Melting rho Meson and Thermal Dileptons

We present a dynamical study of $e^+e^-$ and $\mu^+ \mu^-$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $\rho$-meson spectral function that includes the pion modifications in the nuclear medium as well as the polarization of the $\rho$-meson due to resonant $\rho$$-N$ scattering. We find that the experimental data from the CERES and HELIOS-3 Collaborations c...

Thermal dilepton radiation from the hot fireballs created in high-energy heavy-ion collisions provides unique insights into the properties of the produced medium. We first show how the predictions of hadronic many-body theory for a melting $\rho$ meson, coupled with QGP emission utilizing a modern lattice-QCD based equation of state, yield a quantitative description of dilepton spectra in heavy-ion collisions at the SPS and the RHIC beam energy scan program. We utilize these ...

Aspects of the dilepton spectrum in heavy-ion collisions are discussed, with special emphasis on using lattice computations to guide the phenomenology of finite temperature hadronic matter. The background rates for continuum dileptons expected in forthcoming experiments are summarised. Properly augmented by data from ongoing measurements at HERA, these rates will serve as a calibrating background for QGP searches. Recent results on the temperature dependence of the hadronic s...

The medium modification of vector meson properties and its effects on thermal photon and dilepton emission rates from hot/dense hadronic matter are discussed. The role of spectral changes of hadrons in explaining the WA98 photon data and the CERES/NA45 dilepton data with similar initial conditions is demonstrated.

Recent developments in the evaluation of vector-meson spectral functions in hot and dense matter are discussed with emphasis on connections to the chiral phase transition in QCD. Model independent approaches including chiral low-density expansions, lattice QCD, chiral and QCD sum rules are put into context with model predictions for in-medium vector-spectral function utilizing effective Lagrangians. Hadronic many-body calculations predict a strong broadening (and little mass ...

The medium modification of vector meson properties in hot/dense hadronic matter and its role in explaining the CERES/NA45 dilepton data at different centralities are discussed.

In the last years much work has been done to learn about the properties of the $\rho$-meson in nuclear matter. In a calculation of the in-medium spectralfunction of the $\rho$ we find that it is strongly modified by baryonic resonances, especially the $D_{13}(1520)$, through its coupling to resonance-hole states. In order to test the predictions of a model for the in-medium properties of the $\rho$-meson so far mostly heavy-ion collision have been used. We argue that the cohe...

We study the possibility of $\rho-a_0$ mixing via N-N excitations in dense nuclear matter. This mixing is found to induce a peak in the dilepton spectra at an invariant mass equal to that of the $a_0$. We calculate the cross section for dilepton production through mixing and we compare its size with that of $\pi-\pi$ annihilation. In-medium masses and mixing angles are also calculated. Some preliminary results of the mixing effect on the dilepton production rates at finite te...

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy-ion collisions. They allow to study meson properties, like mass and decay width, at various density and temperature regimes. Present days models allow generally a good description of dilepton spectra in ultra-relativistic heavy ion collision. For the energy regime of a few GeV/nucleon, important discrepancies between theory and experiment, known as the DLS puzzle, have been observe...

Theoretical approaches to assess modifications of vector mesons in the medium, as well as their experimental identification via electromagnetic probes, are discussed. Implications for the nature of chiral symmetry restoration in hot/dense matter are outlined and put into context with the axialvector channel.