. A comparison of integrals shows as many as three additional low-$p_{\rm T}$ particles per unit of $p_{\rm T}^{\rm trk}$ in central PbPb collisions relative to pp reference. In addition to these charged-particle density studies, correlations are constructed with each track weighted by its $p_{\rm T}^{\rm trk}$ in order to study the overall distribution of $p_{\rm T}^{\rm trk}$ with respect to the jet axis. This is used to measure the jet shape $\rho(\Delta r)$ to a distance of $\Delta r = $ 1 from the jet axis, presented differentially as a function of $p_{\rm T}^{\rm trk}$. A comparison of central PbPb versus pp jet shape shows redistribution of $p_{\rm T}^{\rm trk}$ from small angles ($\Delta r < $ 0.3) near the jet axis to large angles ($\Delta r = $ 1) away from the jet axis. In all cases, the modifications observed both in terms of yield modification and jet shapes are similar between the measurements at $\sqrt{\rm s_{NN}} = $ 2.76 and 5.02 TeV when directly comparing PbPb to pp differences in corresponding centrality bins. |
| Additional Figures | |
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Additional Figure 1:
Jet-track correlation signal shape $S(\Delta \eta ,\Delta \phi )$ for tracks with 1 $ < p_{\rm T}^{\rm trk} < $ 2 GeV in 0-10% central events (left), and corresponding mixed event shape $ME(\Delta \eta ,\Delta \phi )$ for the same centrality and $p_{\rm T}^{\rm trk}$ bin (center). Their ratio gives the acceptance-corrected yield (right). |
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Additional Figure 2:
Acceptance-corrected 2D jet-track correlation yield (left) is projected over the range 1.5 $ < |\Delta \eta | < $ 2.5 and re-propagated in $\Delta \eta $ to produce a 2D background distribution (center). This background is subtracted from the total yield to obtain the 2D background-subtracted yield shown on the right (for tracks with 1 $ < p_{\rm T}^{\rm trk} < $ 2 GeV). |
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Additional Figure 3:
Jet peak correlated yield or tracks with 1 $ < p_{\rm T}^{\rm trk} < $ 2 GeV, after background subtraction. |
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Additional Figure 4:
Illustration of charged particle correlation observables, shown for tracks with 1 $ < p_{\rm T}^{\rm trk} < $ 2 GeV. Left: $\Delta \eta $ distribution of correlated yield (projected over $|\Delta \phi | < $ 1). Center: $\Delta \phi $ distribution of correlated yield (projected over $|\Delta \eta | < $ 1). Right: $\Delta r$ distribution of correlated yield (integrated in rings of $\Delta r$) |
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Additional Figure 5:
Top row: Transverse momentum profile of jets ${\rm P}(\Delta r)$ in pp and PbPb, shown differentially in $p_{\rm T}^{\rm trk}$. Middle row: jet shapes $\rho (\Delta r)$ (normalized to unity over $\Delta r < 1$) in PbPb and pp. Bottom row: jet shape ratio $\rho (\Delta r)_{\rm PbPb} / \rho (\Delta r)_{\rm pp}$. Hatched lines on $p_{\rm T}^{\rm trk}$-inclusive points show total systematic uncertainties. Generator-level PYTHIA 6 is shown with green crosses. |
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