In a lepton-proton collision, an intermediate virtual photon scatters off the proton, creating hadronic particles. If the momentum transfered into the collision is large, the process is called deep inelastic scattering and the photon-proton collision can be described using perturbative QCD. I will focus on the description of so-called diffractive events, for which the proton remains intact after the collision. Since their observation ten years ago, their understanding from QCD has been a challenge.
I will investigate the limit of very high center-of-mass energy. It is then convenient to describe the colliding particles in terms of $quark-antiquark$ dipoles, because in the high-energy limit, they are eigenstates of the QCD S-matrix. I will show how to express the diffractive cross-section through $qbar q$ dipole scattering amplitudes. These amplitudes should then be obtained from the solution of the so-called Pomeron-loop equation which gives exact QCD results at leading-logarithmic accuracy and large $N_c$. I will discuss the resulting predictions and show that, in the energy range of the HERA collider, they are in good agreement with the data.

SPhT, CEA/Saclay