  Factorization is possible due to the universal behavior of Yang-Mills
theories in soft and collinear limits. Here, we take a small step towards a
more transparent understanding of these limits by proving a form of
perturbative factorization at tree- level using on-shell spinor helicity
methods. We present a concrete and self-contained expression of factorization
in which matrix elements in QCD are related to products of other matrix
elements in QCD up to leading order in a power-counting parameter determined by
the momenta of certain physical on-shell states. Our approach uses only the
scaling of momenta in soft and collinear limits, avoiding any assignment of
scaling behavior to unphysical (and gauge-dependent) fields. The proof of
factorization exploits many advantages of helicity spinors, such as the freedom
to choose different reference vectors for polarizations in different collinear
sectors. An advantage of this approach is that once factorization is shown to
hold in QCD, the transition to Soft-Collinear Effective Theory is effortless.
