Heavy quark parton distributions: Mass-dependent or mass-independent evolution?

In a consistently formulated PQCD framework incorporating nonzero mass heavy quark partons, there is still the freedom to define parton distributions obeying either mass-independent or mass-dependent evolution equations, contrary to statements made in a recent paper. With properly matched hard cross sections, different choices merely correspond to different factorization schemes; they yield the same physical cross sections. We demonstrate this principle in a concrete order α_s calculation of the Deeply Inelastic Scattering charm structure function. We also examine the proper matching between parton definitions and subtractions in the hard cross section near threshold where the calculation is particularly sensitive to mass effects of the heavy quark. The results obtained from the general-mass formalism are quite stable against different choices of scale and exhibit a smooth transition in the threshold region (using either mass-independent or mass-dependent evolution), in contrast with results of another recently proposed scheme.