We present the heavy-to-light form factors with two different non-vanishing
masses at next-to-next-to-leading order and study its expansion in the small
mass. The leading term of this small-mass expansion leads to a factorized
expression for the form factor. The presence of a second mass results in a new
feature, in that the soft contribution develops a factorization anomaly. This
cancels with the corresponding anomaly in the collinear contribution. With the
generalized factorization presented here, it is possible to obtain the leading
small-mass terms for processes with large masses, such as muon-electron
scattering, from the corresponding massless amplitude and the soft
contribution.Comment: 20 pages, 4 figures, 1 ancillary file, published versio
