Sivers Mechanism for Gluons

Recent experiments performed by the HERMES collaboration have demonstrated that a small but nonzero Sivers mechanism for transverse single-spin asymmetries exists [1]. Although the presence of such effects has been conjectured long ago [2], only more recent studies [3] have clarified the role of final and initial state interactions as a source of such T-odd asymmetries at the parton level and have thus demonstrated that a nonzero Sivers mechanism is possible in QCD. Following these insights, several model estimates for the Sivers asymmetry have been made [4] but only few theoretical constraints exist regarding the anticipated magnitude or even the sign of the resulting asymmetries in QCD. The main purpose of this letter is to derive a constraint on the Sivers mechanism for gluons. For the final (initial) state interactions, previous results for the Sivers mechanism for quarks can be summarized in a relatively compact form by means parton densities with T − odd contributions. In the case of unintegrated quark densities one finds [5, 6, 7]