Transverse single-spin asymmetries are among the most intriguing observables in hadronic physics. Though such asymmetries were already measured for the first time about four decades ago, their origin is still under debate. Here we consider transverse single-spin asymmetries in semi-inclusive lepton-nucleon scattering, in nucleon-nucleon scattering, and in inclusive lepton-nucleon scattering. It is argued that the single-spin asymmetries for those three processes may be simultaneously described in perturbative QCD, where the re-scattering of the active partons plays a crucial role. A comparison of single-spin asymmetries in different reactions can also shed light on the universality of transverse momentum dependent parton correlation functions. In particular, we discuss what existing data tells us about the predicted process dependence of the Sivers function.
Gluon Sivers function and transverse single spin asymmetries in
e+p↑→γ+X