Determination of the Longitudinal Proton Structure Function F_L(x,Q^2) at Low x

TL;DR

This paper reports the first measurement of the longitudinal proton structure function F_L at low x and moderate Q^2, using deep-inelastic scattering data from HERA and a QCD fit to separate F_L from F_2 contributions.

Contribution

It introduces a novel method to determine F_L at low x by subtracting F_2 contributions, providing new insights into proton structure at small Bjorken-x.

Findings

First measurement of F_L at low x and moderate Q^2.

F_L value of 0.52 ± 0.03 (stat) +0.25/-0.22 (syst) at Q^2=15.4 GeV^2.

Supports QCD predictions of proton structure functions.

Abstract

A measurement of the inclusive cross section for the deep-inelastic scattering of positrons off protons at HERA is presented at momentum transfers $8.5 \leq Q^2 \leq 35 GeV^2$ and large inelasticity $y = 0.7$, i.e. for the Bjorken-x range $0.00013 \leq x \leq 0.00055$. 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 $x=0.000243$.