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| CMS-HIN-15-015 ; CERN-EP-2016-242 | ||
| Charged-particle nuclear modification factors in PbPb and pPb collisions at $\sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV | ||
| CMS Collaboration | ||
| 5 November 2016 | ||
| JHEP 04 (2017) 039 | ||
| Abstract: The spectra of charged particles produced within the pseudorapidity window $| \eta | < $ 1 at $\sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV are measured using 404 $ \mu $b$^{-1}$ of PbPb and 27.4 pb$^{-1}$ of pp data collected by the CMS detector at the LHC in 2015. The spectra are presented over the transverse momentum ranges spanning 0.5 $ < p_{\mathrm{T}} < $ 400 GeV in pp and 0.7 $ < p_{\mathrm{T}} < $ 400 GeV in PbPb collisions. The corresponding nuclear modification factor, ${R_\mathrm{AA}} $, is measured in bins of collision centrality. The ${R_\mathrm{AA}} $ in the 5% most central collisions shows a maximal suppression by a factor of 7-8 in the $p_{\mathrm{T}}$ region of 6-9 GeV. This dip is followed by an increase, which continues up to the highest $ p_{\mathrm{T}}$ measured, and approaches unity in the vicinity of $p_{\mathrm{T}}= $ 200 GeV. The ${R_\mathrm{AA}} $ is compared to theoretical predictions and earlier experimental results at lower collision energies. The newly measured pp spectrum is combined with the pPb spectrum previously published by the CMS Collaboration to construct the pPb nuclear modification factor, ${R_{\mathrm{ p }\mathrm{A}}} $, up to 120 GeV. For $ p_{\mathrm{T}} > $ 20 GeV, ${R_{\mathrm{ p }\mathrm{A}}} $ exhibits weak momentum dependence and shows a moderate enhancement above unity. | ||
| Links: e-print arXiv:1611.01664 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Left: ratio of the leading jet $ {p_{\mathrm {T}}} $ distributions in PbPb collisions in the 0-30% centrality range from various triggers, after the data have been normalized to one another. Lines have been added to guide the eye. Right: contributions from the various jet triggers (colored histograms) to the combined, but otherwise uncorrected, track spectrum (black markers) in the 0-5% centrality range in PbPb collisions. The statistical uncertainties are smaller than the size of the data markers. |
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Figure 1-a:
Ratio of the leading jet $ {p_{\mathrm {T}}} $ distributions in PbPb collisions in the 0-30% centrality range from various triggers, after the data have been normalized to one another. Lines have been added to guide the eye. |
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Figure 1-b:
Contributions from the various jet triggers (colored histograms) to the combined, but otherwise uncorrected, track spectrum (black markers) in the 0-5% centrality range in PbPb collisions. The statistical uncertainties are smaller than the size of the data markers. |
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Figure 2:
(Top panel) Charged-particle per-event yields measured in various PbPb centrality classes, as well as in pp data. A factor of 70 mb is used to scale the pp spectrum from a differential cross section to a per-event yield for direct comparison. The statistical uncertainties are smaller than the size of the markers for most points. (Bottom panel) Systematic uncertainties as a function of $ {p_{\mathrm {T}}} $ for representative data sets. The pp uncertainty contains a 2.3% fully correlated uncertainty in the pp integrated luminosity. |
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Figure 3:
Charged-particle $ R_{\mathrm {AA}} $ measured in six different centrality ranges at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11] (all centrality bins), ALICE [9] (in the 0-5% and 5-10% centrality ranges), and ATLAS [10] (in the 0-5% centrality range). The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-a:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 0-5% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11], ALICE [9], and ATLAS [10]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-b:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 5-10% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11], and ALICE [9]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-c:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 10-30% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-d:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 30-50% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-e:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 50-70% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 3-f:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 70-90% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to results at $ \sqrt { s_{\mathrm {NN}} } = $ 2.76 TeV from CMS [11]. The yellow boxes represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 4:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 0-10% (left) and 30-50% (right) centrality ranges at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to predictions of models from Refs. [36,37,38,39]. The yellow band represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 4-a:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 0-10% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to predictions of models from Refs. [36,37,38,39]. The yellow band represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 4-b:
Charged-particle $ R_{\mathrm {AA}} $ measured in the 30-50% centrality range at $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV compared to predictions of models from Refs. [36,37,38,39]. The yellow band represents the systematic uncertainty of the 5.02 TeV CMS points. |
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Figure 5:
Measurements of the nuclear modification factors in central heavy-ion collisions at four different center-of-mass energies, for neutral pions (SPS, RHIC), charged hadrons ($h^{\pm }$) (SPS, RHIC), and charged particles (LHC), from Refs. [3,4,9,10,11,44,45,46], compared to predictions of four models for $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV PbPb collisions from Refs. [36,37,38,39]. The error bars represent the statistical uncertainties. The yellow band around the 5.02 TeV CMS data points show the systematic uncertainties of this measurement, including that of the integrated luminosity. The $T_\mathrm {AA}$ uncertainties, of the order of $\pm $5%, are not shown. Percentage values in parentheses indicate centrality ranges. |
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Figure 6:
Measurements of the nuclear modification factor for an inclusive centrality class for both PbPb and pPb collisions. The $ {R_{\mathrm{ p } \mathrm {A}}} $ values are formed using the previously published CMS pPb data [13] and the pp reference spectrum described in this paper. Please refer to the main text about the exact procedure followed. The green and orange boxes show the systematic uncertainties for $ {R_{\mathrm{ p } \mathrm {A}}} $ and $ R_{\mathrm {AA}} $, respectively, while the $T_{\mathrm{ p } \mathrm {A}}$, $T_\mathrm {AA}$, and pp luminosity uncertainties are shown as boxes at low $ {p_{\mathrm {T}}} $ around unity. |
| Tables | |
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Table 1:
The values of $< N_\text {coll} > $ and $T_\mathrm {AA}$ and their uncertainties in $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV PbPb collisions for the centrality ranges used in this paper. |
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Table 2:
Summary of the $ {E_{\mathrm {T}}} $ and $ {p_{\mathrm {T}}} $ thresholds of the various L1 and HLT triggers used in the analysis for the two colliding systems. Please refer to the text about the exact meaning of the thresholds. Only the highest-threshold triggers collected data unprescaled. The MB symbol refers to seeding by a minimum-bias trigger. |
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Table 3:
Systematic uncertainties associated with the measurement of the charged-particle spectra and $ R_{\mathrm {AA}} $ using $ \sqrt { s_{\mathrm {NN}} } = $ 5.02 TeV pp and PbPb collision data. The ranges quoted cover both the $ {p_{\mathrm {T}}} $ and the centrality dependence of the uncertainties. The combined uncertainty in $ R_{\mathrm {AA}} $ does not include the integrated luminosity and the $T_\mathrm {AA}$ uncertainties. |
| Summary |
| The transverse momentum spectra of charged particles in pp and PbPb collisions at $\sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV have been measured in the pseudorapidity window $ | \eta | < $ 1 in the $ p_{\mathrm{T}} $ ranges of 0.5-400 (pp) and 0.7-400 GeV (PbPb). Using these spectra, the nuclear modification factor ${R_\mathrm{AA}} $ has been constructed in several bins of collision centrality. In the 0-5% bin, the ${R_\mathrm{AA}} $ shows a maximum suppression of a factor of 7-8 around $ p_{\mathrm{T}}= $ 7 GeV. At higher $p_{\mathrm{T}}$, it exhibits a rise, reaching a value of ${R_\mathrm{AA}} = $ 0.86 $\pm$ 0.28 in the $ p_{\mathrm{T}}$ bin from 250 to 400 GeV. As collisions become more peripheral, a weakening of both the magnitude and $p_{\mathrm{T}}$ dependence of this suppression is observed. Comparisons of the measured ${R_\mathrm{AA}} $ values to the 2.76 TeV results reveal similar $ p_{\mathrm{T}}$ dependence and similar suppression. Predictions of the high-$ p_{\mathrm{T}} $ $ {R_\mathrm{AA}} $ coming from the scet$_G$ and v-usphydro+BBMG models are found to approximately reproduce the present data. The cujet 3.0 model and a model parametrizing the departure of the medium transport coefficient, $\hat{q}$, from an ideal estimate, both predict ${R_\mathrm{AA}} $ suppressions that are slightly larger than seen in data. The nuclear modification factor in pPb collisions has been recomputed switching from an interpolation-based reference to the newly measured pp data at $\sqrt{s}= $ 5.02 TeV. In the pPb system, in contrast to the PbPb system, no suppression is observed in the 2-10 GeV region. A weak momentum dependence is seen for $p_{\mathrm{T}}> $ 10 GeV in the pPb system, leading to a moderate excess above unity at high $ p_{\mathrm{T}} $. The pPb and PbPb nuclear modification factors presented in this paper, covering $ p_{\mathrm{T}}$ ranges up to 120 and 400 GeV, respectively, provide stringent constraints on cold and hot nuclear matter effects. |
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