Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum $dN/ (dM_\perp^2 dM^2)$ at fixed transverse mass $M_\perp$ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of $dN/ (dM_\perp^2 dM^2)$ against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum $dN/dM^2$, in the spectrum $dN/dM^2 dM_\perp^2$ with fixed $M_\perp$ the $\omega$ signal is clearly seen as bump riding on the $\rho$ background even in case of strong in-medium modifications.Comment: 11 pages with figs. (style changed to RevTeX

Jet-induced gauge field instabilities in the quark-gluon plasma: A kinetic theory approach

We discuss the properties of the collective modes of a system composed by a thermalized quark-gluon plasma traversed by a relativistic jet of partons. The transport equations obeyed by the components of the plasma and of the jet are studied in the Vlasov approximation. Assuming that the partons in the jet can be described with a tsunami-like distribution function we derive the expressions of the dispersion law of the collective modes. Then the behavior of the unstable gauge modes of the system is analyzed for various values of the velocity of the jet, of the momentum of the collective modes and of the angle between these two quantities. We find that the most unstable modes are those with momentum orthogonal to the velocity of the jet and that these instabilities appear when the velocity of the jet is higher than a threshold value, which depends on the plasma and jet frequencies. The results obtained within the Vlasov approximation are compared with the corresponding results obtained using a chromohydrodynamical approach.The effect we discuss here suggests a possible collective mechanism for the description of the jet quenching phenomena in heavy ion collisions.Comment: 13 pages, 6 figure

Confinement in the Big Bang and Deconfinement in the Little Bangs at CERN-SPS

The evolution of strongly interacting matter during the cosmological confinement transition is reviewed. Despite of many proposed relics no specific signal from the rearrangement of quarks and gluons into hadrons has been identified by observations. In contrast to this, several observables in heavy-ion collisions at CERN-SPS energies point to the creation of a matter state near or slightly above deconfinement. We focus here on the analysis of dileptons and direct photons. Similarities and differences of the Big Bang and the Little Bang confinement dynamics are elaborated.Comment: 8 pages, invited talk at Luederitz 2000, "Fundamental and Applied Aspects of Modern Physics", Nov. 13 - 17, 200

Evaluation of QCD sum rules for light vector mesons at finite density and temperature

QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of mass parameters for the lightest vector mesons $\rho$, $\omega$ and $\phi$ in a strongly interacting medium. For conditions relevant for the starting experiments at HADES we find that the in-medium mass shifts of the $\rho$ and $\omega$ mesons are governed, within the Borel QCD sum rule approach, by the density and temperature dependence of the four-quark condensate. In particular, the variation of the strength of the density dependence of the four-quark condensate reflects directly the decreasing mass of the $\rho$ meson and can lead to a change of the sign of the $\omega$ meson mass shift as a function of the density. In contrast, the in-medium mass of the $\phi$ meson is directly related to the chiral strange quark condensate which seems correspondingly accessible

Probing in-medium vector meson decays by double-differential di-electron spectra in heavy-ion collisions at SIS energies

Within a transport code simulation for heavy-ion collisions at bombarding energies around 1 AGeV, we demonstrate that double-differential di-electron spectra with suitable kinematical cuts are useful to isolate (i) the $\rho$ meson peak even in case of strong broadening, and (ii) the in-medium $\omega$ decay contribution. The expected in-medium modifications of the vector meson spectral densities can thus be probed in this energy range via the di-electron channel

Probing the strange quark condensate by di-electrons from phi meson decays in heavy-ion collisions at SIS energies

QCD sum rules predict that the change of the strange quark condensate $<\bar s s>$ in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from $\phi$ meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the $\phi$ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as effective broadening of the di-electron spectrum in the $\phi$ region. The emerging broadening is sensitive to the in-medium change of $<\bar s s>$. This allows to probe directly in-medium modifications of $$ via di-electron spectra in heavy-ion collisions at SIS energies with HADES

A unique large thermal source of real and virtual photons in the reactions Pb(158 AGeV) + Pb, Au

The data of direct single-photon measurements of the WA98 collaboration in the reaction Pb(158 AGeV) + Pb are analyzed within a thermal model with a minimum number of parameters adjusted to the dilepton data obtained by the CERES and NA50 collaborations in the reactions Pb(158 AGeV) + Au, Pb. The agreement of our model with the WA98 data points to a unique large thermal source emitting electromagnetic radiation observable in both the real and virtual photon channels.Comment: 11 pages including figure