Abstract:
The effective mass of phi meson at non-zero temperature is re-examined with an effective chiral Lagrangian. We find that the phi mass decreases with temperature but the effect is small compared to the result obtained from calculations using QCD sum rules. The leading contributions come from kaon loop corrections but vector meson contributions are also important as temperature increases. We discuss consequences of these changes to the phenomena of chiral phase transition in hot hadronic matter.

Abstract:
The properties of $\rho$-mesons at finite temperature ($T$) are examined with an effective chiral lagrangian in which vector and axial-vector mesons are included as massive Yang-Mills fields of the chiral symmetry. It is shown that, at $T^2$ order, the effective mass is not changed but only the mixing effect in vector and axial-vector correlator appears.

Abstract:
Temperature effects on the electromagnetic couplings of pions in hot hadronic matter are studied with an effective chiral Lagrangian. We show that the Ward-Takahashi identity is satisfied at non-zero temperature in the soft pion limit. The in-medium electromagnetic form factor of the pion is obtained in the time-like region and shown to be reduced in magnitude, especially near the vector-meson resonance region. Finally, we discuss the consequences of this medium effect on dilepton production from hot hadronic matter.

Abstract:
We calculate characteristic time scales for chemical equilibration of pions in hot hadronic matter using an effective chiral Lagrangian. We find that inelastic processes involving the vector and axial vector mesons reduce the chemical equilibration time by a factor of $\sim 10$ compared to the result previously calculated in chiral perturbation theory. For a temperature of $T\sim 150 MeV$ we obtain a chemical relaxation time of $\tau_{ch} \simeq 10 fm/c$, which is comparable with typical time scales for a hadronic system generated in SPS-energy heavy ion collisions. The effect of baryons is also estimated and found to be negligible for SPS-energies but important for AGS-energies. We predict, that chemical freeze-out should take place at considerable higher temperatures $\Delta T \simeq 20 MeV$ than thermal freeze-out and that the hadronic phase would not sustain a pion chemical potential larger than $100 MeV$.

Abstract:
We study the effect of the chiral phase transition on pion production in hot hadronic matter. The phase of restored chiral symmetry is characterized by the appearance of the scalar $\sigma$-meson as a chiral partner of the pion as well as by the degeneracy of the vector and axial-vector mesons. We find rapid thermal and chemical equilibration of these degrees of freedom in the symmetric phase. Provided that the chiral transition temperature is not considerably high, the presence of a chirally symmetric phase will result in $\sim 1.6$ times more thermal pions in the final state.

Abstract:
Dilepton production from resonance scattering in hot hadronic matter is studied. Including the widths of these resonances, which enhance the phase space for dilepton production, we find that the production rate is significantly increased if a resonance appears in the extended phase space. For the reaction $\pi+\rho\to l^++l^-$, the finite $\rho$ meson width extends the invariant mass below the $\omega$-meson mass, so a peak at the $\omega$ meson mass is seen in the dilepton spectrum. Similarly, a $\rho$-meson peak appears in the reaction $\rho+\rho\to l^++l^-$. On the other hand, the effect of particle widths in the reaction $\pi+a_1\to l^++l^-$ is small since the extended phase space does not include any resonance.

Abstract:
Dilepton production from hot hadronic matter is studied in an effective chiral Lagrangian with pions, $\rho$-mesons, and $a_1$ mesons, We find that the production rates from reactions that involve axial-vector mesons dominate over contributions from all other reactions when the dilepton invariant mass is above 1.5 GeV.

Abstract:
The vector and axial vector current mixing phenomena at low temperature pion gas by Dey, Eletsky and Ioffe, leads to the low temperature correction of the photon-vector meson coupling ($g_\rho$) at order $\epsilon=T^2/6f_\pi^2$ and the $\rho$ meson mass at order $\epsilon^2$. We show how this {\it low temperature theorems} involving the photon and vector mesons are satisfied in the chiral models based on hidden gauge symmetry and the massive Yang-Mills approach with an explicit $a_1$ meson. We discuss possible phenomenological consequences of the low temperature corrections in RHIC experiments.

Abstract:
The pion electromagnetic form factor at finite temperature is studied using an effective chiral lagrangian that includes explicitly vector mesons. We find that in the time-like region around the rho meson resonance it decreases with increasing temperature and leads to a suppression of dilepton production from pion-pion annihilation in a hot hadronic matter. Effects on dilepton production in high energy heavy ion collisions and its relevance to the phase transition in a hot hadronic matter are discussed.

Abstract:
We study finite temperature effects on dilepton production from pion-pion annihilation in hot hadronic matter. The softening of the pion dispersion relation in a medium is found to enhance the production rate of dileptons with invariant masses in the region of $2m^*_\pi(T)