Diffraction for non-believers
  Diffractive reactions involving a hard scale can be understood in terms of quarks and gluons. These reactions have become a valuable tool for investigating the low-x structure of the proton and the behavior of QCD in the high-density regime, and they may provide a clean environment to study or even discover the Higgs boson at the LHC. In this paper we give a brief introduction to the description of diffraction in QCD. We focus on key features studied in ep collisions at HERA and outline challenges for understanding diffractive interactions at the LHC. 
M. Arneodo and M. Diehl