Exclusive vector photoproduction: confirmation of Regge theory

Abstract

Recent small-t ZEUS data for exclusive ρ photoproduction are in excellent agreement with exchange of the classical soft pomeron with slope α ′ = 0.25 GeV −2 . Adding in a flavour-blind hard-pomeron contribution, whose magnitude is calculated from the data for exclusive J/ψ photoproduction, gives a good fit also to the ZEUS data for ρ photoproduction at larger values of t, and to φ photoproduction.The ZEUS collaboration has recently suggested [1] that their data for exclusive ρ photoproduction at HERA, when combined with lower-energy data [2] lead to a slope α ′ for the trajectory of the soft pomeron that differs significantly from the classical value α ′ = 0.25 GeV −2 . A main message of this paper is to disagree with this conclusion; we show that in fact the classical value is confirmed by the data.

Unified model of exclusiveρ0,φ, andJ/ψelectroproduction

“…For the J/Ψ at Q 2 = 0 the soft pomeron contributes 80% of the amplitude at √ s = 15 GeV and 35% at √ s = 250 GeV. These results for the soft pomeron proportions at Q 2 = 0 are comparable with those obtained in other phenomenological approaches [8,41]. The same general features, a slow onset of the perturbative region where the hard pomeron dominates, are compatible with those of [68,69].…”

“…The parameters in our calculation are the p F for each of the ρ, φ and J/Ψ, which control the transverse size of the vector-meson wave functions; the momentum cut-off Q 2 0 when the gluon structure function is used in the perturbative term, or the overall normalisation of the perturbative term in the Cudell-Royon model. The normalisation of the nonperturbative term is effectively fixed by the ρ photoprodution cross section which it dominates [8]. Two sets of results are shown: S1 refers to non-perturbative plus gluon structure function; S2 refers to non-perturbative plus Cudell-Royon.…”

“…The tdependence is thus automatically provided, but the CR approach does not specify the energy dependence. In order to obtain a global description of vector meson production, Regge energy dependence corresponding to the "hard pomeron" [7,23,8] is introduced into the CR term by hand, in much same way that it was introduced in the nonperturbative term using the soft pomeron trajectory. In this way one obtains summation model S2:…”

Fading out of J/ψ color transparency in high energy heavy ion peripheral collisions

“…The forward elementary cross section for photoproduction of J/ψ meson on nucleon was taken using the fit to experimental The quantity σ g ef f for Q 2 = 4GeV 2 as a function of the Bjorken x for H1 (solid line) and Alvero et al [20] (dashed line) parameterizations of the gluon diffractive density. data presented in [19] (this is preferable to using the theoretical calculations which for photoproduction of J/ψ have theoretical uncertainty of the order of two).…”

Photoproduction of ηʹ-mesons from the Proton

“…Here the t-channel contributions dominate at the low four momentum transfer squared. We also note that at high energies, E γ >5 GeV, the elementary particle exchange should be generalized to the correct expression for the exchange of a Regge trajectory with physical coupling constants comprising the residue [47][48][49]. Thus, the availability of precision data at photon energies 2<E γ <5 GeV is very crucial for the theoretical analysis of t-channel exchanges.…”