On a possible node in the Sivers and Qiu-Sterman functions

Abstract: The possibility of a node in the x dependence of the Sivers and Qiu-Sterman functions is discussed in light of its importance for the experimental check of the overall sign change of the Sivers effect between semi-inclusive DIS and the Drell-Yan process. An x-dependent version of the Ehrnsperger-Schaefer-Greiner-Mankiewicz relation between the Qiu-Sterman function and a twist-3 part of g_2 is presented, which naturally suggests a node in the Qiu-Sterman function. This relation could be checked experimentally as well and could provide qualitative information on the gluonic field strength inside the proton. Satisfying the Burkardt sum rule by means of a node is briefly discussed and the importance of modelling the Sivers function including its full Wilson line is pointed out.