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| 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 | |
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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). |
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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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