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 $⟨dE/dL⟩\approx 0.5\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}$ for a 10-GeV quark in an Au nucleus. Including the effect of expansion, analysis of the ${\pi }^0$ spectra in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s}=130\mathrm{G}\mathrm{e}\mathrm{V}$ yields an averaged energy loss equivalent to $⟨dE/dL⟩\approx 7.3\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}$ in a static medium. Predictions for central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s}=200\mathrm{G}\mathrm{e}\mathrm{V}$ are also given.