Jet Tomography in Heavy Ion Collisions

Experiments at the Relativistic Heavy Ion Collider RHIC in Brookhaven show for the first time a significant quenching of high-p⊥ leading hadron spectra. In particular, transverse momentum spectra for neutral pion 1,2 and charged hadron 1,3 are suppressed if compared to spectra in p+p collisions rescaled by the number of binary collisions. This suppression is most pronounced (up to a factor ∼ 5) in central Au+Au collisions and smoothly approaches the binary scaling case with decreasing centrality. The azimuthal anisotropy v2(p⊥) of hadroproduction stays close to maximal up to the highest transverse momentum 4. Moreover, the disappearance of back-to-back high-p⊥ hadron correlations 5 provides an additional indication that final state medium effects play a decisive role in hadroproduction up to p⊥ ∼ 10 GeV. Parton energy loss has been proposed to account for the small nuclear modification factor 6, the azimuthal anisotropy 7 and the disappearance of dijets 8,9. Several studies (see references given in 10) indicate, however, that in the kinematical regime relevant for RHIC (p⊥ < 12 GeV), p⊥-broadening, shadowing, formation time and possibly other effects contribute significantly to the high-p⊥ nuclear modification as well. Here, we discuss the status of parton energy loss calculations and their comparison to data in the kinematical p⊥-range probed at RHIC.