CMS-PAS-HIN-15-014

CMS-PAS-HIN-15-014
High-$p_\mathrm{T}$ track $v_{n}$ harmonics in PbPb collisions at 5.02 TeV
CMS Collaboration
May 2016

Abstract: The Fourier coefficients (n = 2, 3) of the azimuthal distributions of charged particles produced in PbPb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV is measured with the CMS detector at the LHC over an extended transverse momentum ($p_\mathrm{T}$) range up to approximately 100 GeV/$c$. The data cover both the low-$p_\mathrm{T}$ region (1 $ < p_\mathrm{T} <$ 3 GeV/$c$) associated with hydrodynamic flow phenomena and the high-$p_\mathrm{T}$ region where anisotropic azimuthal distributions may reflect the path-length dependence of parton energy loss in the created medium. Several methods were used to extract the Fourier coefficients, such as the scalar product method that correlating charged tracks with the energy deposited in the hadronic forward calorimeters, or multi-particle cumulant method. For the seven bins of collision centrality studied, spanning the range of 0-60% most-central events, the observed $v_{2}$ values are found to first increase with $p_\mathrm{T}$, reaching a maximum around $p_\mathrm{T} =$ 3 GeV/$c$ and then to gradually decrease. In the most central (0-5%) events, non-zero $v_{2}$ is observed above $\approx$ 30 GeV/$c$ and it is found to be constant as function of $p_\mathrm{T}$.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 776 (2017) 195. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $v_2$ and $v_3$ results from SP method as a function of ${p_{\mathrm {T}}} $ in seven centrality ranges of PbPb collisions at ${\sqrt {s_{_{\text {NN}}}}} =$ 5.02 TeV. Shaded boxes represent systematic uncertainties.
png pdf	Figure 2: The $v_2$ results from SP and EP method as a function of ${p_{\mathrm {T}}} $ in seven centrality ranges of PbPb collisions at ${\sqrt {s_{_{\text {NN}}}}} =$ 5.02 TeV and 2.76 TeV respectively. Shaded boxes represent systematic uncertainties and dashed lines are predictions from CUJET3.0 model [26,27].
png pdf	Figure 3: Comparison between the $v_2$ results from SP and cumulant methods as a function of ${p_{\mathrm {T}}} $ in six centrality ranges of PbPb collisions at ${\sqrt {s_{_{\text {NN}}}}} =$ 5.02 TeV. Shaded boxes represent systematic uncertainties.
png pdf	Figure 4: The $v_{2}$ values from different centrality ranges at high ${p_{\mathrm {T}}}$ : 12 $< {p_{\mathrm {T}}} <$ 14 GeV/$c$, 20 $< {p_{\mathrm {T}}} <$ 26 GeV/$c$ and 26 $< {p_{\mathrm {T}}} <$ 35 GeV/$c$ vs. low ${p_{\mathrm {T}}} $ (1 $ < {p_{\mathrm {T}}} < $ 1.25 GeV/$c$) obtained from SP (closed circles) and cumulant (open squares) methods in PbPb collisions at ${\sqrt {s_{_{\text {NN}}}}} =$ 5.02 TeV.

Tables
png pdf	Table 1: Sources of systematic uncertainties and range of uncertainties in percent.
png pdf	Table 2: Total systematic uncertainties in percentage

Summary

In summary, the azimuthal anisotropy of charged particles with respect to the event plane has been studied in PbPb collisions at ${\sqrt {s_{_{\text {NN}}}}} =$ 5.02 TeV using the CMS detector. The $v_2$ and $v_3$ coefficients were determined over a wide range in $p_{\mathrm{T}}$ from 1 GeV/$c$ to approximately 100 GeV/$c$, as a function of collision centrality using scalar product method. The results reported in this paper significantly improve the statistical precision of previous $v_{2}$ measurements at lower energy for 12 $< p_{\mathrm{T}} <$ 60 GeV/$c$, and explore for the first time the very high $p_{\mathrm{T}}$ region beyond 60 GeV/$c$. Furthermore, these results provide a $v_{3}$ measurement up to a $p_{\mathrm{T}}$ region that has never before been studied (up to 100 GeV/$c$). The $v_2(p_{\mathrm{T}})$ behavior shows a trend of rapid rise to a maximum at $ p_{\mathrm{T}} \approx$ 3 GeV/$c$ and a subsequent fall for all centrality ranges. Beyond $ p_{\mathrm{T}}\approx$ 10 GeV/$c$, the observed $v_{2}$ values still show some $p_{\mathrm{T}}$ dependence but with a more moderate decrease with $p_{\mathrm{T}}$, remaining finite up very high $p_{\mathrm{T}}$. The $v_3(p_{\mathrm{T}})$ behavior is very similar to $v_{2}$ at low $p_{\mathrm{T}}$ but its value is consistent with zero over the full centrality range for $p_{\mathrm{T}} >$ 20 GeV/$c$. A common trend in the centrality dependence of $v_{2}$ is observed for particles over a wide range of $p_{\mathrm{T}}$, suggesting a potential connection to the initial-state geometry. To further investigate the origin of the correlation at high $p_{\mathrm{T}}$, a multi-particle correlation analysis was performed to extract $v_{2}$ from 1 GeV/$c$ to, at maximum, 100 GeV/$c$. At low $p_{\mathrm{T}}$, $v_{2}\{4\}$, $v_{2}\{6\}$ and $v_{2}\{8\}$ have similar values and smaller than $v_{2}\{SP\}$. This follows hydrodynamic expectations and is a sign of collectivity. The same behavior is observed at high $p_{\mathrm{T}}$ and reflects the collective nature of these correlations. Finally, we have investigated the correlations between $v_{2}$ values at low and high $p_{\mathrm{T}}$ as a function of centrality and they appeared to be strongly correlated. The low and high $p_{\mathrm{T}}$ correlations are respectively driven by the collective expansion and by the path lenght dependence of hard-scattered partons in the medium. These results indicate that the initial geometry and its fluctuations are responsible for both low and high $p_{\mathrm{T}}$ correlations. The precision data over a wide kinematic range presented here will provide important constraints on models of parton energy loss, particularly in terms of its dependence on the initial conditions, parton energy and path length through the medium.

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