Direct calculation of the triple-pomeron coupling for diffractive DIS and real photoproduction

We present a unified direct calculation of the triple-pomeron coupling $A_{3\text{Pom}}(Q^2)$ for diffractive real photoproduction ($Q^2=0$) and deep inelastic scattering at large $Q^2$ in the framework of the dipole approach to the generalized BFKL pomeron. The small phenomenological value of $A_{3\text{Pom}}(0)\approx 0.16$,GeV${}^2$, which was a mystery, is related to the small correlation radius $R_c\approx 0.3$,fm for the perturbative gluons. We confirm the early expectations of weak $Q^2$ dependence of the dimensionfull coupling $A_{3\text{Pom}}(Q^2)$ and predict that it rises by the factor $\sim 1.6$ from real photoproduction to deep inelastic scattering.