MRST partons generated in a fixed - flavour scheme

We generate fixed three-and four-light-flavour sets of partons using MRST2004 par-tons as input. We show that it is important to set n f = 3 in the strong coupling, as well as in the splitting and coefficient functions, in order to obtain a consistent set of fixed-flavour partons. We compare the description of data using partons in both variable-and fixed-flavour-number-schemes. In deep inelastic scattering, and " hard " proton-proton (or antiproton-proton) high-energy collisions, the scattering proceeds via the partonic constituents of the hadron. To predict the rates of the various processes a set of universal parton distribution functions is required. These quark and gluon distributions are usually given in a variable-flavour-number-scheme (VFNS) in which the number of active quark flavours increases from n f = 3 to n f = 4, and then to n f = 5, as Q 2 increases above about m 2 c , and then above about m 2 b. Here Q 2 is the magnitude of the square of the momentum transfer in the process. However there are practical reasons for also generating a set of up-to-date partons in the fixed three-light-flavour (n f = 3) and fixed four-light-flavour (n f = 4) schemes. For many exclusive or semi-inclusive processes the theoretical predictions for the hard scattering cross sections are, as yet, only available in the fixed-flavour-number-scheme (FFNS) where all the heavy flavours are produced in the hard cross-section. Thus fixed-flavour partons are needed in existing Monte Carlos, see, for example, [1, 2, 3]. Also, despite the availability of a number of general mass variable-flavour number schemes, extractions of partons from fits to structure function data are often performed using a FFNS, so it is useful to have available an up-to-date