We revise the bââ model for the Collins-Soper-Sterman resummed form factor
to improve description of the leading-power contribution at nearly
nonperturbative impact parameters. This revision leads to excellent agreement
of the transverse momentum resummation with the data in a global analysis of
Drell-Yan lepton pair and Z boson production. The nonperturbative contributions
are found to follow universal quasi-linear dependence on the logarithm of the
heavy boson invariant mass, which closely agrees with an estimate from the
infrared renormalon analysis.Comment: published version; 14 pages, 4 figures, additional discussio