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Densities, Parton Distributions, and Measuring the Non-Spherical Shape of the Nucleon
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.Comment: 7 pages, two figure
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