Semihard scattering unraveled from collective dynamics by 
two-pion azimuthal correlations in 158A GeV/c Pb+Au collisions

The azimuthal anisotropy is a consequence of early partonic rescattering in
the dense overlap zone of the nuclei at the time of collision and carries
information on such important issues as the equation of state and the level
of equilibration achieved. We report about successful attempts to trace
primeval partonic scattering and the onset of thermalization in the medium
together with and separated from elliptic flow by two-particle azimuthal
correlations of pions at moderately high transverse momentum
(p$_T>$1.2~GeV/$c$) as a function of the centrality of the collision.

Using the large data set of 158A~GeV/c Pb+Au collisions measured by the
CERES/NA45 experiment close to midrapidity we have extracted elliptic flow
anisotropies (v$_2$) of high-p$_T$ pions (p$_T>$1.0~GeV/c) and charged
particles (p$_T>$0.3~GeV/c) employing standard flow analysis with respect to
the event plane and two-particle azimuthal correlations. Elliptic flow
($v_2$) obtained by the standard analysis rises about linearly with $p_T$ to
a value of about 10$\%$ at 1.5~GeV/$c$. Beyond $p_T\approx$~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 mid-central collisions and
reveal non-harmonic contributions at $\Delta\phi$=0 ('close-angle') and
$\Delta\phi=\pi$ ('back-to-back') which are attributed to dijet-like
semihard scattering. While the fragmentation peak does not change its width,
the back-to-back peak is considerably broadened and escapes detection in
central collisions. The results are discussed with the perspective of $p_T$
broadening and jet quenching and related information from inclusive particle
spectra at the SPS and RHIC.