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

A first measurement is reported of the longitudinal proton structure function FL(x,Q2)FL(x,Q2) at the ep collider HERA. It is based on inclusive deep inelastic e+pe+p scattering cross sectionmeasurements with a positron beam energy of 27.5 GeV27.5 GeV and proton beam energies of 920, 575 and 460 GeV460 GeV. Employing the energy dependence of the cross section, FLFL is measured in a range of squared four- momentum transfers12⩽Q2⩽90 GeV212⩽Q2⩽90 GeV2 and low Bjorken x0.00024⩽x⩽0.0036. The FLFL values agree with higher order QCD calculations based on parton densities obtained using cross section datapreviously measured at HERA.