New 5-flavor LO analysis and parametrization of parton distributions in the real photon

New, radiatively generated, leading order (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 Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equations, to all available data for the structure function F2gamma(x,Q2). We adopt a new theoretical approach called ACOT(chi), originally introduced for the proton, to deal with the heavy-quark thresholds. This defines our basic model [Cornet-Jankowski-Krawczyk-Lorca (CJKL) model], which gives a very good description of the experimental data on F2gamma(x,Q2), for both Q2 and x dependences. For comparison we perform a standard fit using the fixed flavor-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 F2gamma(x,Q2) data. The CJKL model gives also the best description of the CERN LEP data on the Q2 dependence of the F2gamma, averaged over various x regions, and the F2,cgamma, which were not used directly in the fit. Finally, a simple analytic parametrization of the resulting parton densities obtained with the CJKL model is given.