  The single transverse spin asymmetry in D meson production at RHIC can
provide a clean measure of the gluon Sivers distribution function. At
intermediate rapidity, D production is largely dominated by the elementary gg
-> c c-bar channel, where there cannot be any transverse spin transfer.
Therefore, any transverse single spin asymmetry observed for D's produced in
polarized proton-proton interactions can only originate from the Sivers effect
in the gluon distribution functions. A sizeable transverse single spin
asymmetry measured by PHENIX or STAR experiments would then give direct
information on the size of the gluon Sivers distribution function.
