Medium Modifications of Hadron Properties and Partonic Processes

TL;DR

This paper reviews experimental efforts at Jefferson Lab to investigate how hadron properties and partonic processes are modified in nuclear media, providing insights into chiral symmetry restoration and in-medium effects on mesons and nucleons.

Contribution

It presents new experimental data on in-medium modifications of proton form factors, rho meson properties, and quark fragmentation functions, advancing understanding of QCD in nuclear environments.

Findings

Indications of modified proton electromagnetic form factors in 4He.

No significant mass shift but broadening of rho meson in nuclei.

High-precision measurements of in-medium quark fragmentation functions.

Abstract

Chiral symmetry is one of the most fundamental symmetries in QCD. It is closely connected to hadron properties in the nuclear medium via the reduction of the quark condensate <bar{q}q>, manifesting the partial restoration of chiral symmetry. To better understand this important issue, a number of Jefferson Lab experiments over the past decade have focused on understanding properties of mesons and nucleons in the nuclear medium, often benefiting from the high polarization and luminosity of the CEBAF accelerator. In particular, a novel, accurate, polarization transfer measurement technique revealed for the first time a strong indication that the bound proton electromagnetic form factors in 4He may be modified compared to those in the vacuum. Second, the photoproduction of vector mesons on various nuclei has been measured via their decay to e+e- to study possible in-medium effects on the properties of the rho meson. In this experiment, no significant mass shift and some broadening consistent with expected collisional broadening for the rho meson has been observed, providing tight constraints on model calculations. Finally, processes involving in-medium parton propagation have been studied. The medium modifications of the quark fragmentation functions have been extracted with much higher statistical accuracy than previously possible.