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| CMS-HIN-16-020 ; CERN-EP-2018-011 | ||
| Jet properties in PbPb and pp collisions at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 28 February 2018 | ||
| JHEP 05 (2018) 006 | ||
| Abstract: Modifications of the properties of jets in PbPb collisions, relative to those in pp collisions, are studied at a nucleon-nucleon center-of-mass energy of $\sqrt{\smash[b]{s_{\mathrm{NN}}}} = $ 5.02 TeV via correlations of charged particles with the jet axis in relative pseudorapidity ($\Delta \eta$), relative azimuth ($\Delta \phi$), and relative angular distance from the jet axis ${\Delta \mathrm{r}} = \sqrt{\smash[b]{(\Delta\eta)^{2}+(\Delta\phi)^{2}}}$. This analysis uses data collected with the CMS detector at the LHC, corresponding to integrated luminosities of 404 $\mu$b$^{-1}$ and 27.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Charged particle number densities, jet fragmentation functions, and jet shapes are presented as a function of PbPb collision centrality and charged-particle track transverse momentum, providing a differential description of jet modifications due to interactions with the quark-gluon plasma. | ||
| Links: e-print arXiv:1803.00042 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
The distribution of jet-correlated charged-particle tracks with $ | \Delta \phi | < $ 1.0 as a function of $\Delta \eta $ in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the difference between the PbPb and pp data. The intervals in $ {p_{\mathrm {T}}^{\text {trk}}} $ are indicated by the stacked histograms. The inclusive points (0.7 $ < {p_{\mathrm {T}}^{\text {trk}}} < $ 20 GeV) are shown by open white circles. The grey bands surrounding these points show the total systematic uncertainties. |
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Figure 2:
The distribution of jet-correlated charged-particle tracks with $ | \Delta \eta | < $ 1.0 as a function of $\Delta \phi $ in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the difference between the PbPb and pp data. The intervals in $ {p_{\mathrm {T}}^{\text {trk}}} $ are indicated by the stacked histograms. The inclusive points (0.7 $ < {p_{\mathrm {T}}^{\text {trk}}} < $ 20 GeV) are shown by open white circles. The grey bands surrounding these points show the total systematic uncertainties. |
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Figure 3:
The distribution of jet-correlated charged-particle tracks as a function of $ {\Delta \mathrm {r}} $ in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the difference between the PbPb and pp data. The intervals in $ {p_{\mathrm {T}}^{\text {trk}}} $ are indicated by the stacked histograms. The inclusive points (0.7 $ < {p_{\mathrm {T}}^{\text {trk}}} < $ 20 GeV) are shown by open white circles. The grey bands surrounding these points show the total systematic uncertainties. |
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Figure 4:
The distribution of jet-correlated charged-particle tracks with $ {\Delta \mathrm {r}} < $ 1 (top row) as a function of $ {p_{\mathrm {T}}^{\text {trk}}} $ in PbPb and pp collisions. The PbPb results are shown for different centrality regions. Statistical uncertainties are shown by the vertical error bars and systematic uncertainties by the shaded boxes. The corresponding results for the difference between the PbPb and pp data are shown in the bottom row, with the analogous difference observed at 2.76 TeV [13]. |
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Figure 5:
The radial jet momentum distribution $ {\mathrm {P}(\Delta \mathrm {r})} $ of jets in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the ratio between PbPb and pp data for the indicated intervals of $ {p_{\mathrm {T}}^{\text {trk}}} $. The shaded bands show the total systematic uncertainties. |
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Figure 6:
The jet shape $\rho ({\Delta \mathrm {r}})$ in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the ratio between the PbPb and pp data for the inclusive range 0.7 $ < {p_{\mathrm {T}}^{\text {trk}}} < $ 300 GeV. The shaded band in the ratio plots shows the total systematic uncertainty. |
| Tables | |
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Table 1:
Systematic uncertainties in percentage for the measurements of the jet-track correlations in PbPb and pp events. The PbPb results are given in intervals of centrality. Where an uncertainty range is given, the upper edge of the range corresponds to the bin with the smallest $ {p_{\mathrm {T}}^{\text {trk}}} $ values. |
| Summary |
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In this paper, measurements are presented of the modifications to charged-particle track yields and jet shapes in PbPb collisions with respect to pp collisions using data collected with the CMS detector at the LHC at a nucleon-nucleon center-of-mass energy of $\sqrt{\smash[b]{s_{\mathrm{NN}}}} = $ 5.02 TeV. The correlations of charged particles having transverse momentum ${p_{\mathrm{T}}}trk > $ 0.7 GeV and pseudorapidity $ | {\eta} | < $ 2.4 with the jet axis of jets having ${p_{\mathrm{T}}} > $ 120 GeV and $ | {\eta} | < $ 1.6 are studied. Charged particle yields associated with jets are shown as a function of relative angular distance ${\Delta \mathrm{r}} = \sqrt{\smash[b]{(\Delta\eta)^{2}+(\Delta\phi)^{2}}}$ from the jet axis, as well as individually in $\Delta\eta$ and $\Delta\phi$. In these studies, a strong enhancement of tracks with ${p_{\mathrm{T}}}trk < $ 3 GeV, extending to large angles, is found in PbPb collisions with respect to pp collisions. This low $ {p_{\mathrm{T}}} $ excess remains correlated with the jet axis, but the distribution is broader in PbPb than that observed in the corresponding $ {p_{\mathrm{T}}} $ bin of pp collisions, which could indicate additional in-medium gluon radiation and/or a medium backreaction, i.e., a wake-like response of the QGP to the propagating parton. For PbPb events with centrality 0-10%, the correlated yield is increased relative to pp collisions at low $ {p_{\mathrm{T}}} $ by up to a factor of two. In addition to these charged-particle yields, we examine the jet transverse momentum profile ${\mathrm{P}(\Delta\mathrm{r})} $ and the jet shape $\rho({\Delta \mathrm{r}} )$ variables, defined using the distribution of charged-particle tracks in annular rings around the jet axis, with each particle weighted by its $ {p_{\mathrm{T}}}^{\text{trk}} $ value. A redistribution of energy from small to large angles from the jet axis is observed for PbPb relative to pp events, with the most pronounced effect seen for central collisions. The energy flow within the jet is modified by shifting the momentum away from the jet axis out to large relative angular distances such that in central PbPb collisions, the ratio of the jet shapes in PbPb to pp collisions approaches a value of two. These measurements provide a comprehensive picture of the modifications of the parton shower evolution and the quark-gluon plasma response to the propagating jets in 5 TeV PbPb collisions. |
| Additional Figures | |
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Additional Figure 1:
The ratio of the radial jet momentum distribution $ {\mathrm {P}(\Delta \mathrm {r})} $ of jets between PbPb and pp collisions for the indicated intervals of ${p_{\mathrm {T}}^{\text {trk}}}$ shown for the 0-10% (most central) bin. The shaded bands show the total systematic uncertainties. |
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