Photoproduction of Vector Mesons Off Nuclei

This experiment is designed to measure the properties of vector mesons in nuclear matter. At high baryon density the masses of vector mesons are predicted to change due to chiral symmetry restoration. The predicted downward shift of the vector meson masses is 10 to 20%. The photoproduction of vector mesons near threshold can be used to measure the masses and widths of vector mesons embedded in nuclear matter. It may also be possible to measure the mass shift for narrow vector mesons. Because o f the long decay length cτ , only a small fraction of these vector mesons will decay inside the nucleus. However, since the mass shift is predicted to be substantial relative to the natural width, these decays will be easier t o separate from vector mesons decaying outside of the nucleus. Detecting the leptonic decays of vector mesons is the only reliable way t o measure the mass shift of vector mesons because the hadronic decay in nuclear matter is always distrubed by final state interactions. The small cross sections for incident photons and for secondary interactions of the outgoing electrons with nuclear matter makes this reaction the ideal probe for testing the properties of the dense central region of the nucleus without significant input and output distortions. We plan to measure the A(γ , e + e - ) ' A reactions by identifying the coincident electron pairs in the CLAS detector. Energy deposition in the electromagnetic calorimeter, the Cerenkov counter signal, and transverse momentum compensation define clear cuts f or the separation of the e + e - events from the large hadronic background.