Measurement of Direct-Photon Elliptic Flow in Pb-Pb Collisions at sqrt(NN) = 2.76 TeV

TL;DR

This paper reports the first measurement of direct-photon elliptic flow in Pb-Pb collisions at 2.76 TeV, revealing a non-zero v2 that challenges existing hydrodynamical models and suggests late-stage photon emission.

Contribution

It provides the first experimental evidence of direct-photon elliptic flow at LHC energies, offering new insights into photon production timing in heavy-ion collisions.

Findings

Evidence for non-zero direct-photon v2 in 1-3 GeV/c range

v2 magnitude similar to charged pion v2

Implications for photon emission timing in the collision evolution

Abstract

We present the first measurement of the direct-photon elliptic flow v_{2}^{\gamma,dir} in Pb-Pb collisions at sqrt(NN) = 2.76 TeV with data taken by the ALICE experiment at the LHC. The measurement provides evidence for a non-zero v_{2}^{\gamma,dir} for 1 < p_T < 3 GeV/c with a magnitude similar to the observed charged pion v_2. 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_{2}^{\gamma,dir} is expected to be small compared to hadrons. A large v_{2}^{\gamma,dir} might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed.