Multiple Parton Scattering in Nuclei Twist Four Nuclear Matrix Elements and O Forward Parton Distributions

Multiple parton scatterings inside a large nucleus generally involve higher twist nuclear parton matrix elements The gluon bremsstrahlung induced by multiple scattering depends not only on direct parton matrix elements but also on momentum crossed ones due to the Landau Pomeranchuk Migdal in terference e ect We show that both types of twist four nuclear parton matrix elements can be factorized approximately into the product of twist two nu cleon matrix elements in the limit of extremely large nuclei A as assumed in previous studies Due to the correlative nature of the twist four matrix elements under consideration it is actually the o forward parton distributions that appear naturally in this decomposition rather than the or dinary diagonal distributions probed in deeply inelastic scattering However we argue that the di erence between these two distribution classes is small in certain kinematic regimes In these regions the twist four nuclear parton ma trix elements are evaluated numerically and compared to the factorized form for di erent nuclear sizes within a schematic model of the two nucleon corre lation function The nuclear size dependence is found to be A in the limit of large A as expected We nd that the factorization is reasonably good when the momentum fraction carried by the gluon eld is moderate The deviation can be more than a factor of however for small gluon momentum fractions where the gluon distribution is very large Typeset using REVTEX