Rikard Enberg

The High energy asymptotics of scattering processes in QCD

High energy scattering in the QCD parton model was recently shown to be a reaction-diffusion process, and thus to lie in the universality class of the stochastic Fisher-Kolmogorov-Petrovsky-Piscounov equation. We recall that the latter appears naturally in the context of the parton model. We provide a thorough numerical analysis of the mean field approximation, given in QCD by the Balitsky-Kovchegov equation. In the framework of a simple stochastic toy model that captures the relevant features of QCD, we discuss and illustrate the universal properties of such stochastic models. We investigate in particular the validity of the mean field approximation and how it is broken by fluctuations. We find that the mean field approximation is a good approximation in the initial stages of the evolution in rapidity.

Transversity GPD in photoproduction and electroproduction of two vector mesons

The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photoproduction or electroproduction of two vector mesons on a polarized nucleon target, γ(*)N→ρ1ρ2N’, where ρ1 is produced at large transverse momentum, ρ2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal ρ2 production. To this end, we propose a model for the transversity GPD HT(x,ξ,t) and give an estimate of the relative sizes of the transverse and longitudinal ρ2 cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.

Soft colour interactions and diffractive hard scattering at Tevatron

We make a brief presentation of the soft colour interactions models, the Soft Colour Interaction and the Generalised Area Law, and summarise the results when they are applied to p-pbar scattering. The models give a good description of the Tevatron data on production of W, bottom and jets in diffractive events, as well as jets with two rapidity gaps, alternatively leading particles. We also give predictions for diffractive J/psi production and discuss diffractive Higgs production at the Tevatron and LHC.

Testing the dynamics of high energy scattering using vector meson production

I review work on diffractive vector meson production in photon–proton collisions at high energy and large momentum transfer, accompanied by proton dissociation and a large rapidity gap. This process provides a test of the high energy scattering dynamics, but is also sensitive to the details of the treatment of the vector meson vertex. The emphasis is on the description of the process by a solution of the non-forward BFKL equation, i.e. the equation describing the evolution of scattering amplitudes in the high-energy limit of QCD. The formation of the vector meson and the nonperturbative modeling needed is also briefly discussed.

Hard diffraction and parton rescattering

We present a new model where hard diffraction is an effect of soft gluon rescattering between partons emerging from the hard interaction and the color field of the proton. A resummation to all orders of multiple gluon exchanges leads to an overall color singlet exchange amplitude which reproduces the precise HERA data on diffractive deep inelastic scattering.

Hard diffractive scattering from soft color screening effects

We construct a QCD-based model where soft gluon rescattering between final state partons in deep inelastic scattering leads to events with large rapidity gaps and a leading proton. The model successfully describes the precise HERA data on the diffractive deep inelastic structure function in the whole available kinematical range.