Measurement of the proton structure function FL(x,Q2) at low x

Abstract A first measurement is reported of the longitudinal proton structure function F L ( x , Q 2 ) 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, F L is measured in a range of squared four-momentum transfers 12 ⩽ Q 2 ⩽ 90 GeV 2 and low Bjorken x 0.00024 ⩽ x ⩽ 0.0036 . The F L values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA.