and gluon polarization †

In exclusive J/ψ production by polarized photons incident on polarized protons , a finite polarization asymmetry arises because of c¯ c Fermi-motion and binding-energy effects. The asymmetry depends on the polarized nonfor-ward gluon distribution of the proton and thus gives information on gluon polarization in the proton. The analyzing power, however, is rather small. Gluon polarization in the proton is a topic of current interest in hadronic physics (see e.g. [1, 2]). A useful probe of gluon distributions is the production of open or hidden charm, because charm quarks dominantly couple to gluons and not to light quarks in the proton. In this paper we discuss what the exclusive photoproduction of J/ψ mesons could tell about the polarized gluon content of the proton.