Instantons and Saturation in the Colour Dipole Picture

Abstract

We pursue the intriguing possibility that larger-size instantons build up diffractive scattering, with the marked instanton-size scale $<ρ> \approx 0.5$ fm being reflected in the conspicuous ``geometrization'' of soft QCD. As an explicit illustration, the known instanton contribution to DIS is transformed into the intuitive colour dipole picture. With the help of lattice results, the $q \bar{q}$-dipole size r is carefully increased towards hadronic dimensions. Unlike pQCD, one now observes a competition between two crucial length scales: the dipole size r and the size $ρ$ of the background instanton that is sharply localized around $<ρ> \approx 0.5$ fm. For r exceeding $<ρ>$, the dipole cross section indeed saturates towards a geometrical limit, proportional to the area $π<ρ>^2$, subtended by the instanton.