CMS-PAS-HIN-16-018

CMS-PAS-HIN-16-018
Measurement of mixed higher order flow harmonics in PbPb collisions
CMS Collaboration
February 2017

Abstract: The mixed higher order flow harmonics and nonlinear response coefficients of charged particles are measured for the first time as a function of $p_{\rm{T}}$ and centrality in PbPb collisions at $\sqrt{s_{NN}} =$ 2.76 TeV and 5.02 TeV with the CMS detector. The results are obtained using the scalar product method, and cover a $p_{\rm{T}}$ range from 0.3 GeV/ $c$ to 8.0 GeV/ $c$ , pseudorapidity $\|\eta\| <$ 2.4, and a centrality range of 0-60%. At 5.02 TeV, results for mixed harmonics are compared to the matching higher order flow harmonics from two-particle correlations, which measure $v_n$ values with respect to the $n$ -th order event plane. It is observed that the nonlinear response coefficients of the odd harmonics are larger than the even harmonics ones. The results are compared with hydrodynamic predictions with different shear viscosity to entropy density ratios and different initial conditions.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The mixed higher order flow harmonics, $v_4\{\Psi _{22}\}$ , $v_5\{\Psi _{23}\}$ , $v_6\{\Psi _{222}\}$ , $v_6\{\Psi _{33}\}$ and $v_7\{\Psi _{223}\}$ from the scalar product method at 2.76 and 5.02 TeV as a function of ${p_{\mathrm {T}}}$ with $\|\eta \|<$ 2.4 in the 0-20% (top row) and 20-60% (bottom row) centrality ranges. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown.
png pdf	Figure 2: Comparison of mixed higher order flow harmonics and flow from two-particle correlations at 5.02 TeV as a function ${p_{\mathrm {T}}}$ with $\|\eta \|<$ 2.4 in the 0-20% (top row) and 20-60% (bottom row) centrality ranges. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown.
png pdf	Figure 3: The nonlinear response coefficients, $\chi _{422}$ , $\chi _{523}$ , $\chi _{6222}$ , $\chi _{633}$ and $\chi _{7223}$ from the scalar product method at 2.76 and 5.02 TeV as a function of ${p_{\mathrm {T}}}$ with $\|\eta \|<$ 2.4 in the 0-20% (top row) and 20-60% (bottom row) centrality ranges. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown.
png pdf	Figure 4: The mixed higher order flow harmonics, $v_4\{\Psi _{22}\}$ , $v_5\{\Psi _{23}\}$ , $v_6\{\Psi _{222}\}$ , $v_6\{\Psi _{33}\}$ and $v_7\{\Psi _{223}\}$ from the scalar product method at 2.76 and 5.02 TeV as a function of centrality with $\|\eta \|<$ 2.4 in the 0.3 $< {p_{\mathrm {T}}} <$ 3.0 GeV/ $c$ range. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown. The hydrodynamic predictions [12] with $\eta /s =$ 0.08 (magenta lines) are compared with data.
png pdf	Figure 5: The nonlinear response coefficients, $\chi _{422}$ , $\chi _{523}$ , $\chi _{6222}$ , $\chi _{633}$ and $\chi _{7223}$ from the scalar product method at 2.76 and 5.02 TeV as a function of centrality with $\|\eta \|<$ 2.4 in the 0.3 $< {p_{\mathrm {T}}} <$ 3.0 GeV/c range. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown. The results are compared with predictions from AMPT and hydrodynamics with a deformed symmetric Gaussian density profile as the initial condition using $\eta /s =$ 0.08 in Ref. [12], and from iEBE-VISHNU hydrodynamics with Glauber and KLN initial conditions using the same $\eta /s$ [14].
png pdf	Figure 6: The nonlinear response coefficients, $\chi _{422}$ , $\chi _{523}$ , $\chi _{6222}$ , $\chi _{633}$ and $\chi _{7223}$ from the scalar product method at 2.76 and 5.02 TeV as a function of centrality with $\|\eta \|<$ 2.4 in the 0.3 $< {p_{\mathrm {T}}} <$ 3.0 GeV/c range. Statistical (error bars) and systematic (shaded boxes) uncertainties are shown. The results are compared with predictions from AMPT in Ref. [12] and from iEBE-VISHNU hydrodynamics with KLN initial condition using $\eta /s =$ 0, 0.08 and 0.2 [14].

Tables
png pdf	Table 1: Summary of different sources of systematic uncertainties for each mixed higher order flow harmonic.

Summary

The mixed higher order flow harmonics and nonlinear response coefficients of charged particles has been studied for the first time as a function of

$p_{\mathrm{T}}$ and centrality in PbPb collisions at

$\sqrt{s_{NN}} =$ 2.76 TeV and 5.02 TeV using the CMS detector. The measurements are done with the scalar product method, covering a

$p_{\mathrm{T}}$ range from 0.3 GeV/

$c$ to 8.0 GeV/c,

$|\eta|<$ 2.4 and centrality range of 0-60%. Additionally, as a comparison,

$v_n$ harmonics (

$n=$ 4, ... , 7) are measured with the two-particle correlation method over 0.3

$< p_{\mathrm{T}} <$ 8.0 GeV/

$c$ and

$|\eta| <$ 2.4 and within the same centrality range. The shape of the mixed higher order flow harmonics,

$v_4\{\Psi_{22}\}$ ,

$v_5\{\Psi_{23}\}$ ,

$v_6\{\Psi_{222}\}$ ,

$v_6\{\Psi_{33}\}$ and

$v_7\{\Psi_{223}\}$ , and nonlinear response coefficients,

$\chi_{422}$ ,

$\chi_{523}$ ,

$\chi_{6222}$ ,

$\chi_{633}$ and

$\chi_{7223}$ as a function

$p_{\mathrm{T}}$ are similar, first increases at low

$p_{\mathrm{T}}$ , reach maximum at about 3-4 GeV/

$c$ then decreases at higher

$p_{\mathrm{T}}$ . The contribution of nonlinear part for

$v_5$ and

$v_7$ are larger than other harmonics in the centrality range 20-60%. It is clearly observed that the nonlinear response coefficients of the odd harmonics,

$\chi_{523}$ and

$\chi_{7223}$ , are larger than the even harmonics. The data are compared with AMPT and hydrodynamic predictions with different shear viscosity to entropy density ratios and initial condition models. The predictions from AMPT are favored by the measurement. These results will provide constraints on the theoretical description of the medium close to the freeze-out temperature, which is poorly understood so far.

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