A New 5 Flavour LO Analysis and Parametrization of Parton Distributions in the Real Photon

New, radiatively generated, LO quark (u, d, s, c, b) and gluon densities in a real, unpolarized photon are presented. We perform a global 3 parameter fit, based on LO DGLAP evolution equations, to all available data for the structure function F γ 2 (x,Q 2). We adopt a new theoretical approach called ACOT(χ), originally introduced for the proton, to deal with the heavy-quark thresholds. This defines our basic model (CJKL model) which gives a very good description of the experimental data on F γ 2 (x,Q 2), for both Q2 and x dependences. For comparison we perform a standard fit using the Fixed Flavour-Number Scheme (FFNScjkl model), updated with respect to the previous fits of this type. We show the superiority of the CJKL fit over the FFNScjkl one and other LO fits to the F γ 2 (x,Q 2) data. The CJKL model gives also the best description of the LEP data on the Q2 dependence of the F γ 2 , averaged over various x-regions, and the F γ 2,c, which were not used directly in the fit. Finally, a simple analytic parametrization of the resulting parton densities obtained in the CJKL model is given. IFT-22/2002 UG-FT-138/02 CAFPE-8/02 DESY 02-118