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

TL;DR

This paper measures the elliptic flow of direct photons in Pb-Pb collisions at 2.76 TeV, revealing results similar to RHIC and comparing them with hydrodynamic models.

Contribution

First measurement of direct photon elliptic flow in Pb-Pb collisions at LHC energies using combined detection methods.

Findings

Direct photon elliptic flow observed in Pb-Pb collisions.

Results are similar in magnitude to RHIC measurements.

Hydrodynamic models are systematically lower but compatible with 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 $0.9 < p_{\rm T} < 6.2$ GeV/$c$. A comparison to RHIC data shows a similar magnitude of the measured direct-photon elliptic flow. Hydrodynamic and transport model calculations are systematically lower than the data, but are found to be compatible.