Diffractive vector mesons beyond the s-channel helicity conservation

Abstract

We derive a full set, and determine the twist, of helicity amplitudes for diffractive production of light to heavy vector mesons in deep inelastic scattering. For large $Q^2$ all helicity amplitudes but the double-flip are calculable in perturbative QCD and are proportional to the gluon structure function of the proton at a similar hard scale. We find a substantial breaking of the $s$-channel helicity conservation which must persist also in real photoproduction.

Comments: 7 pages, 1 postscript figure. Discussion of double-flip updated, typos corrected, reference to the ZEUS, H1, HERMES presentations at ICHEP-98 added