Quark correlations and single-spin asymmetries

Many hadron production reactions give rise to significant ⊥ single-spin asymmetries (SSA) which persist at high energies. For example, hyperons produced in hadronic collisions typically show a large polarization perpendicular to the production plane. More recently, the HERMES collaboration found a significant SSA (left-right asymmetry in the transverse momentum distribution of the produced mesons in the directions perpendicular to the nucleon spin) in semi-inclusive production of π and K mesons [1]. Theoretically, two mechanisms have been proposed to explain the asymmetry: the Sivers mechanism, where the final state interactions (FSI) give rise to a ⊥ momentum asymmetry of the active quark already before it fragments into hadrons [2], and the Collins mechanism, where the asymmetry arises when a ⊥ polarized quark fragments into mesons (see Ref. [3] and references therein). Both mechanisms have recently been observed by the HERMES collaboration (they can be disentangled by also measuring the scattering plane of the electron) [1]. Simple model calculations [4,5] have revealed that, even at high energies, the FSI can indeed give rise to a non-vanishing transverse single-spin asymmetry for the active quark (Sivers mechanism). Those FSI can formally be incorporated into a definition of unintegrated parton densities by introducing appropriate Wilson line phase factors [6,7].