High Energy Physics, Cavendish Laboratory, University of Cambridge
Abstract
At NNLO it is particularly important to have a Variable-Flavour Number Scheme
(VFNS) to deal with heavy quarks because there are major problems with both the zero
mass variable-flavour number scheme and the fixed-flavour number scheme. I illustrate
these problems and present a general formulation of a Variable-Flavour Number Scheme
(VFNS) for heavy quarks that is explicitly implemented up to NNLO in the strong coupling
constant S, and may be used in NNLO global fits for parton distributions. The
procedure combines elements of the ACOT( ) scheme and the Thorne-Roberts scheme.
Despite the fact that at NNLO the parton distributions are discontinuous as one changes
the number of active quark flavours, all physical quantities are continuous at flavour
transitions and the comparison with data is successful