Critical examination is made on the relation between the mass shift of vector mesons in nuclear medium and the vector-meson − nucleon scattering length. We give detailed comparison between the QCD sum rule approach by two of the present authors (Phys. Rev. C46 (1992) R34) and the scatteringlength approach by Koike (Phys. Rev. C51 (1995) 1488. It is shown that the latter approach is mortally flawed both technically and conceptually.

Effective masses of ρ and ω mesons in nuclear medium are studied in a hadronic effective theory. Both the pole position and the screening mass decrease in nuclear matter due to the polarization of the nucleon Dirac sea. The physical origin of the decrease is a reduction of the wave function renormalization constant induced by the tensor (vector) interaction of the ρ (ω) with the nucleon. Relation to the results of the QCD sum rules is also discussed.

We summarize the current theoretical and experimental status of the spectral changes of vector mesons ($\rho$, $\omega$, $\phi$) in nuclear medium. Various approaches including QCD sum rules, effective theory of hadrons and bag models show decreasing of the vector meson masses in nuclear matter. Possibility to detect the mass shift through lepton pairs in $\gamma-A$, $p-A$ and $A-A$ reactions are also discussed.Comment: 33 pages, latex, 14 figures, invited paper (Prog. Theor. Phys.

The external-field QCD Sum Rules method is used to evaluate the coupling constant of the light isoscalar-scalar meson ("σ" or ε) to the nucleon. The contributions that come from the excited nucleon states and the response of the continuum threshold to the external field are calculated. The obtained value of the coupling constant is compatible with the large value required in one-boson exchange potential models of the two-nucleon interaction.

Induced tensor charge of the nucleon g T , which originates from G-parity violation, is evaluated from QCD sum rules. We find that g T /g A with g A being the axial charge is −0.0152 ± 0.0053 which is proportional to u-d quark mass difference. This result is small compared to preliminary analysis of the experiment, but is consistent with the estimate in the MIT bag model.

We summarize the current theoretical and experimental status of the spectral change of the vector mesons in dense matter.