Medium Modifications of Hadron Properties and Partonic Processes

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 qq >, 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 ρ meson. In this experiment, no significant mass shift and some broadening consistent with expected collisional broadening for the ρ 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.