Jörg Ruppert

Waking the colored plasma

Abstract We calculate the wake induced in a hot, dense QCD medium by a fast parton in the framework of linear response theory. We discuss two different scenarios: a weakly coupled quark–gluon plasma (pQGP) described by hard-thermal loop (HTL) perturbation theory and a strongly coupled QGP (sQGP), which had the properties of a quantum liquid.

Mass and width of the {rho}-meson in a nuclear medium from Brown-Rho scaling and QCD sum rules

We explore the range of values of the in-medium width of a {rho}-meson at rest which is compatibale with the QCD sum rule approach in a nuclear medium assuming vector meson dominance and a Brown-Rho scaling law of the {rho}-meson mass with the chiral condensate. The lower and upper bounds for the in-medium width are found to be strongly increasing with the decreasing mass of the {rho}-meson (increasing nuclear density). We also study the bounds for the in-medium width in models not satisfying the Brown-Rho scaling law. It is shown that the in-medium width depends on how rapidly the mass decreases in comparison to the change of the quark condensate. The bounds for the in-medium width increase with density only if the relative change of the quark condensate is stronger than the relative decrease in mass. This is important for experimental tests of the Brown-Rho scaling paradigm and other dropping {rho}-mass scenarios.

The rapidity structure of Mach cones and other large angle correlations in heavy-ion collisions

The pattern of angular correlations of hadrons with a (semi-)hard trigger hadron in heavy-ion collisions has attracted considerable interest. In particular, unexpected large angle structures on the away side (opposite to the trigger) have been found. Several explanations have been brought forward, among them Mach shockwaves and Cherenkov radiation. Most of these scenarios are characterized by radial symmetry around the parton axis, thus angular correlations also determine the rapidity dependence of the correlation. If the observed correlations are remnants of an away side parton after interaction with the medium created in the collision, pQCD allows to calculate the distribution P(y) of the away side partons in rapidity. The measured correlation then arises as a folding of P(y) and the rapidity structure of the correlation taking into account the detector acceptance. This places non-trivial and rather stringent constraints on the underlying scenario. We investigate these dependences and demonstrate that Mach shockwaves survive this folding procedure well whereas Cherenkov radiation scenarios face new challenges.

The Quark mass dependence of T(c) in QCD: Working up from m = 0 or down from m = infinity?

We analyze the dependence of the QCD transition temperature on the quark (or pion) mass. We find that a linear sigma model, which links the transition to chiral symmetry restoration, predicts a much stronger dependence of T{sub c} on m{sub {pi}} than seen in present lattice data for m{sub {pi}} > or approx. 0.4 GeV. On the other hand, working down from m{sub {pi}}={infinity}, an effective Lagrangian for the Polyakov loop requires only small explicit symmetry breaking, b{sub 1}{approx}exp(-m{sub {pi}}), to describe T{sub c}(m{sub {pi}}) in the above mass range. Physically, this is a consequence of the flat potential (large correlation length) for the Polyakov loop in the three-color pure gauge theory at T{sub c}. We quantitatively estimate the end point of the line of first-order deconfining phase transitions: m{sub {pi}}{approx_equal}4.2{radical}({sigma}){approx_equal}1.8 GeV and T{sub c}{approx_equal}240 MeV for three flavors and three colors.

Self-consistent calculations of spectral densities in the O(N) model: improving the Hartree–Fock approximation by including nonzero decay widths

Abstract We study the linear sigma model with O ( N ) symmetry at nonzero temperature in the framework of the Cornwall–Jackiw–Tomboulis formalism. Extending the set of two-particle irreducible diagrams by adding sunset diagrams to the usual Hartree–Fock contributions, we derive a new approximation scheme which extends the standard Hartree–Fock approximation by the inclusion of nonzero decay widths. In this approximation, in-medium modifications of the meson masses as well as decay and collisional broadening effects are self-consistently taken into account. As compared to the standard Hartree–Fock approximation, the temperature for the chiral symmetry restoring phase transition is lowered by ∼ 20 – 25 % . For large temperatures, the spectral densities of the σ-meson and the pion become degenerate, as required for the chirally symmetric phase.

Properties of theϕmeson at high temperatures and densities

The spectral density of the phi meson in a hot bath of nucleons and pions is calculated by relating the vector meson self-energy to the forward scattering amplitude, which is constrained by experimental data. Dispersion techniques are used to verify the relationship between real and the imaginary parts of the in-medium self energy. The position of the spectral peak of the phi meson is found to be shifted from its vacuum position by only a small amount, but its width is considerably increased.

Focus talk on interactions between jets and medium

The energy and momentum lost by a hard parton propagating through hot and dense matter has to be redistributed during the nuclear medium evolution. Apart from heating the medium, there is the possibility that collective modes are excited leading to the emergence of Mach cones or Cherenkov radiation. Recent two-particle correlation measurements by STAR [F. Wang [STAR Collaboration], J. Phys. G 30 , S1299 (2004) [ arXiv:nucl-ex/0404010 ]; C. Gagliardi, these proceedings] and PHENIX [S. S. Adler et al. [PHENIX Collaboration], arXiv:nucl-ex/0507004 ; N. Ajitanand, these proceedings] at RHIC indicate that such phenomena may play an important role in understanding the jet-medium interactions. Possible collective modes are discussed and it is demonstrated that Mach cones as created by colorless or colored sound are a possible explanation of the hardronic two-particle correlation data.

Probing color response?wakes in a color plasma

The wake induced in a hot QCD medium by a high momentum parton (jet precursor) is calculated in the framework of linear response theory. Two different scenarios are discussed: a weakly coupled quark gluon plasma (pQGP) as described by hard-thermal loop (HTL) perturbation theory and a strongly cupled QGP (sQGP) with the properties of a quantum liquid. In the latter case the wake could exhibit a pronounced Mach cone structure. This physical mechanism could be important for the understanding of preliminary data from the PHENIX and STAR experiments at RHIC on the angular distribution of low-pt secondaries stemming from the away-side jet which indicate maxima at ∇ = π ± 1.1.

PROSPECTS OF MEDIUM TOMOGRAPHY USING 2-, 3- AND 4-PARTICLE CORRELATIONS FOR A (SEMI)-HARD TRIGGER

Hard partons propagating through hot and dense matter lose energy, leading to the observed depletion of hard hadron spectra in nucleus nucleus collision as compared to scaled proton proton collisions. This lost energy has to be redistributed in the medium due to the conservation of energy, which is manifest in the p_T dependence of the angular correlation pattern of hadrons associate with a (semi-) hard trigger. While at low p_T a splitting of a broad peak is observed, at high p_T the structure shows vacuum width, albeit with reduced yield. This sugests a transfer of energy from hard partons to a collectively recoiling medium. We present a systematic study of these phenomena using a realistic medium evolution and a Monte-Carlo simulation of the experimental trigger and show what information about the medium can be derived from multiparticle correlations.

Reply to 'Comment on: 'Non-perturbative finite T broadening of the rho meson and dilepton emission in heavy ion-collisions''

Nota bene: the numerical calculation underlying Version 1 of this reply and the original work Phys.Rev. C71:064903,2005 contains a basic numerical error (wrong factor in the self-energy formulas of Phys. Rev.C71:064903,2005). This renders the numerical results presented there and in Version 1 of the reply invalid and enforces a careful reinvestigation of Phys. Rev.C71:064903,2005 and the reply. Calculations for an erratum of Phys. Rev. C71:064903,2005 are in progress.