First Measurements of FL at Low Bjorken x

Abstract

The first measurements of the longitudinal proton structure function FL in deep inelastic positron-proton scattering at low Bjorken x are described. Theoretical predictions in higher order QCD and using dipol model calculations are consistent with the measurements. The data were taken by the HERA experiments H1 and ZEUS in a series of runs with different proton beam energies in 2007.