Direct photon elliptic flow in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV
Phys.Lett. B789 (2019) 308-322
The collaboration

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.