A determination of FL at xmin with HERA data

TL;DR

This paper introduces a new method to determine the longitudinal structure function FL at low Q2 using HERA data, revealing unexpectedly large FL values and supporting the dipole model interpretation.

Contribution

A novel extrapolation technique for FL determination at low x and Q2, providing new insights into the structure functions and their behavior in the low Q2 regime.

Findings

FL at low Q2 is larger than expected

FL and F2 show properties consistent with the dipole picture

Method accounts for statistical and systematic errors

Abstract

It is well known that there are persistent statistical tensions with the standard model in the low Q2 HERA deep inelastic scattering neutral current data characterized by a turn-over of F2(x, Q2) at low x and low Q2. One important experimental signature that sheds light on this low Q2 region is the determination of the longitudinal structure function FL(x, Q2). This paper describes a novel method to determine FL based on an extrapolation of the reduced NC cross section at fixed s and Q to the minimum value of x given by Q2/s. At this kinematic point, the reduced cross section equals 2xF1 = F2 - FL so that a determination of both this value and the value of F2, determines FL. Since the polarization of the exchanged photon is transverse at this kinematic point, we expect FL to be small because its dominate gluon component is strongly suppressed. Surprisingly, we find FL at low Q2 to be much larger than expectation and observe that both FL and F2 at x = Q2/s show several properties consistent with the dipole picture. We discuss the statistical as well as chief systematic errors of our method and we tabulate our determinations of F2, 2xF1 and FL in the Appendix.