High energy factorization predictions for the charm structure function F2^c at HERA.pdf

a r X i v : h e p - p h / 9 8 0 2 2 3 0 v 1 3 F e b 1 9 9 8 High energy factorization predictions for the charm structure function F c2 at HERA S. Munier and R. Peschanski CEA, Service de Physique The?orique, CE-Saclay F-91191 Gif-sur-Yvette Cedex, France February 1, 2008 Abstract High energy factorization predictions for F c2 are derived using BFKL descriptions of the proton structure function F2 at HERA. The model parameters are fixed by a fit of F2 at small x. Two different approaches of the non perturbative proton input are shown to correspond to the factorization at the gluon or quark level, respectively. The predic- tions for F c2 are in agreement with the data within the present error bars. However, the photon wave-function formulation (factorization at quark level) predicts significantly higher F c2 than both gluon fac- torization and a next-leading order DGLAP model. 1 1 Motivation High energy factorization [1, 2] is a QCD factorization scheme suited for high- energy hard processes - and in particular for deep-inelastic e??p scattering in the small x-regime (x ? Q 2 W 2 ) (W 2 is the energy squared in the virtual photon-proton center of mass frame). This scheme takes into account the resummation of the ( αs log 1 x )n terms in the QCD perturbative expansion of the structure functions. It amounts to proving the factorization of the perturbative amplitude in terms of a gluon-gluon BFKL kernel [3] convoluted with a first order (virtual)photon-(virtual)gluon cross section. A graphical illustration of this property is given in figure 1. The main difference between this scheme and the renormalization group factorization [4] which is valid at finite x is that the former involves a bidimensional integration in transverse momentum k⊥, whereas the latter is a convolution in energy. Interestingly enough, high energy factorization can be applied to the resummation of the( αs log W 2 M2 )n terms in heavy quark pair photoproduction, and the combined resummation for