Triangular flow of thermal photons from an event-by-event hydrodynamic model for 2.76A TeV Pb + Pb collisions at the CERN Large Hadron Collider

Abstract

We calculate the triangular flow parameter v3 of thermal photons from an event-by-event ideal hydrodynamic model for 0–40% central collisions of Pb nuclei at √sNN = 2.76 TeV at the CERN Large Hadron Collider. v3 determined with respect to the participant plane (PP) is found to be nonzero and positive, and its pT dependence is qualitatively similar to the elliptic flow parameter v2(PP) of thermal photons in the range 1 pT 6 GeV/c. In the range pT 3 GeV/c, v3(PP) is found to be about 50–75% of v2(PP) and for pT > 3 GeV/c the two anisotropy parameters become comparable. The value of v3 is driven by local density fluctuations both directly via the creation of triangular geometry and indirectly via additional flow. As expected, the triangular flow parameter calculated with respect to the reaction plane v3(RP) is found to be close to zero. We show that v3(PP) strongly depends on the spatial size of fluctuations, especially in the higher pT (3GeV/c) region where a larger value of σ results in a smaller v3(PP). In addition, v3(PP) is found to increase with the assumed formation time of the thermalized system.