Search for Medium Modifications of the $\rho$ Meson

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, $\rho$, $\omega$, and $\varphi$, are observed via their decay to $e^{+}{e}^{-}$, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the $\rho$ meson. The $\rho$ mass spectrum is extracted from the data on various nuclei, ${}^2\mathrm{H}$, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the $\rho$ meson.

Figure 1

Normalized combinatorial background (blue line) for all and individual targets compared to data (black crosses).

Figure 2

Result of the fit to the $e^{+}{e}^{-}$ invariant mass obtained for the carbon (top) and Fe-Ti (bottom) data. The curves are calculations by the BUU model [48, 49] for various $e^{+}{e}^{-}$ channels.

Figure 3

The result of a simultaneous fit (solid lines) to the $\rho$ mass spectra and the ratios for ${}^2\mathrm{H}$ (top), C (middle), and Fe-Ti (bottom).

Figure 4

Fits to the $\rho$ mass distribution from ${}^2\mathrm{H}$ data using a Breit-Wigner function (dashed line) and a Breit-Wigner function divided by ${\mu }^3$ (solid line).