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

Abstract A measurement of the inclusive cross section for the deep-inelastic scattering of positrons off protons at HERA is presented at momentum transfers 8.5 ≤ Q 2 ≤ 35 GeV 2 and large inelasticity y > = 0.7, i.e. for the Bjorken- x range 0.00013 ≤ x ≤ 0.00055. Using a next-to-leading order QCD fit to the structure function F 2 at lower y values, the contribution of F 2 to the measured cross section at high y is calculated and, by subtraction, the longitudinal structure function F L is determined for the first time with an average value of F L = 0.52 ± 0.03 (statt) −0.22 +0.25 (syst) at Q 2 = 15.4 GeV 2 and x = 0.000243.