SPIN EFFECTS AND FACTORIZATION IN THE DRELL-YAN PROCESS

We discuss the idea that the nontrivial vacuum structure in QCD may induce transverse momentum and spin correlations of the initial state partons in Drell-Yan type reactions, i.e. in hadron-hadron collisions with production of a vector boson V = gamma*, Z, W+/-. Transverse momentum correlations are found to have practically no effect on observable quantities, but spin correlations have drastic consequences for the polarization density matrices of the V-bosons. We therefore propose measurements of the V polarization as a good test for the basic factorization hypothesis which, so far, has been assumed to be valid in numerous applications of the naive and QCD-improved parton model. We compare our ansatz for a spin correlation violating factorization with data from the NA 10 collaboration on gamma* production in pi- N scattering. We find that the data give an indication of spin correlations of the partons in the initial state to be present.