Jet Tomography of Dense and Nuclear Matter

The medium modification of parton fragmentation functions induced by multiple scattering and gluon bremsstrahlung can be calculated within a systematic expansion of higher-twist contributions. Such a modification is shown to describe the recent HERMES data in deeply inelastic scattering (DIS) very well, providing the first evidence of $A^{2/3}$-dependence of the modification. The energy loss is found to be $ dE/dx \approx 0.5$ GeV/fm for 10-GeV quark in a $Au$ nucleus. Including the effect of expansion, analysis of the $\pi^0$ spectra in central $Au+Au$ collisions at $\sqrt{s}=130$ GeV yields an averaged energy loss equivalent to $ \approx 7.3$ GeV/fm in a static medium. This amounts to an initial parton density (at $\tau_0=0.2$ fm/$c$) about 15 times that in a cold nuclear matter. Predictions for central $Au+Au$ collisions at $\sqrt{s}=200$ GeV are also given.