Light Vector Mesons in the Nuclear Medium

The light vector mesons ($ρ$, $ω$, and $ϕ$) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the $ρ$ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The $ρ$ meson mass spectrum was extracted after the $ω$ and $ϕ$ signals were removed in a nearly model-independent way. Comparisons were made between the $ρ$ mass spectra from the heavy targets ($A > 2$) with the mass spectrum extracted from the deuterium target. With respect to the $ρ$-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

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