Probes of initial-state interactions in dilepton angular distributions

We discuss the angular distribution of dileptons d sigma/d4Qd OMEGA, emphasizing phase sensitivity as a probe of initial-state interactions in QCD. The coherent nature of Sudakov effects is discussed, along with the presence of imaginary parts related by analyticity. Angular-distribution structure functions which describe interference between longitudinal and transverse virtual photons, e.g., can be used to probe phase differences that depend on large momenta. These evolve according to exp(ic ln ln(Q2/lambda/sub QCD2)) where Q2 is a large scale. We report on a complete calculation at O(α/sub s/2) of the q anti q → #betta#* + gluons channel which confirms the cancellation of small (cutoff) scales, and describe a complementary experiment involving spin. We discuss the limit x → 1 of the distribution d sigma/dQ2dxdcos theta, and point out an unusual and interesting effect that a momentum-dependent phase can produce here