Connections between collinear and transverse-momentum-dependent polarized observables within the Collins–Soper–Sterman formalism

We extend the improved Collins–Soper–Sterman (iCSS) W+Y construction recently presented in [1] to the case of polarized observables, where we focus in particular on the Sivers effect in semi-inclusive deep-inelastic scattering. We further show how one recovers the expected leading-order collinear twist-3 result from a (weighted) qT-integral of the differential cross section. We are also able to demonstrate the validity of the well-known relation between the (TMD) Sivers function and the (collinear twist-3) Qiu–Sterman function within the iCSS framework. This relation allows for their interpretation as functions yielding the average transverse momentum of unpolarized quarks in a transversely polarized spin-12 target. We further outline how this study can be generalized to other polarized quantities. Keywords: CSS formalism, TMDs, Collinear twist-3, Transverse spin