Multiple Parton Scattering in Nuclei with pQCD
In a nuclear medium, the propagating parton will experience additional scatterings with other partons from the nucleus. The rescatterings may induce additional parton radiation and cause the leading parton to lose energy. Such multiple parton scattering is related to the hight-twist process in pQCD and effectively leads to a modification of partons fragmentation functions in a medium. Within the frame work of generalized factorization theorem of twist-4 processes the modifications to quark (antiquark) fragmentation functions due to quark-gluon double scattering and quark-quark (antiquark) double scattering in nuclear medium are studied systematically. The contribution of quark-gluon double scattering is found to be proportional to gluon distribution and give dominant contribution to the leading quark energy loss, which depends quadratically on the nuclear size. Compared to quark-gluon scattering, the modification due to quark-quark double scattering is smaller by a factor of C_F / C_A = 4/9 times the ratio of quark and gluon distributions in the medium. Such a modification is not negligible for realistic kinematics and finite medium size. The modification to quark (antiquark) fragmentation functions from quark-antiquark annihilation processes are shown to be determined by the antiquark (quark) correlation with the leading quark. The asymmetry in quark and antiquark distributions in nuclei will naturally lead to different modifications of quark and antiquark fragmentation functions in a nucleus. The quark-antiquark annihilation processes also mix quark and gluon fragmentation functions in the large fractional momentum region, leading to a flavor dependence of jet quenching in heavy-ion collisions. The extension of this twist expansion approach to other processes is also discussed.