Physical reason behind the mass-shift of vector mesons ($\rho$, $\omega$, $\phi$) in nuclear matter is discussed in the Walecka model and in QCD sum rules. Using analytic formulas for the mass-shift valid at low densities, it is shown that the energy dependent part of the sels-energy (wave function renomalization) in medium is a main source for the negative mass-shift, while the energy independent part (the plasma frequency) has relatively a small effect. Future experiments in GSI ($\pi^- + A \to X + \omega$ ) and KEK-PS ($p + A \to X + \phi$ ) to detect the twin peak structure of vector mesons in $e^+e^-$ spectrum are reviewed. Also, it is proposed a new possible experiment using a $S (strangeness) = 1$ vector meson $K^{ *}(892)$ with its radiative decay $K^{ *+}(892) \to K^+ + \gamma$.
Categories: nucl-th hep-ph