Semihard Scattering Unraveled from Collective Dynamics by Two-Pion Azimuthal Correlations in 158A GeV/c Pb + Au Collisions

Elliptic flow and two-particle azimuthal correlations of charged hadrons and high-p T pions (p T > I GeV/c) have been measured close to midrapidity in 158A GeV/c Pb + Au collisions by the CERES experiment. Elliptic flow (v 2 ) rises linearly with p T to a value of about 10% at 2 GeV/c. Beyond p T 1.5 GeV/c, the slope decreases considerably, possibly indicating a saturation of v 2 at high p T . Two-pion azimuthal anisotropies for p T > 1.2 GeV/c exceed the elliptic flow values by about 60% in midcentral collisions. These nonflow contributions are attributed to nearside and back-to-back jetlike correlations, the latter exhibiting centrality dependent broadening.