Jet tomography of hot and cold nuclear matter

Modification of parton fragmentation functions by multiple scattering and gluon bremsstrahlung in nuclear media is shown to describe very well the recent HERMES data in deeply inelastic scattering, giving the first evidence of the A(2/3) dependence of the modification. The energy loss is found to be approximate to0.5 GeV/fm for a 10-GeV quark in an Au nucleus. Including the effect of expansion, analysis of the pi(0) spectra in central Au+Au collisions at roots=130 GeV yields an averaged energy loss equivalent to approximate to7.3 GeV/fm in a static medium. Predictions for central Au+Au collisions at roots=200 GeV are also given.