Direct measurement of the longitudinal proton structure function at HERA

The first direct measurements of the longitudinal proton structure function FL(x,Q2) at HERA are reported. The measurements were performed by the H1 and ZEUS collaborations. They are based on inclusive deep inelastic e+p scattering cross sections with a positron beam energy of 27.5 GeV and proton beam energies of 920, 575 and 460 GeV. The FL values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA.