Determination of the longitudinal proton structure function FL at low Q2 at HERA

TL;DR

The paper presents a measurement of the longitudinal proton structure function FL at very low Q2 and low x using H1 data from 1999–2000. It introduces two extraction methods, a derivative approach and a shape fit, and shows they are consistent, with the shape method delivering higher precision. FL is found to be positive across the explored kinematic range, and the data favor GBW dipole and BKS models while challenging MRST at low Q2. These results extend FL measurements to Q2 as low as ~0.75 GeV2, informing the gluon distribution in the proton at small x.

Abstract

An extraction of the longitudinal proton structure function FL(x,Q2) from H1 data at low Q2 (about 1 GeV2) and low Bjorken x (about 0.00005) is reported. The analysis is based on the data collected in a dedicated low Q2 running period in 1999 and during shifted vertex runs in 2000. Two methods of extracting FL(x,Q2) are discussed and shown to give consistent results. Theoretical predictions are compared with the data.

Figures (4)

• Figure 1: The reduced cross section as a function of $x$ for different $Q^2$ bins. The dashed lines show a function of the form $\sigma_r = c\cdot x^{-\lambda}$ representing the $F_2$ contribution to the fitted cross section. The solid lines show fits of the form $\sigma_r = c\cdot x^{-\lambda} - F_L\cdot y^2/Y_+$ , from which $F_L$ is extracted in the shape method.
• Figure 2: Comparison of $F_L(x,Q^2)$ results, for fixed $Q^2$, from the 2000 shifted vertex data as extracted by the derivative (triangles) and the shape (points) methods, see text.
• Figure 3: $F_L(x,Q^2)$, for fixed $Q^2$, as extracted by the shape method. The solid, thin line represents the QCD fit to previous H1 cross section data and the dashed line the QCD fit extrapolated backwards to $Q^2$ below $Q^2_{min}$=3.5 GeV$^2$. Other curves show predictions of different theoretical models.
• Figure 4: $Q^2$ dependence of $F_L(x,Q^2)$ (at fixed W=276 GeV), summarizing the data from the H1 experiment.