Determine whether HERA observes high-density QCD effects

Determine whether the kinematic reach of HERA is sufficient to observe high-density gluon effects and whether such effects have already been observed in HERA data.

Background

The paper discusses the possibility that the rapid small-x growth of the gluon density leads to gluon recombination and saturation, requiring non-linear evolution equations such as GLR, BK, or JIMWLK. It emphasizes that the scales at which non-perturbative, BFKL, and high-density regimes begin are debated and that HERA reaches only relatively low saturation scales.

The question remains unresolved because saturation-like features, including geometrical scaling and the flattening of low-Q² observables, can also be described by non-saturating models or by higher-twist and non-perturbative effects. The authors therefore identify the existence of observable high-density effects at HERA as an open issue rather than a settled conclusion.

References

One of the many hotly argued questions raised by the HERA data is does HERA have the reach to see high density effects and have they been seen?

— HERA at low-x  (2610.26393 - Cooper-Sarkar et al., 19 Oct 2026) in Section "Non-linear evolution equations, saturation" (subsection of Section 5, Low x physics at HERA)

Whether saturation effects have been demonstrated at HERA is controversial, but one needs the $\ln(1/x)$ resummations in order to preserve the high density gluon system that is a pre-requisite of saturation.

— HERA at low-x  (2610.26393 - Cooper-Sarkar et al., 19 Oct 2026) in Section "Non-linear evolution equations, saturation" and Summary