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| CMS-HIN-15-011 ; CERN-EP-2016-199 | ||
| Decomposing transverse momentum balance contributions for quenched jets in PbPb collisions at $\sqrt{s_\mathrm{NN}} = $ 2.76 TeV | ||
| CMS Collaboration | ||
| 8 September 2016 | ||
| JHEP 11 (2016) 055 | ||
| Abstract: Interactions between jets and the quark-gluon plasma produced in heavy ion collisions are studied via the angular distributions of summed charged-particle transverse momenta ($p_{\mathrm{T}}$) with respect to both the leading and subleading jet axes in high-$p_{\mathrm{T}}$ dijet events. The contributions of charged particles in different momentum ranges to the overall event $p_{\mathrm{T}}$ balance are decomposed into short-range jet peaks and a long-range azimuthal asymmetry in charged-particle $p_{\mathrm{T}}$. The results for PbPb collisions are compared to those in pp collisions using data collected in 2011 and 2013, at collision energy $\sqrt{s_\mathrm{NN}} =$ 2.76 TeV with integrated luminosities of 166 $\mu$b$^{-1}$ and 5.3 pb$^{-1}$, respectively, by the CMS experiment at the LHC. Measurements are presented as functions of PbPb collision centrality, charged-particle $p_{\mathrm{T}}$, relative azimuth, and radial distance from the jet axis for balanced and unbalanced dijets. | ||
| Links: e-print arXiv:1609.02466 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Jet-track correlation distributions for unbalanced dijet events ($A_\mathrm {J} > $ 0.22), projected over $ {| \Delta \eta | }< $ 1. and overlaid with a long-range distribution projected over 1.5 $ < {| \Delta \eta | }< $ 2.5. The region $ {| \Delta \phi | }<\pi /2$ is taken from the leading jet correlation, while the region $\pi /2 < \Delta \phi < 3\pi /2$ is taken from the subleading jet correlation. The top row shows the low ${ {p_{\mathrm {T}}} ^\text {trk}}$ bin 1 $ < { {p_{\mathrm {T}}} ^\text {trk}} < $ 2 GeV , while the bottom row shows the high ${ {p_{\mathrm {T}}} ^\text {trk}}$ bin 4 $ < { {p_{\mathrm {T}}} ^\text {trk}} < $ 8 GeV. Statistical uncertainties are shown with vertical bars. |
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Figure 2:
Top row: leading jet shape $\rho (\Delta r)$ for the pp reference, and peripheral and central PbPb data, shown for all tracks with $ {p_{\mathrm {T}}} > $ 0.5 GeV and decomposed by ${ {p_{\mathrm {T}}} ^\text {trk}}$ (with ${ {p_{\mathrm {T}}} ^\text {trk}}$ ranges denoted by different color shading). Shapes are normalized to unity over the region $\Delta r<0.3$ for comparison with the published reference shown (Ref. [11]). Bottom row: leading jet shape ratio $\rho (\Delta r)_\mathrm {PbPb}/\rho (\Delta r)_{\mathrm{ p } \mathrm{ p } }$, again with the published reference. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 3:
Top row: subleading jet shape $\rho (\Delta r)$ for pp reference and peripheral and central PbPb data, shown for all tracks with $ {p_{\mathrm {T}}} > $ 0.5 GeV and decomposed by ${ {p_{\mathrm {T}}} ^\text {trk}}$ (with ${ {p_{\mathrm {T}}} ^\text {trk}}$ ranges denoted by different color shading), normalized to unity over the region $\Delta r< $ 0.3. Bottom row: subleading jet shape ratio $\rho (\Delta r)_\mathrm {PbPb}/\rho (\Delta r)_{\mathrm{ p } \mathrm{ p } }$. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 4:
Top row: difference of the total ${ {p_{\mathrm {T}}} ^\text {trk}}$ distributions between subleading and leading jet hemispheres, projected on $\Delta \phi $, for balanced dijet events with $A_\mathrm {J} < $ 0.22 shown differentially by ${ {p_{\mathrm {T}}} ^\text {trk}}$ for pp reference, peripheral PbPb, and central PbPb data. Bottom row: PbPb-pp difference in these $\Delta \phi $ momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 5:
Top row: difference of the total ${ {p_{\mathrm {T}}} ^\text {trk}}$ distributions between subleading and leading jet hemispheres, projected on $\Delta \phi $, for unbalanced dijet events with $A_\mathrm {J} > $ 0.22, shown differentially by ${ {p_{\mathrm {T}}} ^\text {trk}}$ for the pp reference data, peripheral PbPb, and central PbPb data. Bottom row: PbPb-pp difference in these $\Delta \phi $ momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 6:
Top row: jet-peak (long-range subtracted) distribution in $\Delta \phi $ of ${ {p_{\mathrm {T}}} ^\text {trk}}$ about the subleading (plotted positive) and leading (plotted negative) jets for balanced dijet events with $A_\mathrm {J} < $ 0.22. Middle rows: PbPb-pp momentum distribution differences for subleading and leading jets. Bottom row: PbPb-pp, subleading-leading double difference in these $\Delta \phi $ momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 7:
Top row: jet-peak (long-range subtracted) distribution in $\Delta \phi $ of ${ {p_{\mathrm {T}}} ^\text {trk}}$ about the subleading (plotted positive) and leading (plotted negative) jets for unbalanced dijet events with $A_\mathrm {J} > $ 0.22. Middle rows: PbPb-pp momentum distribution differences for subleading and leading jets. Bottom row: PbPb-pp, subleading-leading double difference in these $\Delta \phi $ momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 8:
Top row: long-range distribution in $\Delta \phi $ of excess ${ {p_{\mathrm {T}}} ^\text {trk}}$ in the subleading relative to leading sides for balanced dijet events with $A_\mathrm {J} < $ 0.22. Bottom row: PbPb-pp difference in these $\Delta \phi $ long-range momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 9:
Top row: long-range distribution in $\Delta \phi $ of excess ${ {p_{\mathrm {T}}} ^\text {trk}}$ in the subleading relative to leading sides for unbalanced dijet events with $A_\mathrm {J} > $ 0.22. Bottom row: PbPb-pp difference in these $\Delta \phi $ long-range momentum distributions. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 10:
Integrated ${p_{\mathrm {T}}}$ in the long-range $\Delta \phi $-correlated distribution as a function of track-${ {p_{\mathrm {T}}} ^\text {trk}}$ integrated over $ {| \Delta \phi | } < \pi /2$ and $ {| \Delta \eta | } < $ 2.5 for pp reference, peripheral PbPb and central PbPb data for balanced compared to unbalanced dijet events. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 11:
Modifications of jet-track correlated ${ {p_{\mathrm {T}}} ^\text {trk}}$ in central PbPb collisions with respect to the pp reference data, integrated over $ {| \Delta \phi | } < \pi /2$ and $ {| \Delta \eta | } < $ 2.5. Top row: subleading and leading jet peaks PbPb-pp. Bottom row: relative contributions from jet peaks and long-range asymmetry to the double difference (PbPb-pp, subleading-leading) in total hemisphere ${ {p_{\mathrm {T}}} ^\text {trk}} $. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
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Figure 12:
Modifications of jet-track correlated ${ {p_{\mathrm {T}}} ^\text {trk}}$ in peripheral PbPb collisions with respect to the pp reference data, integrated over $ {| \Delta \phi | } < \pi /2$ and $ {| \Delta \eta | } < $ 2.5. Top row: subleading and leading jet peaks PbPb-pp. Bottom row: relative contributions from jet peaks and two-dimensional asymmetry to the double difference PbPb-pp, subleading-leading in total hemisphere ${ {p_{\mathrm {T}}} ^\text {trk}} $. Statistical uncertainties are shown with vertical bars, and systematic uncertainties are shown with shaded boxes. |
| Tables | |
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Table 1:
This table summarizes the systematic uncertainties in the measurement of the jet-track correlations in PbPb and pp collisions. Upper and lower limits are shown as a function of collision centrality. Upper values correspond to the uncertainties at lowest ${ {p_{\mathrm {T}}} ^\text {trk}} $. |
| Summary |
| In this analysis, the redistribution of momentum in dijet events is studied via two-dimensional $\Delta\eta$-$\Delta\phi$ jet-track correlations in PbPb and pp collisions at $\sqrt{s_\mathrm{NN}} =$ 2.76 TeV, using data sets with integrated luminosities of 166 $\mu$b$^{-1}$ and 5.3 pb$^{-1}$, respectively. Events are selected to include a leading jet with $p_\mathrm{T,1} > $ 120 GeV and subleading jet with $p_\mathrm{T,2} > $ 50 GeV, with an azimuthal separation of at least $ \Delta\phi_{1,2} > 5\pi/6 $. Subtracting the long-range part of the correlation from the jet peaks, this work extends the studies of jet shape modifications in PbPb events relative to pp collisions to large radial distances ($\Delta r \approx $ 1) from the jet axis. These modifications are found to extend out to the largest radial distance studied in central PbPb events for both leading and subleading jets. The jet modifications are further studied differentially for balanced and unbalanced dijet events, and as a function of $p_{\mathrm{T}}^{\text{trk}} $ and collision centrality for each of these selections in PbPb and pp reference data. Transverse momentum redistribution around the subleading and leading jets, as well as differences in long-range correlated background asymmetry, are separately analyzed. An excess of transverse momentum carried by soft particles ($p_{\mathrm{T}}^{\text{trk}} < $ 2 GeV) is found for both leading and subleading jets in central PbPb collisions relative to the pp reference, consistent with previous studies of charged-particle yields correlated to high-$p_{\mathrm{T}}$ jets. For unbalanced dijet events, this low-$p_{\mathrm{T}}^{\text{trk}}$ excess is greater on the subleading-jet side than on the leading-jet side. However, the difference in $p_{\mathrm{T}}^{\text{trk}}$ contained in the Gaussian-like jet peaks is found only partially to account for the total $p_{\mathrm{T}}^{\text{trk}}$ redistribution in the most central PbPb collisions with dijet events. In the long-range correlated distribution of $p_{\mathrm{T}}^{\text{trk}}$ under the jet peaks, it is found that the excess of relatively high-$p_{\mathrm{T}}$ particles (2 $ < p_{\mathrm{T}}^{\text{trk}} < $ 8 GeV) on the subleading side relative to the leading side, which is observed in asymmetric pp collisions and mainly attributed to three-jet events, is absent in most central PbPb collisions. This indicates that the fraction of events with additional jets in the asymmetric dijet sample is significantly lower than in an identical selection of dijet events in pp data. A long-range asymmetry in the low-$p_{\mathrm{T}}$ particles in dijet events is also observed, providing further input for the theoretical understanding of jet-medium coupling. |
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