Diffraction in QCD

Abstract

This lecture presents a short review of the main features of diffractive processes and QCD inspired models. It includes the following topics: (1) Quantum mechanics of diffraction: general properties; (2) Color dipole description of diffraction; (3) Color transparency; (4) Soft diffraction in hard reactions: DIS, Drell-Yan, Higgs production; (5) Why Pomerons interact weakly; (6) Small gluonic spots in the proton; (7) Diffraction near the unitarity bound: the Goulianos-Schlein "puzzle"; (8) Diffraction on nuclei: diffractive Color Glass; (9) CGC and gluon shadowing.

Diffractive gauge bosons production beyond QCD factorization

“…This can be understood as follows. According to the general theory of diffraction [1][2][3][4], the off-diagonal diffractive channels are possible only if different Fock components of the projectile (eigenstates of interaction) interact with different elastic amplitudes. Clearly, the two Fock states consisting of just a quark and of a quark plus a gauge boson interact equally, if their elastic amplitudes are integrated over impact parameter.…”

“…In this case, diffraction vanishes everywhere, except the periphery of the disk, so the the fraction of diffractive cross section is expected to fall with energy as diff = tot / 1= lns [4].…”

“…This results in a new combination of the Fock components, which can be projected to a new physical state (e.g., see the review on QCD diffraction in Ref. [4]). …”

Breakdown of QCD factorization in hard diffraction

“…Making a plausible (not proven) assumption about universality of the former, one can measure it with electro-weak hard probes (DIS, Drell-Yan process) and apply to hard hadronic processes. Although it is tempting to extend this factorization scheme to diffractive, large rapidity gap processes, it turns out to be heavily broken [2,3], as is demonstrated below.…”

Hard hadronic diffraction is not hard

“…One may naturally be tempted to extend such a factorization scheme to diffractive reactions with a large rapidity gap. However, such diffractive factorisation turns out to be unavoidably broken and does not hold in practice [2,3].…”