Two-gluon form factor of the nucleon andJ/ψphotoproduction

Abstract

We argue that the t-dependence of the two-gluon form factor of the nucleon should be given by Γ(t) = (1 − t/m 2 2g ) −2 with m 2 2g ≈ 1GeV 2 . We demonstrate that this form provides a good description of the t-dependence of the cross section of the elastic photoproduction of J/ψ-mesons between the threshold region of E γ = 11 GeV (Cornell), E γ = 19 GeV (SLAC) and E γ = 100 GeV (FNAL) including the strong energy dependence of the t-slope. It is also well matched with the recent HERA data. The same assumption explains also the t-dependence of φ-meson electroproduction near threshold at W = 2.3 GeV, Q 2 = 1.0 GeV 2 . Theoretical expectationsIt was demonstrated in [1] for the case of small x and in [2] in a general case that in the limit of large Q 2 the t-dependence of the process γ * L + N → V + N at fixed x is factorized into the convolution of a hard interaction block calculable in perturbative QCD, the short-distance qq wave function of the meson, and the generalized/skewed parton distribution (GPD) in the nucleon.In the scaling limit the t-dependence is originating solely from the GPD's since the meson wave function is highly squeezed in the direction transverse to the reaction axis. In the case of the valence quark exchanges these t-dependences are constrained by the sum rules for unpolarized [3] and polarized quarks [4] and could be also modeled in the chiral soliton models, see review in [5]. The t-dependence of GPD's provides unique information about the impact parameter distribution of the partons in nucleons. The knowledge of the transverse gluon distribution is especially important since it provides a key ingredient for the understanding of relative importance of soft and hard physics for high energy nucleonnucleon interactions at different impact parameters and has to be implemented in realistic Monte Carlo generators of nucleon-nucleon collisions at collider energies.In this paper we will first discuss theoretical expectations for the t-dependence of gluon GPD's. Next we will use the 11 ≤ E γ ≤ 100GeV data to check these expectations and hence 1

Chiral dynamics and the growth of the nucleon’s gluonic transverse size at smallx

“…Using a simple two-component picture, we estimate the magnitude of the increase at roughly 20% of the total transverse size of the nucleon. Such an increase is indeed observed when comparing the slope of the t-dependence of the J/ψ photoproduction cross section at HERA with that at fixed-target energies [12]. Third, we ask how the glue in the pion cloud could be probed directly in hard exclusive processes.…”

“…In the present investigation we concentrate on the unpolarized gluon distribution, the t-dependence of which is well-known in the regions of x covered by J/ψ photoproduction at fixed-target and collider energies [12]. The approach developed here can easily be extended to the quark distributions (singlet and non-singlet), as well as to polarized quark and gluon distributions.…”

“…For the discussion of the t-dependence of the nonforward gluon density it is convenient to define the socalled two-gluon form factor of the nucleon [12,24] …”

“…We concentrate on the region of moderately small x (∼ 10 −2 ), for which the t-dependence of the gluon GPD (the so-called "two-gluon form factor" of the nucleon) has been extracted from a number of electro-/photoproduction experiments, see Ref. [12] for a compilation. The investigation consists of three parts.…”

Generalized parton distributions and rapidity gap survival in exclusive diffractiveppscattering

“…[31,32] for a review of the data. This process probes the two-gluon formfactor at an effective scale Q 2 ≈ 3 GeV 2 .…”