Abstract

A first measurement is reported of the longitudinal proton structure function FL(x,Q2) at the ep collider HERA. It is based on inclusive deep inelastic e+p scattering cross section measurements with a positron beam energy of 27.5 GeV and proton beam energies of 920, 575 and 460 GeV. Employing the energy dependence of the cross section, FL is measured in a range of squared four-momentum transfers 12less-than-or-equals, slantQ2less-than-or-equals, slant90 GeV2 and low Bjorken View the MathML source. The FL values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA.