Sivers Function Research Articles

Asymmetric jet correlations inpp↑scattering

We propose that back-to-back correlations in the azimuthal angle of jets produced in collisions of unpolarized with transversely polarized proton beams could be used to determine Sivers functions. The corresponding single-spin asymmetry is not power suppressed, but is subject to Sudakov suppression. We present estimates of the asymmetry (without and with Sudakov effects) for BNL RHIC at jet transverse momenta of $\ensuremath{\sim}10\mathrm{GeV}$ and show that it may reach a few percent or more and could provide access to the gluon Sivers function.

Sivers function in a spectator model with axial-vector diquarks

We perform a calculation of the Sivers function in a spectator model of the nucleon, with scalar and axial-vector diquarks. We make use of gluon rescattering to produce the nontrivial phases necessary to generate the Sivers function. The inclusion of axial-vector diquarks enables us to obtain a nonzero Sivers function for down quarks. Using the results of our model, we discuss the phenomenology of transverse single spin asymmetries in π+, π−, and π0 production, which are currently analysed by the HERMES and COMPASS Collaborations. We find that the inclusion of axial-vector diquarks substantially reduces the asymmetries.

Sivers function in the MIT bag model

The Sivers function, an asymmetric transverse-momentum distribution of the quarks in a transversely polarized nucleon, is calculated in the MIT bag model. The bag quark wave functions contain both S-wave and P-wave components, and their interference leads to nonvanishing Sivers function in the presence of the final state interactions. We approximate these interactions through one-gluon exchange. An estimate of another transverse momentum dependent distribution h1⊥ is also performed in the same model.

Sivers vs. Collins effect in azimuthal single spin asymmetries in pion production in SIDIS

Recently it has been argued that the transverse momentum dependent twist-2 Sivers distribution function does not vanish in QCD. Therefore both, the Collins and Sivers effects, should be considered in order to explain the azimuthal single spin asymmetries AUL in pion production in semi-inclusive deeply inelastic lepton scattering of a longitudinally polarized target. On the basis of presently available phenomenological information on the Sivers function we estimate that for those asymmetries AUL in the kinematic region of the HERMES experiments the Sivers effect can be neglected with respect to the Collins effect. It is argued that the same feature holds also for the COMPASS and CLAS experiments. This justifies theoretical approaches to understand the HERMES data on the basis of the Collins effect only.

Transverse single-spin asymmetries in gauge boson production

Transverse single-spin asymmetries (SSA) in inclusive reactions are now considered to be directly related to the transverse momentum kT of the fundamental partons involved in the process. Several possible leading-twist QCD mechanisms have been proposed to explain the available data, in particular the Sivers effect which was resurrected recently. We show that from the measurement of transverse SSA in inclusive production of gauge bosons, one can learn more about the Sivers functions and possibly to achieve a reliable flavor separation.