Vector mesons in medium

Abstract

Based on an effective Lagrangian which combines chiral SU(3) dynamics with vector meson dominance, we have developed a model for the forward vector meson-nucleon scattering amplitudes. We use this as an input to calculate the low energy part of the current-current correlation function in nuclear matter. For the isovector channel we find a significant enhancement of the in-medium spectral density below the $ρ$ resonance while the $ρ$ meson mass itself changes only slightly. The situation is different in the isoscalar channel, where the mass and peak position of the $ω$ meson move downward while its width increases moderately. For the $ϕ$ meson we find almost no mass shift, but the width of the peak increases significantly. We use these spectra as ``left hand side'' of in medium QCD sum rules and observe a remarkable degree of consistency with the operator product expansion ``right hand''side in all three channels. We point out, however, that these results cannot simply be interpreted, as commonly done, in terms of a universal rescaling of vector meson masses in matter. We also compare the resulting thermal dilepton rates of our model with CERES data. We find satisfactory agreement but we point out that the dilepton rates from completely uncorrelated quark-antiquark pairs also fit the data quite well. Considering the strongly attractive potential for the $ω$ together with its comparably small width, we discuss the possible formation of bound $ω$ states in nuclei. Using a Green's function approach we investigate the possibility of detecting these bound states in the $A(d, ^3He) $ pick-up reaction.