CMS-HIN-15-014

CMS-HIN-15-014 ; CERN-EP-2017-001
Azimuthal anisotropy of charged particles with transverse momentum up to 100 GeV/ $c$ in PbPb collisions at $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV
CMS Collaboration
2 February 2017
Phys. Lett. B 776 (2017) 195
Abstract: The Fourier coefficients $v_2$ and $v_3$ characterizing the anisotropy of the azimuthal distribution of charged particles produced in PbPb collisions at $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV are measured with data collected by the CMS experiment. The measurements cover a broad transverse momentum range, $p_{\mathrm{T}}=$ 1-100 GeV/ $c$ . The analysis focuses on $p_{\mathrm{T}} >$ 10 GeV/ $c$ range, where anisotropic azimuthal distributions should reflect the path-length dependence of parton energy loss in the created medium. Results are presented in several bins of PbPb collision centrality, spanning the 60% most central events. The $v_2$ coefficient is measured with the scalar product and the multiparticle cumulant methods, which have different sensitivities to the initial-state fluctuations. The values of both methods remain positive up to $p_{\mathrm{T}} \approx$ 70 GeV/ $c$ , in all examined centrality classes. The $v_3$ coefficient, only measured with the scalar product method, tends to zero for $p_{\mathrm{T}} \gtrsim$ 20 GeV/ $c$ . Comparisons between theoretical calculations and data provide new constraints on the path-length dependence of parton energy loss in heavy ion collisions and highlight the importance of the initial-state fluctuations.
Links: e-print arXiv:1702.00630 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $v_2$ and $v_3$ results from the SP method as a function of ${p_{\mathrm {T}}}$ , in seven collision centrality ranges from 0-5% to 50-60%. The vertical bars (shaded boxes) represent the statistical (systematic) uncertainties. The curves represent calculations made with the CUJET3.0 [38] and the SHEE models [23] (see text).
png pdf	Figure 2: Comparison between the $v_2$ results from the SP and the 4-, 6-, and 8-particle cumulant methods, as a function of ${p_{\mathrm {T}}}$ , in six centrality ranges from 0-5% to 50-60%. The vertical bars (shaded boxes) represent the statistical (systematic) uncertainties.
png pdf	Figure 3: Correlation between the high- ${p_{\mathrm {T}}}$ $v_{2}$ measured in the 14-20 (left), 20-26 (middle), and 26-35 GeV/ $c$ (right) ${p_{\mathrm {T}}}$ ranges and the low- ${p_{\mathrm {T}}}$ $v_{2}$ measured in the 1 $< {p_{\mathrm {T}}} <$ 1.25 GeV/ $c$ range, with the SP (closed circles) and cumulant (open squares) methods.

Summary

The azimuthal anisotropy of charged particles produced in PbPb collisions at

$\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV has been studied using data collected by the CMS experiment. The

$v_2$ and

$v_3$ coefficients are determined, as a function of collision centrality, over the widest transverse momentum range studied to date (from 1 up to 100 GeV/

$c$ ). For the first time, the multiparticle cumulant method is used for

$p_{\mathrm{T}} >$ 20 GeV/

$c$ . Over the measured centrality range, positive

$v_2$ values are found up to

$p_{\mathrm{T}} \approx$ 70 GeV/

$c$ , while the

$v_3$ values are consistent with zero for

$p_{\mathrm{T}} >$ 20 GeV/

$c$ . For

$p_{\mathrm{T}} <$ 3 GeV/

$c$ ,

$v_{2}\{\mathrm{SP}\} > v_{2}\{4\} \approx v_{2}\{6\} \approx v_{2}\{8\}$ , consistent with a collective behavior arising from the hydrodynamic expansion of a quark-gluon plasma. The similarity of

$v_{2}\{4\}$ ,

$v_{2}\{6\}$ , and

$v_{2}\{8\}$ at high

$p_{\mathrm{T}}$ suggests that

$v_{2}$ originates from the path-length dependence of parton energy loss associated with an asymmetric initial collision geometry. In addition, a common trend in the centrality dependence of

$v_{2}$ is observed over the full

$p_{\mathrm{T}}$ range, further supporting a common connection to the initial-state geometry. A model calculation (SHEE) incorporating initial-state fluctuations with a linear path-length dependence of parton energy loss is found to be in good agreement with the data, over the wide

$p_{\mathrm{T}}$ and centrality ranges probed in this analysis.

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