Comparison between the various descriptions for charm electroproduction and the HERA - data

We examine the charm component F 2,c (x, Q 2 , m 2) of the proton structure function F 2 (x, Q 2) in three different schemes and compare the results with the data in the x and Q 2 region explored by the HERA experiments. Studied are (1) the three flavour number scheme (TFNS) where the production mechanisms are given by the photon-gluon fusion process and the higher order reactions with three light-flavour parton densities as input (2) the four flavour number scheme (FFNS) where F 2,c is expressed in four light flavour densities including one for the charm quark and (3) a variable-flavour number scheme (VFNS) which interpolates between the latter two. Both the VFNS and the TFNS give good descriptions of the experimental data. However one cannot use the FFNS for the description of the data at small Q 2 .