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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.

1163¢-meson mass in nuclear matter (m:) is investigated using an effective Lagrangian of .P interacting with octet baryons. m: decreases in nuclear matter due to the current conservation and effective nucleon/hyperon masses. Its implication to the P-A and A-A collisions are briefly discussed.In recent years, the effective meson masses in hadronic matter at finite density (p) and temperature ( T) acquire wide interests both theoretically and experimentally (see the recent reviews. 1 >) In particular, the ¢-meson, which is an ss resonance in r =1-channel with a narrow width (m,=1019.4 MeV and r,=4.4 MeV), is a unique probe for partial restoration of chiral symmetry in hot/dense hadronic matter. 2 H> Detection of¢> through the dacays ¢>->KK, e+e-, f-L+f.F in nucleus-nucleus (A-A) and proton-nucleus (p-A) collisions could give experimental information on the spectral change of ¢>in matter: preliminary data on ¢-> K+ K-in A-A collisions at AGS-BNL has been recently reported, 5 > and an experiment using P-A reactions is planned at KEK. 6 > In this paper, we will report our recent study on the ¢-meson mass in nuclear matter at zero T. Our starting point is an effective hadronic model composed of ¢-meson, nucleon and hyperons. This is a generalization of the recent works by Shiomi and Hatsuda 7 > and others 8 Ho> who studied the effect of nucleon-loops to the properties of rho and omega mesons in nuclear matter.Let us start with the vector coupling of¢> with octet baryons (B=N, A 0 , :£±,where g,s is the ¢-baryon coupling constant listed in Table I. Some remarks are in order here: (i) ¢-A and ¢-I couplings do not break the OZI rule, since the quark lines at the vertices are connected. On the other hand, the ¢>-N coupling is OZI violating. (ii) E is neglected, since its effect to the ¢> self-energy is doubly suppressed by the mass of E and by the OZI violating nature of ¢>-E coupling. (iii) If one relies on the quark counting rule ,1 1 > the ¢-hyperon couplings are related to the w-hyperon couplings as g,A(,.E)= UwA(w.E)/ j2 with UwA(w.E) being determined by the fit of the hypernuclear levels. 12 > This is assumed in Table I. (iv) ¢-nucleon coupling, which is OZI violating, is not known experimentally. Table I. Y11s, Yws and g,s denote 11-B scalar coupling, w-B vector coupling and ¢-B vector coupling, respectively. 911B and 9ws are taken from Ref. 12). The number with * should be considered an upper bound. Baryons 911B 9wB g,B

The restoration of chiral symmetry has recently become one of the central issues related to the quark-gluon plasma (QGP). In a high temperature, high density environment, such as that created in the Relativistic Heavy Ion Collider (RHIC), the QCD vacuum condensate is dissolved, and therefore it is believed that a phase transition to QGP is realized. The order parameter characterizing the transition is the chiral quark condensate qq . The reduction of qq is regarded as the partial restoration of chiral symmetry in QCD.It is suggested that the mass shift of the light vector mesons (ρ, ω, φ · · ·) is one way to experimentally detect the restoration of chiral symmetry in medium. The mass shift of light vector mesons is believed to be directly experimentally observable through the production of vector mesons in media (nuclei) and detecting lepton pairs, which themselves decay. Among light vector mesons, the φ-meson is a useful probe for finding the signal of its mass shift, because of its small width.The purpose of this paper is to show that a shift of vector meson masses, in particular the φ-meson mass m * φ , can occur even in ordinary nuclear media at zero temperature prior to the critical point of the QGP phase transition, using an effective theory. Adopting effective chiral quark dynamics as effective theories, we investigate the possibility for the vector meson mass to become light.The model is treated for the one-loop level. The results reveal a density dependence of the masses to decrease in medium, as a result of the decrease of the chiral quark condensate. We also analyze the physical reason of this meson mass shift. We can use f K /f π to eliminate one more parameter, m 8 or m 0 . If we use the relation m 8 /m 0 = 1/(2f K /f π − 1), 24) together with the experimental value f K /f π = 113/92.4 = 1.22, we obtain m 8 /m 0 = 0.69, which is 15% larger than the value assumed in this paper.at