The spectral function of a rho meson that is at rest in dense hadronic matter and couples strongly to the pion is studied in the vector dominance model by including the effect of the delta hole polarization on the pion. With the free rho-meson mass in the Lagrangian, we find that both the rho-meson peak and width increase with increasing nuclear density, and that a low-mass peak appears at invariant mass around three times the pion mass. Including the decreasing densitydependent hadron masses in the Lagrangian as suggested by the scaling law of Brown and Rho, we find instead that the rho peak moves to smaller invariant masses with diminishing strength when the nuclear density increases. The low-mass peak also shifts down with increasing density and becomes more pronounced. The relevance of the rho-meson property in dense matter to dilepton production in heavy-ion collisions is discussed.PACS number(s): 25.75+r, 14.40.Cs, 12.40.Vv In heavy-ion collisions the nuclear matter can be compressed to many times the normal nuclear matter density [1]. The study of the property of hadrons in dense matter has recently attracted a lot of attention [2]. We have previously studied dilepton production from the pion-pion annihilation in heavy-ion collisions to see whether one can learn about the pion dispersion relation in a dense medium [3 -5]. In particular, we have considered dileptons of invariant mass around twice the pion mass. Gale and Kapusta [6] were the first to show that the dilepton yield around this invariant mass would be enhanced if the softening of the pion spectrum in the nuclear matter is included. This is due to the large momentum range available for the two pions when the pion dispersion relation develops at high densities a minimum at energy less than the pion mass. We have obtained a similar result when the pion self-energy in the nuclear matter is calculated in the delta-hole model [3,4]. But for dileptons with zero three-momentum the enhancement is canceled by the delta-hole polarization correction to the zero vertex as shown by Korpa and Pratt [7], and one obtains instead a suppression of the dilepton yield for invariant mass around twice the pion mass. This cancellation is, however, reduced for dileptons with finite momenta, so there is still some enhancement of the dilepton yield around this invariant mass [5]. The experimental data [8,9] from the Bevalac on the dilepton invariant mass spectra from heavy-ion collisions have already shown clearly the contribution from the pion-pion annihilation [10, 11). But for dileptons with invariant masses below about 500 MeV, it is dominated by the eta Dalitz 'On leave from KFKI, Budapest, Hungary. decay and also has an appreciable bremsstrahlung contribution [12]. This makes the enhancement of dileptons with invariant masses around 280 MeV difBcult if not impossible to be observed.Since the electromagnetic form factor of a pion is dominated by the rho meson, the dilepton invariant mass spectrum should also reveal information about the property of the rho meson i...
The effect of the kaon loop correction to the property of a phi meson in dense matter is studied in the vector dominance model. Using the density-dependent kaon effective mass determined from the linear chiral perturbation theory, we find that with increasing baryon density the phi meson mass is reduced slightly while its width is broadened drastically.
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