On the longitudinal structure function in the dipole model

TL;DR

This paper evaluates various dipole models against new HERA data for the longitudinal structure function, demonstrating the ratio $F_L/F_2$ is well modeled and largely insensitive to fit details, with implications for higher Fock states.

Contribution

It introduces bounds for the ratio $F_L/F_2$ derived from geometrical scaling in dipole models and discusses the impact of higher Fock components on these bounds.

Findings

Dipole models accurately describe the $F_L/F_2$ ratio.

The ratio $F_L/F_2$ is insensitive to fit details.

Higher Fock states may influence the bounds.

Abstract

We compare new HERA data for the longitudinal structure function $F_{\rm L}$ with the predictions of different variants of the dipole model. In particular we show that the ratio $F_{\rm L}/F_2$ is well described by the dipole models and is rather insensitive to the details of the fit. Fits to $F_2$ are performed with the help of geometrical scaling (GS). Using the property of GS we derive the bounds for $F_{\rm L}/F_2$ both for the different versions of the dipole model and in the general case. Finally we briefly discuss how the higher Fock components of the photon wave function may affect these bounds.