Multiple Parton Scattering And Modified Fragmentation Functions In Nuclei

Suppression of jet production or jet quenching in high-energy nuclear collisions has been proposed as a good probe of the hot and dense medium produced in rel-ativistic heavy ion collision. The quenching of an energetic parton is caused by multiple scattering and induced parton energy loss during its propagation through the hot QCD medium. It suppresses the final leading hadron distribution giving rise to modified fragmentation functions and the final hadron spectra. Recently the modified quark fragmentation function inside a nucleus in DIS and the effective parton energy loss have been derived with the approach of helicity amplitude approximation. The results show that the modified fragmentation functions become softer due to multiple scattering in nuclei, which can be directly translated into the energy loss of the leading quark. Also it is found the effective parton energy loss depends quadratically on the nuclear size RA.Here I utilize the generalized factorization theorem of twist-four processes and study the modified quark fragmentation function inside a nucleus in DIS and the effective parton energy loss with the full QCD calculation beyond helicity amplitude approximation. It is shown that such a complete calculation gives rise to new corrections to the modified quark fragmentation function. As a consequence, the effective parton energy loss is found to be reduced by a factor of 5/6 from the result of helicity amplitude approximation and the leading quark energy loss still has a quadratical dependence on nuclear size.Furthermore, I extend the discussion to the case of heavy quark and derive the induced energy loss in terms of the modified fragmentation function of heavy quark and demonstrate the quark mass effect by comparing the result for heavy quark with the one for light quark . It is shown that the effect of heavy quark mass will give a significant suppression to the induced quark energy loss. In particular, the nuclear size dependence of heavy quark energy loss is discussed and it is shown that in some cases the heavy quark energy loss will have a linear dependence on nuclear size instead of quadratical dependence as in the light quark case.