Geometric scaling in leading neutron events at HERA

TL;DR

This paper demonstrates that leading neutron production at HERA exhibits geometric scaling, with the total gamma-star pion-star cross section depending only on a single variable, indicating saturation effects similar to inclusive DIS.

Contribution

First demonstration of geometric scaling in leading neutron events at HERA, extending saturation model applicability to this process.

Findings

Leading neutron data exhibit geometric scaling.

Scaling region spans a broad Q^2 range.

Saturation boundary influences the scaling behavior.

Abstract

This analysis provides new fits of the GBW model and the impact parameter-dependent saturation model (bSat or IP-Sat) to the leading neutron structure function HERA data in one pion exchange approximation. Both parametrizations of the dipole cross section provide good descriptions of the considered data. It is shown here for the first time that the experimental leading neutron production HERA data exhibits geometric scaling, which in this context means that the total $\gamma^* \pi^*$ cross section is a function of only one dimensionless variable $\tau = Q^2/Q_s^2(x)$. The geometric scaling region extends over a broad range of $Q^2$ and can be attributed to the presence of a saturation boundary which manifests at $Q^2\geq Q^2_s$. The scaling behaviour in leading neutron events is profoundly similar to what has been observed for the inclusive DIS events.