Exclusive processes at HERA

Exclusive processes, such as diffractive production of vector mesons, γp → V p with V = ρ, ω, φ, J/ψ, ψ,Υ, and Deeply Virtual Compton Scattering (DVCS), γp → γ p, shown in Figure 1, have been extensively studied at HERA. The variables that describe the exclusive process are (Figure 1): the mass of the vector meson, M , the photon virtuality, Q, the four-momentum transfer squared at the proton vertex, |t|, and the centre-of-mass energy of the γp system, W . The latter is related to the Bjorken scaling variable, x, as W 2 ∝ 1/x. The HERA data span an exceptionally wide range of kinematic variables and thus they can be used to study diffractive interactions in the region where the transition from soft to hard processes occurs, with the hard scale provided by Q, M or |t|. The data can be used to investigate diffraction in terms of pQCD and to test the non-perturbative quantities, such as the generalised or skewed parton densities (GPDs) which describe the correlation between partons in the proton. In the absence of a hard scale the vector meson production process can be described within the Regge formalism and proceeds by the exchange of the soft Pomeron trajectory. In analogy to low energy hadron-hadron interactions, the slow rise of the cross section with energy (W δ with δ ≈ 0.2), the slow rise of the b slope for the exponential |t| dependence with W and the s-channel helicity conservation (SCHC) are expected. In QCD models, at high energies vector meson production factorises into three steps: (i) the emitted photon fluctuates into a qq̄ pair (a colour dipole), (ii) the colour dipole interacts with the proton by the exchange of a two-gluon system (or a gluon ladder) in a colour singlet state, (iii) the colour dipole forms the vector meson. Such a description is valid whenever the transverse size of the qq̄ pair is small, eg. at high values of Q (Q > O(10 GeV)) or for heavy vector mesons. The process is sensitive to the square of the gluon distribution in the proton and the cross section is expected to rise steeply with energy (W δ with δ ≥ 0.7) in accordance with gluons rising steeply at low-x. Furthermore, a universal value of the b slope and violation of the SCHC are expected. In contrast to vector meson production, the DVCS process with a real photon in the final state is free from the uncertainty of modelling the vector meson wave function. This allows access to the skewed parton distribution functions with a better precision. An additional advantage of DVCS is that it interferes with the Bethe-Heitler process (QED ep→ epγ) so that the real part of the amplitude can be measured through the interference term [1]. This paper reviews the recent high precision measurements of light vector mesons (ρ, φ) and DVCS performed by the H1 and ZEUS collaborations.