Abstract
A first measurement is reported of the longitudinal proton structure function FL(x,Q2) 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 GeV2 and low Bjorken x0.00024⩽x⩽0.0036. The FL values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA.
Highlights
This Letter presents the first measurement of the longitudinal structure function F L (x, Q 2) of the proton at low Bjorken x
From experimental determinations by H1 [5,6,7], which used assumptions on the behaviour of F2 in extracting F L, and from theoretical analyses of the inclusive deep inelastic ep scattering (DIS) cross section data [8,9], the longitudinal structure function at low x is expected to be significantly larger than zero. This prediction relies on perturbative Quantum Chromodynamics (QCD) calculations of F L to next-to-leading order (NLO) [10] and NNLO [11]
Complementary tracking information is obtained from the backward silicon tracker (BST), which is positioned around the beam pipe, and from the z drift chamber COZ, which is located in between the two cylinders of the central jet drift chamber (CJC)
Summary
5 Partially Supported by Polish Ministry of Science and Higher Education, grant. 8 Supported by the Swedish Natural Science Research Council. 10 Supported by the Swiss National Science Foundation.
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