Sivers Asymmetry Research Articles

Transverse single spin asymmetries in Drell-Yan processes

Recently, it has been shown, contrary to previous beliefs, that the k_T distribution of quarks in a transversely polarized proton can be asymmetric. This ``Sivers effect'' had already been used to explain transverse single spin asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p --> pion + X and p-bar(transv. polarized) p --> pion + X. In such channels, however, other mechanisms, like the ``Collins effect'' (a k_T asymmetric fragmentation of a transversely polarized quark into pions), may generate SSA. The Sivers asymmetry is used here to compute SSA in Drell-Yan processes; in this case, by considering the differential cross-section in the lepton-pair invariant mass, rapidity and transverse momentum, other mechanisms which may originate SSA cannot contribute. Estimates for RHIC experiments are given.

Leading-twist single-transverse-spin asymmetries: Drell–Yan and deep-inelastic scattering

Recently, Brodsky, Hwang and Schmidt have proposed a new mechanism that gives a transverse spin symmetry at leading twist in semi-inclusive deep-inelastic scattering. I show that the new mechanism is compatible with factorization and is due to a transverse-spin asymmetry in the k T distribution of quarks in a hadron (the “Sivers asymmetry”). An earlier proof that the Sivers asymmetry vanishes because of time-reversal invariance is invalidated by the path-ordered exponential of the gluon field in the operator definition of parton densities. Instead, the time-reversal argument shows that the Sivers asymmetry is reversed in sign in hadron-induced hard processes (e.g., Drell–Yan), thereby violating naive universality of parton densities. Previous phenomenology with time-reversal-odd parton densities is therefore validated.