HERA Data Constrain Pion Structure via Leading Neutrons

A new study uses data from the HERA collider to probe the internal structure of the pion. The analysis combines the Sullivan one-pion-exchange framework with models of target fragmentation to describe leading-neutron production across a broad kinematic range.

Researchers from the H1 and ZEUS experiments at HERA previously measured leading-neutron electroproduction. These measurements have now been used to constrain pion parton distribution functions (PDFs), which describe how momentum is shared among the pion's constituent quarks and gluons. The work focuses on small momentum fractions within the pion, complementing constraints from other experiments that probe larger momentum fractions.

The Sullivan Process and Target Fragmentation

The primary mechanism for producing a leading neutron is the Sullivan one-pion-exchange process. In this framework, a virtual photon from the electron beam interacts with a pion that has been temporarily exchanged between the proton's constituent quarks. This process is most dominant when the detected neutron carries a large fraction of the original proton's longitudinal momentum, denoted as x_L.

At smaller values of x_L, contributions from a different mechanism become significant. This is the target-fragmentation region of deep-inelastic scattering, where the struck proton breaks apart and produces a neutron among its debris. Previous analyses often ignored this region or applied ad hoc corrections to account for it.

A Unified Model for HERA Data

This new work employs the Pythia event generator to model the target-fragmentation contribution. The authors show that combining this modeled contribution with the calculated Sullivan process contribution successfully reproduces the key features of the HERA leading-neutron data across the full measured x_L range. Crucially, this combined model does not require introducing additional normalization factors, providing a more consistent theoretical description.

The success of this approach demonstrates that a broader set of leading-neutron data can be incorporated into global analyses of particle structure. This extended dataset increases sensitivity to smaller pion momentum fractions, x_π, offering a clearer window into the pion's internal dynamics at high energies.

Model Dependence and Future Prospects

The analysis also investigates theoretical uncertainties. The results show sensitivity to the choice of the pion-nucleon vertex form factor, a parameter in the Sullivan model, and to the different treatments of target fragmentation available within Pythia. The HERA data can help distinguish between these modeling choices.

Looking ahead, the study presents projections for leading-neutron production at the future Electron-Ion Collider (EIC) in the United States. The authors identify specific beam-energy configurations and kinematic regions that would maximize sensitivity to pion structure while minimizing contamination from the target-fragmentation background.

These EIC projections suggest that precise measurements of leading neutrons will be a powerful tool for extracting pion PDFs. The future collider's high luminosity and variable center-of-mass energy will allow physicists to isolate the Sullivan process signal more effectively than was possible at HERA.