First Study of the Nuclear Response to Fast Hadrons via Angular Correlations between Pions and Slow Protons in Electron-Nucleus Scattering

TL;DR

This study introduces the first measurement of angular correlations between high-energy pions and slow protons in electron-nucleus scattering, revealing how nuclei respond to fast quarks and testing current theoretical models.

Contribution

It provides novel experimental data on pion-proton correlations in $eA$ scattering and compares these results with state-of-the-art event generators, highlighting areas for model improvement.

Findings

Correlation functions are more spread out in heavier nuclei.

Proton-to-pion yield ratio increases with nuclear mass and saturates.

Current models qualitatively reproduce trends but show discrepancies.

Abstract

We report on the first measurement of angular correlations between high-energy pions and slow protons in electron-nucleus ($eA$) scattering, providing a new probe of how a nucleus responds to a fast-moving quark. The experiment employed the CLAS detector with a 5-GeV electron beam incident on deuterium, carbon, iron, and lead targets. For heavier nuclei, the pion-proton correlation function is more spread-out in azimuth than for lighter ones, and this effect is more pronounced in the $\pi p$ channel than in earlier $\pi\pi$ studies. The proton-to-pion yield ratio likewise rises with nuclear mass, although the increase appears to saturate for the heaviest targets. These trends are qualitatively reproduced by state-of-the-art $eA$ event generators, including BeAGLE, eHIJING, and GiBUU, indicating that current descriptions of target fragmentation rest on sound theoretical footing. At the same time, the precision of our data exposes model-dependent discrepancies, delineating a clear path for future improvements in the treatment of cold-nuclear matter effects in $eA$ scattering.