Triple-pomeron dynamics: valence vs. glue

Three pomerons are known to couple via the gluon loop and interactions of this kind which are responsible for high-mass diffraction are described in terms of the gluonic structure function of the QCD pomeron. We show that the triple-pomeron coupling via the light quark loop associated with the ``valence'' in the pomeron is of the same strength as the purely gluonic coupling. The large $Q^2$ behavior of this new contribution is described by the DLLA evolution from the non-perturbative $f\bar f$ valence state of the pomeron. Numerical estimates of the high-mass diffraction structure functions based on the consistent account of both couplings are in good agreement with experimental data.