The diffraction cone for exclusive vector meson production in deep inelastic scattering

TL;DR

This work develops a color-dipole gBFKL framework to describe exclusive diffractive vector-meson production in deep inelastic scattering, linking the amplitude to the dipole cross section $\sigma(\xi,r)$ and the gluon content of the proton. It predicts substantial diffraction-cone shrinkage with energy due to the running, non-Scale-invariant gBFKL dynamics, and introduces the scanning phenomenon where the effective dipole size is controlled by $Q^{2}$ via $r_S \sim A/\sqrt{m_V^{2}+Q^{2}}$, causing the diffraction slope to decrease with $Q^{2}$ and to exhibit approximate flavor independence in the scaling variable $Q^{2}+m_V^{2}$. The paper further analyzes the soft-hard decomposition, the role of a soft pomeron, and the node effect for radially excited $2S$ states, forecasting the striking result $B(\gamma^{*}\rightarrow 2S) < B(\gamma^{*}\rightarrow 1S)$ in certain regimes. These predictions enable stringent tests against HERA and fixed-target data and provide a framework for extracting the gluon distribution at small $x$ while clarifying the interplay between soft and hard QCD dynamics in diffractive processes.

Abstract

We develop the color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction $γ^{*}N \to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments. We predict a substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$. The $Q^{2}$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^{2}$ (which is supported by the available experimental data). We predict an approximate flavor independence of the diffraction slope in the scaling variable $Q^{2}+m_{V}^{2}$. For diffractive production of the radially excited $2S$ states ($Ψ',Υ'$) the counterintuitive inequality of diffraction slopes $B(2S) \lsim B(1S)$ is predicted, which defies the common wisdom that diffraction slopes are larger for reactions with larger size particles.