Where does the rho go? Chirally symmetric vector mesons in the quark-gluon plasma

First measurement of the rho spectral function in high-energy nuclear collisions

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

Temperature dependence of rho and a1-meson masses and mixing of vector and axial-vector correlators

Within a chiral model which provides a good description of the properties of rho and a1 mesons in vacuum, it is shown that, to order T2, the rho- and a1-meson masses remain constant in the chiral limit, even if at tree level they are proportional to the chiral condensate, sigma0. Numerically, the temperature dependence of the masses turns out to be small also for realistic parameter sets and high temperatures. The weak temperature dependence of the masses is consistent with the Eletsky-Ioffe mixing theorem, and traces of mixing effects can be seen in the spectral function of the vector correlator at finite temperature.

Fate of vector dominance in effective field theory

We reveal the full phase structure of the effective field theory for QCD, based on hidden local symmetry (HLS) through the one-loop renormalization group equation including quadratic divergences. We then show that vector dominance (VD) is not a sacred discipline of the effective field theory but rather an accidental phenomenon peculiar to three-flavored QCD. In particular, the chiral symmetry restoration in the HLS model takes place in a wide phase boundary surface, on which the VD is nowhere realized. This suggests that VD may not be valid for chiral symmetry restoration in hot and/or dense QCD.

Effective restoration of the U(A)(1) symmetry in the SU(3) linear sigma model

The effective restoration of the chiral U(A)(1) symmetry in strong interactions is studied using the linear chiral SU(3)xSU(3) model at finite temperatures. We find that the disappearance of the chiral anomaly causes a considerable change in the meson mass spectrum. We propose several signals for detecting this chiral phase in ultrarelativistic heavy-ion collisions: The eta/pi(0) ratio is enhanced by an order of magnitude, the a(0) is suppressed in the K&Kmacr; mass spectrum, and the scalar kappa meson appears as a peak just below the K(*)(892) in the invariant piK mass spectrum.