  Spin-dependent quark densities, matrix elements of specific density operators
in proton states of definite spin-polarization, indicate that the nucleon may
harbor an infinite variety of non-spherical shapes. We show that these matrix
elements are closely related to specific transverse momentum dependent parton
distributions accessible in the angular dependence of the semi-inclusive
processes electron plus proton goes to electron plus pion plus anything, and
the Drell-Yan reaction proton plus proton goes to a lepton anti-lepton pair
plus anything. New measurements or analyses would allow the direct exhibition
of the non-spherical nature of the proton.
