The New F_L Measurement from HERA and the Dipole Model

TL;DR

This paper analyzes new F_L measurements from HERA within the dipole model framework, comparing data to theoretical bounds, and discusses implications for understanding proton structure and QCD phenomena.

Contribution

It derives fixed-Q^2 averages of F_L/F_2 from HERA data and compares them with rigorous bounds in the dipole model, including charm contributions, highlighting potential deviations.

Findings

Data generally respect the bounds within errors

Some data points are close to or above the bounds

Exceeding bounds would challenge the dipole picture

Abstract

From the new measurement of F_L at HERA we derive fixed-Q^2 averages <F_L/F_2>. We compare these with bounds which are rigorous in the framework of the standard dipole picture. The bounds are sharpened by including information on the charm structure function F_2^(c). Within the experimental errors the bounds are respected by the data. But for 3.5 GeV^2 <= Q^2 <= 20 GeV^2 the central values of the data are close to and in some cases even above the bounds. Data on F_L/F_2 significantly exceeding the bounds would rule out the standard dipole picture at these kinematic points. We discuss, furthermore, how data respecting the bounds but coming close to them can give information on questions like colour transparency, saturation and the dependencies of the dipole-proton cross section on the energy and the dipole size.