SPIN EFFECTS AND FACTORIZATION IN THE DRELL-YAN PROCESS

We discuss the idea that the nontrivial vacuum structure in QCD may in duce transverse momentum and spin correlations of the initial state pa rtons in Drell-Yan type reactions, i.e. in hadron-hadron collisions wi th production of a vector boson V = gamma, Z, W+/-. Transverse moment um correlations are found to have practically no effect on observable quantities, but spin correlations have drastic consequences for the po larization density matrices of the V-bosons. We therefore propose meas urements of the V polarization as a good test for the basic factorizat ion hypothesis which, so far, has been assumed to be valid in numerous applications of the naive and QCD-improved parton model. We compare o ur ansatz for a spin correlation violating factorization with data fro m the NA 10 collaboration on gamma production in pi- N scattering. We find that the data give an indication of spin correlations of the par tons in the initial state to be present.