Search for Medium Modifications of the Light Vector Mesons at Jefferson Lab

The E01-112 experiment at the Thomas Jefferson National Laboratory was an investigation of the properties of light vector mesons in dense nuclear matter, such as a shift in their masses and/or broadening of their widths. Theoretical calculations relate the modifications to partial restoration of chiral symmetry at high density or temperature. In the experiment, the $\rho$, $\omega$, and $\phi$ mesons were photo-produced of off $^{2}$H, C, Ti, Fe, and Pb targets and reconstructed with the CEBAF Large Acceptance Spectrometer (CLAS). The incident beam was tagged photons with energies up to 4~GeV. The mesons were detected via their rare leptonic decay to $e^{+}e^{-}$. This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The $\rho$ meson mass spectrum was extracted after the subtraction of a combinatorial background and after the removal of the $\omega$ and $\phi$ signals in a nearly model-independent way. The $\rho$ mass spectra from the heavy targets (A $>$ 2) were compared with the mass spectrum extracted from the deuterium target. We obtain a mass-shift compatible with zero for the $\rho$ meson. For the $\rho$-mesons widths, our result is consistent with standard nuclear many-body effects, i.e. collisional broadening and Fermi motion. Even though the $\omega$ and $\phi$ mesons have a high probability of decaying outside the nucleus in their vacuum state, their in-medium widths can be accessed through their absorption inside the nucleus. The signature of absorption is a decrease of the nuclear transparencies of these mesons as a function of the number of target nucleons. Preliminary results indicate a substantial widening of the $\omega$ and $\phi$ mesons in the medium.