  Within the collinear twist-3 framework, we study the single-spin asymmetry
(SSA) in collisions between unpolarized protons and transversely polarized
protons with focus on the fragmentation term. The fragmentation mechanism must
be analyzed in detail in order to unambiguously determine the impact of various
contributions to SSAs in hadron production. Such a distinction may also settle
the "sign mismatch" between the transverse SSA in proton-proton collisions and
the Sivers effect in semi-inclusive deep inelastic scattering. We calculate
terms involving quark-quark and quark-gluon-quark correlators, which is an
important step in such an investigation.
