Jet Quenching and Azimuthal Anisotropy of Large $p_T$ Spectra in Non-central High-energy Heavy-ion Collisions

Abstract

Parton energy loss inside a dense medium leads to the suppression of large $p_T$ hadrons and can also cause azimuthal anisotropy of hadron spectra at large transverse momentum in non-central high-energy heavy-ion collisions. Such azimuthal anisotropy is studied qualitatively in a parton model for heavy-ion collisions at the RHIC energies. The coefficient $v_2(p_T)$ of the elliptic anisotropy at large $p_T$ is found to be very sensitive to parton energy loss. It decreases slowly with $p_T$ contrary to its low $p_T$ behavior where $v_2$ increases very rapidly with $p_T$. The turning point signals the onset of contributions of hard processes and the magnitude of parton energy loss. The centrality dependence of $v_2(p_T)$ is shown to be sensitive to both size and density dependence of the parton energy loss and the later can also be studied via variation of the colliding energy. The anisotropy coefficient $v_2/\epsilon$ normalized by the spatial ellipticity $\epsilon$ is found to decrease significantly toward semi-peripheral collisions, differing from the hydrodynamic results for low $p_T$ hadrons. Constrained by the existing WA98 experimental data at the SPS energy on parton energy loss, both hadron spectra suppression and azimuthal anisotropy at high $p_T$ are predicted to vanish for $b>$7-8 fm in $Au+Au$ collisions at $\sqrt{s}=$130-200 GeV when the hadron rapidity density per unit area of the initial overlapped region is less than what is achieved in the central $Pb+Pb$ collisions at the SPS energy.