Measurement of the Proton Structure Function F_L at Low x

TL;DR

This paper reports the first measurement of the longitudinal proton structure function F_L at low x using deep inelastic scattering data from HERA, confirming QCD predictions within the measured kinematic range.

Contribution

It provides the first experimental determination of F_L at low x, expanding understanding of proton structure in previously unexplored kinematic regions.

Findings

F_L measurements agree with higher order QCD calculations

Results cover 12 < Q^2 < 90 GeV^2 and 0.00024 < x < 0.0036

Supports existing parton density models

Abstract

A first measurement is reported of the longitudinal proton structure function F_L(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, F_L is measured in a range of squared four-momentum transfers 12 < Q^2 < 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.