kT factorization of exclusive processes -: art. no. 034001

We prove the k(T) factorization theorem in perturbative QCD (PQCD) for exclusive processes by considering pigamma*-->gamma(pi) and B-->gamma(pi)l (nu) over bar. The relevant form factors are expressed as the convolution of hard amplitudes with two-parton meson wave functions in the impact parameter b space, b being conjugate to the parton transverse momenta k(T). The point is that on-shell valence partons carry longitudinal momenta initially, and acquire k(T) through collinear gluon exchanges. The b-dependent two-parton wave functions with an appropriate path for the Wilson links are gauge-invariant. The hard amplitudes, defined as the difference between the parton-level diagrams of on-shell external particles and their collinear approximation, are also gauge-invariant. We compare the predictions for two-body nonleptonic B meson decays derived from k(T) factorization (the PQCD approach) and from collinear factorization (the QCD factorization approach).