/ 0 41 20 47 v 1 2 1 D ec 2 00 4 Semihard scattering unraveled from collective dynamics at √ s = 17 GeV

We present a study of elliptic flow (v 2) and two-particle azimuthal correlations of charged hadrons and high transverse momentum (p T) pions measured by the CERES experiment in Pb+Au collisions at √ s = 17 GeV. Azimuthal anisotropy v 2 increases linearly with p T to a value of 10% at p T = 1.5 GeV/c. Beyond this p T the slope decreases indicating a possible saturation at high p T. Two-pion azimuthal anisotropies (p T > 1.2 GeV/c) exceed v 2 by about 60% in semicentral collisions and reveal non-harmonic contributions at close (∆φ ≈ 0) and back-to-back (∆φ ≈ π) angles that can be attributed to semihard processes. While the close-angle peak remains unchanged, the back-to-back peak is broadened and disappears in central collisions.