Clarify the need for evolution beyond DGLAP

Determine whether the low-x behavior of the HERA structure function data requires an evolution equation beyond the conventional DGLAP formalism.

Background

The paper explains that conventional DGLAP fits describe much of the HERA data but become less successful at low x and low Q², particularly in describing the longitudinal structure function and the turnover of the reduced cross section. The extracted gluon distribution can also become valence-like while the sea-quark distribution remains steep, challenging the simple interpretation of the gluon driving the sea through ordinary splitting.

The authors suggest that measurements of FL and F2^{c\bar c} can help determine whether the anomalous Q² slope of F2 is caused by the gluon distribution or by the splitting function itself. This leaves unresolved whether the data require a genuinely different evolution framework, such as one incorporating small-x resummation or non-linear dynamics.