Selected topics in diffraction with protons and nuclei: past, present, and future

Abstract

We review a broad range of phenomena in diffraction in the context of hadron-hadron, hadron-nucleus collisions and deep inelastic lepton-proton/nucleus scattering focusing on the interplay between the perturbative QCD and nonperturbative models. We discuss inclusive diffraction in DIS, phenomenology of dipole models, resummation and parton saturation at low x, hard diffractive production of vector mesons, inelastic diffraction in hadron-hadron scattering, formalism of color fluctuations, inclusive coherent and incoherent diffraction as well as soft and hard diffraction phenomena in hadron-hadron/nucleus and photon-nucleus collisions. For each topic we review key results from the past and present experiments including HERA and the LHC. Finally, we identify the remaining open questions, which could be addressed in the continuing experiments, in particular in photon-induced reactions at the LHC and the future Electron-Ion Collider (EIC) in the US, Large Hadron electron Collider (LHeC) and Future Circular Collider (FCC) at CERN.

UPC contribution to forward rapidity gap distribution in pPb collisions at the LHC

“…Strong and electromagnetic mechanisms in pA coherent diffraction. The phenomenon of diffractive dissociation of protons in proton-nucleus scattering at high energies is a classic example of composite structure of hadronic projectiles, which can be conveniently described within the framework of cross section fluctuations [5][6][7][8]. In this approach, the cross section of pA coherent diffraction dissociation, p + A → X + A, can be written in the following form…”

“…In general, P p (σ) should be modeled, see, e.g. [7,8]. However, in the case of diffraction dissociation, the detailed information on the shape of P p (σ) is not needed since one can use the general property that P p (σ) is peaked around σ tot pp (s) = σ ≡ dσP p (σ)σ.…”

“…where ω σ (s) = σ 2 / σ 2 − 1 quantifies the dispersion of cross section fluctuations of the proton. At √ s = √ s N N = 8.16 TeV, we use the COMPETE parametrization [10] giving σ = σ tot pp (s) = 98.6 mb and a simple interpolation from fixed-target to Tevatron and further to LHC energies giving ω σ (s) = 0.092 ± 0.015 [8]. The spread in the values of ω σ (s) reflects the theoretical uncertainty in modeling P p (σ).…”

“…where the photon energy corresponds to the given ∆η F , i.e., to the given M X , see Eqs. (8) and (9). The resulting values of dσ e.m. pA /d∆η F as a function of ∆η F in the 1 ≤ ∆η F ≤ 5 interval are given in Table I.…”