k(T) effects in direct-photon production - art. no. 074007

We discuss the phenomenology of initial-state parton-k(T) broadening in dir ect-photon production and related processes in hadron collisions. After a b rief summary of the theoretical basis for a Gaussian-smearing approach, we present a systematic study of recent results on fixed-target and collider d irect-photon production, using complementary data on diphoton and pion prod uction that provide empirical guidance on the required amount of k(T) broad ening. This approach provides a consistent description of the observed patt ern of deviation of next-to-leading order QCD calculations relative to the direct-photon data, and accounts for the shape and normalization difference between fixed-order perturbative calculations and the data. We also discus s the uncertainties in this phenomenological approach, the implications of these results on the extraction of the gluon distribution of the nucleon, a nd the comparison of our findings to recent related work. [S0556-2821(99)03 805-9].