Azimuthal anisotropy of neutral pion production in Au+Au collisions at sqrt(s_NN) = 200 GeV: Path-length dependence of jet quenching and the role of initial geometry

Understand

We have measured the azimuthal anisotropy of pi0's for 1 < pT < 18 GeV/c for Au+Au collisions at sqrt s_NN = 200 GeV.

• The observed anisotropy shows a gradual decrease in 3 < pT < 7 - 10 GeV/c, but remains positive beyond 10 GeV/c.
• The magnitude of this anisotropy is under-predicted, up to at least 10 GeV/c, by current perturbative QCD (pQCD) energy-loss model calculations.
• An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and initial-geometry fluctuations is insufficient to account for this discrepancy.