On the observability of the quark orbital angular momentum distribution
Aurore Courtoy (Laboratori Nazionali di Frascati, INFN, Frascati, Italy, IFPA, AGO Department, Université de Liège, Bât. B5, Sart Tilman, Liège, B-4000, Belgium); Gary R. Goldstein (Department of Physics and Astronomy, Tufts University, Medford, MA, 02155, USA); J. Osvaldo Gonzalez Hernandez (Istituto Nazionale di Fisica Nucleare (INFN) – Sezione di Torino, via P. Giuria, 1, Torino, 10125, Italy); Simonetta Liuti (Laboratori Nazionali di Frascati, INFN, Frascati, Italy, University of Virginia – Physics Department, 382 McCormick Rd., Charlottesville, VA, 22904, USA); Abha Rajan (University of Virginia – Physics Department, 382 McCormick Rd., Charlottesville, VA, 22904, USA)
We argue that due to parity constraints, the helicity combination of the purely momentum space counterparts of the Wigner distributions – the generalized transverse momentum distributions – that describes the configuration of an unpolarized quark in a longitudinally polarized nucleon can enter the deeply virtual Compton scattering amplitude only through matrix elements involving a final state interaction. The relevant matrix elements in turn involve light-cone operators projections in the transverse direction, or they appear in the deeply virtual Compton scattering amplitude at twist three. Orbital angular momentum or the spin structure of the nucleon was a major reason for these various distributions and amplitudes to have been introduced. We show that the twist three contributions associated with orbital angular momentum are related to the target-spin asymmetry in deeply virtual Compton scattering, already measured at HERMES.
