Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at $\snn=2.76$ TeV

The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\snn = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|\eta|< 0.8$ at different collision centralities and as a function of transverse momentum, $\pt$, out to $\pt=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for $\pt>8$ GeV/$c$. The small $\pt$ dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to $\pt=8$ GeV/$c$. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least $\pt=8$ GeV/$c$ indicating that the particle type dependence persists out to high $\pt$.

Figures

Figure 1

$v_2$, $v_3$, and $v_4$ measured for unidentified charged particles as a function of transverse momentum for various centrality classes. The dashed line represents the WHDG model calculations for neutral pions $v_2$ extrapolated to the LHC collision energy. For clarity, the markers for $v_3$ and $v_{4/\Psi_{2}}$ results are slightly shifted along the horizontal axis. Error bars (shaded boxes) represent the statistical (systematic) uncertainties.
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Figure 2

Comparison of the ALICE results on $v_n(\pt)$ obtained with the event plane method to the analogous measurements from ATLAS and CMS collaborations, as well as $v_2$ measurements by STAR. Only statistical errors are shown.
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Figure 3

Relative event-by-event elliptic flow fluctuations for unidentified charged particles versus transverse momentum for different centrality classes. For clarity, the markers for centrality classes $\ge 10\%$ are slightly shifted along the horizontal axis. Error bars (shaded boxes) represent the statistical (systematic) uncertainties.
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Figure 4

Unidentified charged particle $v_2$, $v_3$, and $v_4$ integrated over the transverse momentum range $10< \pt< 20$ GeV/$c$ as a function of collision centrality, with the more central (peripheral) collisions shown on the left-(right-)hand side, respectively. The dashed line represents the WHDG model calculations for neutral pions extrapolated to the LHC collision energy. Error bars (shaded boxes) represent the statistical (systematic) uncertainties.
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Figure 5

$v_2$ (top) and $v_3$ (bottom) of charged pion and proton as a function of transverse momentum for 10-50% centrality class compared to unidentified charged particles results from the event plane method. For clarity, the markers for $v_2$ and $v_3$ at $\pt>8$ GeV/$c$ are slightly shifted along the horizontal axis. PHENIX $\pi^0$ $v_2$ measurements are also shown. The dashed line represents the WHDG model calculations for neutral pions extrapolated to the LHC collision energy for the 20-50% centrality range. Error bars (shaded boxes) represent the statistical (systematic) uncertainties.
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Additional Figures

Figure 1

Charged pion and proton $v_2$ as a function of transverse momentum for various centrality classes from the event plane method. For clarity, the markers for proton results are slightly shifted along the horizontal axis. Error bars (shaded boxes) represent the statistical (systematic) uncertainties. For details, please see Phys. Lett. B 719, 18 (2013), arXiv:1205.5761.
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