Problems in the resummation of soft-gluon effects in the   transverse-momentum distributions of massive vector bosons in hadronic   collisions
  We consider the resummation of soft-gluon emission in the transverse-momentum distribution of vector mesons in hadronic collisions. We find that the resummed expression in the impact-parameter formulation has an expansion in $\as$ with factorially growing terms with oscillating signs. These diverging terms arise from the small impact-parameter region of integration, and are of a subleading nature. We also obtain a closed expression for the next-to-leading logarithm resummation in $\qt$-space, and we study its analytic structure. We find in this case that, although no factorially growing terms are present, there are geometrical singularities that severely restrict the range of applicability of the resummation formula. 
S. Frixione (ETH Zurich), P. Nason (CERN) and G. Ridolfi (INFN Genova)