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Direct photon elliptic flow in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

The ALICE collaboration
Phys.Lett. B789 (2019) 308-322, 2019

Abstract (data abstract)
The elliptic flow of inclusive and direct photons was measured at mid-rapidity in two centrality classes 0--20\% and 20--40\% in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV by ALICE. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the detector material with the $e^+e^-$ pairs reconstructed in the central tracking system. The results of the two methods were combined and the direct photon elliptic flow was extracted in the transverse momentum range $0.9 < p_\text{T} < 6.2$ GeV/$c$. We test the hypothesis $v_{2}^{\gamma,dir} \equiv 0$ for $0.9 < p_\text{T} < 2.1$ GeV/$c$ and obtain a significance of $1.4\sigma$ for the 0--20\% class and $1.0\sigma$ for the 20--40\% class. A comparison to RHIC data shows a similar magnitude of the measured elliptic flow, while hydrodynamic and transport model calculations predict a smaller flow than observed.

  • Table 1

    Figure 1

    10.17182/hepdata.88050.v1/t1

    Ratio V2{GAMMA,INCLUSIVE,PCM}/V2{GAMMA,INCLUSIVE,COMBINED} as function of $p_\text{T}$, collision centrality 0-20%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 2

    Figure 1

    10.17182/hepdata.88050.v1/t2

    Ratio V2{GAMMA,INCLUSIVE,PHOS}/V2{GAMMA,INCLUSIVE,COMBINED} as function of $p_\text{T}$, collision centrality 20-40%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 3

    Figure 2

    10.17182/hepdata.88050.v1/t3

    V2{GAMMA,DECAY} as function of $p_\text{T}$, collision centrality 0-20%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 4

    Figure 2

    10.17182/hepdata.88050.v1/t4

    V2{GAMMA,DECAY} as function of $p_\text{T}$, collision centrality 20-40%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 5

    Figure 3

    10.17182/hepdata.88050.v1/t5

    V2{GAMMA,INCLUSIVE} as function of $p_\text{T}$, collision centrality 0-20%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 6

    Figure 3

    10.17182/hepdata.88050.v1/t6

    V2{GAMMA,INCLUSIVE} as function of $p_\text{T}$, collision centrality 20-40%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 7

    Figure 5

    10.17182/hepdata.88050.v1/t7

    V2{GAMMA,DIRECT} as function of $p_\text{T}$, collision centrality 0-20%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

  • Table 8

    Figure 5

    10.17182/hepdata.88050.v1/t8

    V2{GAMMA,DIRECT} as function of $p_\text{T}$, collision centrality 20-40%, for Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV.

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