  The main focus of the physics program at PHENIX and STAR that makes use of
RHIC's polarized proton beams is to figure out how and if at all the gluons
inside protons are polarized, or to put it another way, do the spin 1 gluons
prefer to have their spins aligned or anti-aligned with the spin of the proton,
or do they just not care? This question is an important part of the more
general question of how the constituents of protons, gluons and quarks,
conspire to make up the overall spin 1/2 of the proton. Measurements of, e.g,
jet and hadron, production cross-section differences between the two cases
where the two polarized protons colliding have their spins aligned and
anti-aligned are sensitive to the gluon polarization, which is encoded in the
spin--dependent parton distribution function (PDF) for gluons, Delta-g(x).
