CMS-PAS-HIN-15-015

CMS-PAS-HIN-15-015
Measurement of the charged particle nuclear modification factor in PbPb collisions at $\sqrt{s_{\rm NN}}=$ 5.02 TeV
CMS Collaboration
May 2016

Abstract: The nuclear modification factor, $R_{\rm AA}$ , of charged particles produced in the $\|\eta\|<$ 1 pseudorapidity region is measured at $\sqrt{s_{\rm NN}}=$ 5.02 TeV using 404 $\mu$ b $^{-1}$ of PbPb and 25.8 pb $^{-1}$ of pp data collected by the CMS experiment at the end of 2015. The $R_{\rm AA}$ is presented in several bins of collision centrality, over the transverse momentum range of 0.7-400 GeV. The measured $R_{\rm AA}$ in the 0-5% most central collision class shows a maximum suppression by a factor of approximately 7.5 in the 6-9 GeV $p_\mathrm{T}$ region, followed by a rising trend up to the highest transverse momenta measured in the analysis. The measurement is compared to earlier results at lower collision energies, and to theory predictions.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 04 (2017) 039. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1-a: a: ratio of the leading-jet ${p_{\mathrm {T}}}$ distributions in PbPb collisions in the 0-30% centrality bin from various triggers, after the data have been normalized to one another. For better visibility, lines were added to guide the eye. b: contributions from the various jet triggers (colored histograms) to the combined, but otherwise uncorrected, track spectrum (black markers) in the 0-5% centrality bin.
png pdf	Figure 1-b: a: ratio of the leading-jet ${p_{\mathrm {T}}}$ distributions in PbPb collisions in the 0-30% centrality bin from various triggers, after the data have been normalized to one another. For better visibility, lines were added to guide the eye. b: contributions from the various jet triggers (colored histograms) to the combined, but otherwise uncorrected, track spectrum (black markers) in the 0-5% centrality bin.
png pdf	Figure 2-a: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 2-b: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 2-c: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 2-d: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 2-e: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 2-f: Charged particle $R_{\rm AA}$ measured in six different centrality ranges in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around 1 show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.
png pdf	Figure 3-a: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 3-b: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 3-c: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 3-d: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 3-e: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 3-f: Charged particle $R_{\rm AA}$ measured in six different centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to CMS [10] (all centrality bins), ALICE [8] (in the 0-5% and 5-10% centrality bins) and ATLAS [9] (in the 0-5% centrality bin) results at a center-of-mass energy per nucleon pair of 2.76 TeV.
png pdf	Figure 4-a: Charged particle $R_{\rm AA}$ measured in the 0-10% (a) and 30-50% (b) centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to predictions of models from Refs. [26] and [27].
png pdf	Figure 4-b: Charged particle $R_{\rm AA}$ measured in the 0-10% (a) and 30-50% (b) centrality ranges at $\sqrt {s_{\rm NN}}=$ 5.02 TeV compared to predictions of models from Refs. [26] and [27].
png pdf	Figure 5: Measurements of the nuclear modification factors in central heavy-ion collisions at four different center-of-mass energies, for neutral pions ( $\pi ^{0}$ ) (SPS, RHIC), charged hadrons ( $h^{\pm }$ ) (SPS, RHIC), and charged particles (LHC), from Refs. [28-32,8-10], compared to predictions of two models from Refs. [26,27]. The error bars represent the statistical uncertainties. The yellow band around the 5TeV CMS data shows the systematic uncertainties of the measurement, including that of the luminosity. The $T_{\rm AA}$ uncertainties, of the order of $\pm$ 5%, are not shown.
png pdf	Figure 6: Charged particle $R_{\rm AA}$ measured in the inclusive 0-100% centrality range in PbPb collisions at $\sqrt {s_{\rm NN}}=$ 5.02 TeV collision energy. The error bars represent the statistical uncertainties, the blue and gray error boxes around unity show the Glauber-model-related $T_{\rm AA}$ and the pp luminosity uncertainties, respectively, while the yellow band represents all other systematic uncertainties.

Tables
png pdf	Table 1: Summary of the $N_{\rm coll}$ and $T_{\rm AA}$ values and their uncertainties in $\sqrt {s_{NN}}=$ 5.02 TeV PbPb collisions for the centrality bins used in this report.
png pdf	Table 2: Systematic uncertainties associated with the charged particle $R_{\rm AA}$ measurement. The ranges quoted cover both the ${p_{\mathrm {T}}}$ and the centrality dependence of the uncertainties. The uncertainty of $R_{\rm AA}$ quoted in the table does not include the luminosity and the $T_{\rm AA}$ uncertainties.

Summary

The presented charged particle

$R_{\rm AA}$ distributions help to illuminate the mechanisms for jet quenching and the properties of the medium produced in heavy-ion collisions. Together with measurements on high-

$p_\mathrm{T}$ charged particle azimuthal anisotropies, inclusive jet spectra, fragmentation functions,

$\gamma$ -jet and Z-jet correlations, it constrains the possible phase space of parameters for models aiming to describe the main features of heavy-ion collisions.

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