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

TL;DR

This study measures the azimuthal anisotropy of neutral pion production in gold-gold collisions at high energy, revealing discrepancies with existing models and suggesting a steeper path length dependence of jet quenching.

Contribution

It provides the first detailed measurement of pi0 azimuthal anisotropy at high pT in Au+Au collisions and tests the path-length dependence of jet quenching models.

Findings

Anisotropy decreases with pT but remains positive beyond 10 GeV/c.

Current pQCD energy-loss models under-predict the anisotropy up to 10 GeV/c.

Steeper path length dependence models agree better with the data.

Abstract

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. Calculations which implement a path length dependence steeper than what is implied by current pQCD energy-loss models, show reasonable agreement with the data.