Transverse quark motion inside charmonia in diffractive photo- and electroproductions

Abstract

We reexamine the Fermi motion effects on the diffractive photo- and electroproductions of heavy vector-mesons, J/psi and psi', off a nucleon in the leading ln(Q^2/Lambda_QCD^2) approximation (LLA) of QCD. We take into account all the Fermi motion corrections arising from the relative motion of quarks inside the charmonium, which is treated as a nonrelativistic bound state of c and \bar{c}. Our key ingredients are the correct spin structure for the ^3S_1 c\bar{c} bound state, the off-shellness in the c\bar{c} \to J/psi (psi') hadronization vertex, and the modification of the gluon's longitudinal momentum fraction probed by the process, due to the relative motion between c and \bar{c}. We demonstrate that these three contributions produce the new Fermi motion effects in the LLA diffractive amplitude in QCD. It is found that our new effects moderate the strong suppression of the diffractive J/psi (psi') production cross sections, which was reported in the previous works on the Fermi motion effects. We emphasize the role of the transverse quark motion for the heavy meson production, and also discuss the strong helicity dependence of the Fermi motion effects and its implication in the longitudinal to transverse production ratio, sigma_L/sigma_T.