  We advocate the existence of a new type of $k_\perp$-dependent functions. In
contrast to the well-known Boer-Mulders function, the presented new functions
can be associated with the collective alignment of quark spin vectors.
Moreover, the new functions are sensitive to the transverse motion of partons
inside hadrons, which are linked to the spin alignment of partons, and they are
initiated by the interactions encoded in the corresponding correlators.
