ar X iv : h ep - p h / 02 11 32 6 v 1 2 0 N ov 2 00 2 Azimuthal Asymmetries : Access to Novel Structure Functions

One of the most interesting consequence of non-zero intrinsic transverse momentum of partons in the nucleon is the nontrivial azimuthal dependence of the cross section of hard scattering processes. Many of the observable asymmetries contain unknown functions which provide essential information on the quark and gluon structure. Several of them have been studied in the last few years; we discuss their qualitative and quantitative features in semi-inclusive DIS. Introduction The study of the structure of hadrons, bound states of quarks and gluons, in the context of quantum chromodynamics (QCD), is one of the challenges of elementary particle physics requiring new nonperturbative approaches in the field theory. This talk focuses on a discussion of correlations between the spins of hadron or quark and the momentum of the quark with respect of that of the hadron in polarized hard scattering processes. Signatures of these correlations appear in high-energy scattering processes as correlations between the azimuthal angles of the (transverse) spin and the (transverse) momentum vectors. In particular, we discuss single-spin (transversely polarized target or longitudinally polarized beam) and double-spin (longitudinally polarized beam and transversely polarized target) azimuthal asymmetries in single hadron electroproduction in DIS. The general form of the factorized cross sections of hard scattering process is written as [1] dσ = H ⊗ f2 ⊗ f ′ 2 + 1 Qn H ⊗ f2 ⊗ f ′ 2+n +O (