Parton evolution in the fixed flavor factorization scheme

It is argued that while the scale dependence of the parton distributions in the fixed flavor factorization scheme is governed by three active flavors, the scale dependence of the running coupling should nevertheless be better governed by a variable number of active flavors. The fixed flavor factorization schme (FFS) is characterized by considering the heavy quarks (h = c, b, t) always as external particles which are not included among the partons in the colorless hadrons. Their participation in deep inelastic scattering processes like, say, eN → eX is considered to be due to production subprocesses such as γ∗g → hh̄ rather than γ∗h → h. The latter subprocess becomes relevant in the so called variable flavor factorization scheme where, besides the light u, d, s quarks, the heavy quarks are also considered to form an intrinsic part of colorless hadrons. Considering both subprocesses together would amount to double counting and thus the FFS dictates setting the number of flavors nf = 3 in the flavor–singlet QCD evolution equations d d lnQ2 ~q (x,Q) = αs(Q ) 2π ∫ 1