Measurement of Direct-Photon Elliptic Flow in Pb-Pb Collisions at √SNN = 2.76 TeV

We present the first measurement of the direct-photon elliptic flow vγ,dir2 in Pb-Pb collisions at √SNN = 2.76 TeV with data taken by the ALICE experiment at the LHC. The measurement provides evidence for a non-zero vγ,dir2 for 1 < pT < 3GeV/c with a magnitude similar to the observed charged pion ν2π±. In order to explain the large inverse slope parameter Teff of the low pT direct-photon spectrum observed at LHC and RHIC, recent hydrodynamical descriptions of the direct-photon production include a substantial portion of thermal photons from the hot plasma phase. As a consequence of the early production time, vγ,dir2 is expected to be small compared to hadrons. A large vγ,dir2 might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed