Recent results on exclusive production at HERA

Abstract

Exclusive processes at HERA can lead to a deeper insight into production mechanisms in QCD. A series of processes is presented here. A first measurement is presented of exclusive photoproduction of ρ 0 mesons associated with leading neutrons at HERA. The H1 data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a ρ 0 meson on the virtual pion. The value of the elastic cross section indicates significant absorptive corrections for the exclusive reaction. Production of exclusive dijets in diffractive deep inelastic e ± p scattering has been measured by ZEUS. The results are compared to predictions from models based on different assumptions about the nature of the diffractive exchange. The exclusive deep inelastic electroproduction and photoproduction of ψ(2S) and J/ψ(1S) has been studied by ZEUS. The cross-section ratio σ (ψ(2S))/σ (J/ψ(1S)) has been measured as a function of Q 2 , W and t. The results are compared to many predictions of QCD-inspired models of exclusive vector-meson production.