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.