Comment on "The New F_L Measurement from HERA and the Dipole Model"

TL;DR

This paper critiques a recent measurement of the ratio of proton structure functions, arguing that the claimed model-independent test relies on assumptions and that the results support the existing color-dipole model prediction.

Contribution

It clarifies that the recent upper bound on $F_L/F_2$ is not a model-independent test and reinterprets the measurement as confirmation of the color-dipole model prediction.

Findings

The upper bound depends on an ad hoc assumption.

The measurement supports the prediction $F_L = 0.27 F_2$.

The test is not model-independent.

Abstract

The upper bound on the ratio of the proton structure functions $F_L/F_2$ tested in the recent paper "The New $F_L$ Measurement from HERA and the Dipole Model", contrary to what is said therein, does not provide a model-independent "rigorous" experimental test of the color-dipole picture. The validity of the theoretical upper bound depends on an ad hoc assumption on the dipole cross section. -- The analysis in the paper "The New $F_L$ Measurement from HERA and the Dipole Model" can be reinterpreted as an additional confirmation of the absolute model-independent prediction from the color-dipole picture of $F_L = 0.27 F_2$.