  We calculate the cross section of a pion diffraction dissociation in two jets
with large transverse momenta originating from a hard gluon exchange between
the pion constituents. To the leading logarithmic accuracy (in energy), the
contribution coming from small transverse separations between the quark and the
antiquark in the pion acquires the expected factorized form, the longitudinal
momentum distribution of the jets being proportional to the pion distribution
amplitude. The hard gluon exchange can in this case be considered as a part of
the unintegrated gluon distribution. Beyond the leading logarithms (in energy)
this proportionality does not hold. Moreover, the collinear factorization
appears to be broken by the end-point singularities. Remarkably enough, the
longitudinal momentum distribution of the jets for the non-factorizable
contribution is calculable, and turns out to be the same as for the
factorizable contribution with the asymptotic pion distribution amplitude.
