  We consider the production of a generic system of non-strongly interacting
particles with a high total invariant mass M in hadron collisions. We examine
the transverse-momentum (q_T) distribution of the system in the small-q_T
region (q_T << M), and we present a study of the perturbative QCD contributions
that are enhanced by powers of large logarithmic terms of the type ln
(M^2/q_T^2). These terms can be resummed to all orders in QCD perturbation
theory. The partonic production mechanism of the final-state system can be
controlled by quark-antiquark (qqbar) annihilation and/or by gluon fusion. The
resummation formalism for the qqbar annihilation subprocess is well
established, and it is usually extrapolated to the gluon fusion subprocess. We
point out that this naive extrapolation is not correct, and we present the
all-order resummation formula for the q_T distribution in gluon fusion
processes. The gluon fusion resummation formula has a richer structure than the
resummation formula in qqbar annihilation. The additional structure originates
from collinear correlations that are a specific feature of the evolution of the
colliding hadrons into gluon partonic states. In the q_T cross section at small
values of q_T, these gluon collinear correlations produce coherent spin
correlations between the helicity states of the initial-state gluons and
definite azimuthal-angle correlations between the final-state particles of the
observed high-mass system.
