A first measurement is reported of the longitudinal proton structure function FL(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, FL is measured in a range of squared four-momentum transfers 12 <= Q2 <=, 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.
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