Low-pT collective flow induces high-pT jet quenching -: art. no. 064910

Data on low-p(T) hadronic spectra are widely regarded as evidence of a hydrodynamic expansion in nucleus-nucleus collisions. In this interpretation, different hadron species emerge from a common medium that has built up a strong collective velocity field. Here we show that the existence of a collective flow field implies characteristic modifications of high-p(T) parton fragmentation. We generalize the formalism of parton energy loss to the case of flow-induced oriented momentum transfer. We also discuss how to embed this calculation in hydrodynamic simulations. Flow effects are found to result generically in characteristic asymmetries in the eta x phi plane of jet-energy distributions and of multiplicity distributions associated with high-p(T) trigger particles. However, collective flow also contributes to the medium-induced suppression of single inclusive high-p(T) hadron spectra. In particular, we find that low-p(T) elliptic flow can induce a sizable additional contribution to the high-p(T) azimuthal asymmetry by selective elimination of those hard partons that propagate with significant inclination against the flow field. This reduces, at least partially, the recently observed problem that models of parton energy loss tend to underpredict the large azimuthal asymmetry v(2) of high-p(T) hadronic spectra in semiperipheral Au + Au collisions.