  The small-$t$ behaviour of the deep inelastic diffractive dissociation cross
section in the triple Regge region is investigated, using the BFKL
approximation in perturbative QCD. We show that the cross section is finite at
$t=0$, but the diffusion in $\ln{k_t^2}$ leads to a large contribution of small
momenta at the triple Pomeron vertex. We study the dependence upon the total
energy and the invariant mass. At $t=0$, there is a decoupling of the three
BFKL singularities which is a consequence of the conservation of the conformal
dimension. For large invariant masses, the four gluon state in the upper
t-channel plays an important role and cannot be neglected.

High Energy Physics - Phenomenology