FZJ - IKP - 99 / 04 Diffractive S and D – wave vector mesons in deep inelastic scattering

We derive helicity amplitudes for diffractive leptoproduction of the S and D wave states of vector mesons. We predict a dramatically different spin dependence for production of the S and D wave vector mesons. We find very small R = σL/σT and abnormally large higher twist effects in production of longitudinally polarized D-wave vector mesons. Diffractive vector meson production γ∗ + p→ V + p′, in deep inelastic scattering (DIS) at small x = (Q +mV )/(W 2 +Q) is a testing ground of ideas on the QCD pomeron exchange and light-cone wave function (LCWF) of vector mesons ([1, 2, 3, 4, 5], for the recent review see [6]). (For the kinematics see fig. 1, Q = −q2 and W 2 = (p+ q) are standard DIS variables). The ground state vector mesons, V = ρ, ω, φ, J/Ψ,Υ are usually supposed to be the Swave spin-triplet qq̄ states. However, all the previous theoretical calculations used the V q̄q vertex φV Vμq̄Γμq with the simplest choice Γμ = γμ, which corresponds to a certain mixture of the Sand D-wave states, and any discussion of the impact of the D-wave admixture in the literature is missing (here Vμ is the vector meson polarization vector and φV is the vertex function related to the vector meson LCWF as specified below.). We report here a derivation of helicity amplitudes for diffractive production of pure S and D-wave qq̄ systems for small to moderate momentum transfer ∆ within the diffraction cone. Understanding production of D-wave states is a topical issue for several reasons. First, the Dwave admixture may affect predictions for the ratio R = σL/σT , in which there is a persistent departures of theory from the experiment. To this end we recall that the nonperturbative long-range pion-exchange between light quarks and antiquarks [7] is a natural source of S-D