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

We present the first measurement of the direct-photon elliptic flow v^γ,dir₂ in Pb-Pb collisions at = 2.76 TeV with data taken by the ALICE experiment at the LHC. The measurement provides evidence for a non-zero v^γ,dir₂ for 1 < p_T < 3GeV/c with a magnitude similar to the observed charged pion . In order to explain the large inverse slope parameter T_eff of the low p_T 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, v^γ,dir₂ is expected to be small compared to hadrons. A large v^γ,dir₂ might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed.