Speaker
Description
Elliptic flow of direct photons in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV from ALICE
Direct photons are produced in all stages of a ultrarelativistic heavy-ion collisions and are sensitive to details of the space-time evolution of the produced medium. The low ${\textit p}_{\rm T}$ part of the direct photon spectrum is expected to be dominated by thermal direct photons - thermal radiation of hot matter, i.e., of the quark-gluon plasma and the hadron gas. At RHIC energies it is a challenge for hydrodynamical models to simultaneously described the yield and the elliptic flow of direct photons which is often referred to as the direct-photon puzzle.
The ALICE detector is equipped with two electromagnetic calorimeters and a central tracking system that make it well suited to study direct photon production at low and intermediate ${\textit p}_{\rm T}$. Photons are also detected via conversions in the ALICE detector material with the $e^{+}e^{-}$ pair reconstructed in the central tracking system.
In this talk the final results on the direct photon elliptic flow ($v_{2}^{\gamma ,dir}$) at $\sqrt{s_{\rm NN}}=2.76$ TeV from the Pb-Pb run in 2010 will be presented for central and semi-central collisions. This is the first time that the direct photon flow is calculated by combining conversion and calorimetric measurements in the ${\textit p}_{\rm T}$ range of $0.9<{\textit p}_{\rm T}<6.2$ GeV/c. Comparisons to PHENIX results and to predictions of several theoretical models will be presented in order to shed light on the direct photon puzzle at LHC energies.
