  Multiple parton scattering and induced parton energy loss in deeply inelastic
scattering (DIS) off heavy nuclei is studied within the framework of
generalized factorization in perturbative QCD with a complete calculation
beyond the helicity amplitude (or soft bremsstrahlung) approximation. Such a
calculation gives rise to new corrections to the modified quark fragmentation
functions. The effective parton energy loss is found to be reduced by a factor
of 5/6 from the result of helicity amplitude approximation.
