Determination of the Longitudinal Proton Structure Function F_L(x,Q^2) 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 $8.5\le Q^2\le 35Ge{V}^2$ and large inelasticity $y=0.7$, i.e. for the Bjorken-x range $0.00013\le x\le 0.00055$. 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 $F_L=0.52\pm 0.03(stat)$^ {+0.25}_{-0.22}$(syst)at$Q^2=15.4 GeV^2$and$x=0.000243$.

17 pages, latex, 4 Figures

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