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

TL;DR

This paper reports measurements of the elliptic flow of inclusive and direct photons in Pb-Pb collisions at 2.76 TeV, revealing results that challenge existing theoretical models and align with previous RHIC findings.

Contribution

The study provides the first detailed measurement of direct photon elliptic flow at LHC energies, using combined detection methods and decay photon simulations.

Findings

Agreement with earlier RHIC results

Theoretical models under-predict the measured flow

Photon flow observed in the specified transverse momentum range

Abstract

The elliptic flow of inclusive and direct photons was measured by ALICE for central and semi-central Pb--Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=2.76$ TeV. The photons were reconstructed using the electromagnetic calorimeter PHOS and the central tracking system. The inclusive photon flow reconstructed with both methods are combined and used to extract the direct photon flow, using a decay photon simulation and the direct photon excess R$_{\gamma}$, in the transverse momentum range $0.9 < p_\mathrm{T} < 6.2$ GeV/c. We find agreement to earlier results obtained at RHIC, and the theoretical predictions generally under-predict the results.