arXiv subjects

R. Rapp

Publications and source records attributed to R. Rapp.

At least 109 records · Page 6Linked to original sources

arXiv2001

Regeneration of Anti-Protons in Ultrarelativistic Heavy-Ion Collisions

The production and annihilation of antiprotons in the hadronic phase of heavy-ion collisions is evaluated within a thermal equilibrium approach. It is shown that the inverse reaction of $p\bar p$ annihilation (i.e., multi-pion annihilation $N_ππ\to p\bar p$ with $N_π\simeq$~5-7) in connection with oversaturation of pion phase space (i.e., finite pion chemical potentials) plays an important role in understanding the observed antiproton yields at SpS energies within the standard picture of subsequent chemical and thermal freezeout. Implications for RHIC energies are also addressed.

hep-ph↗

arXiv2001

Resolving the Antibaryon-Production Puzzle in High-Energy Heavy-Ion Collisions

We argue that the observed antiproton production in heavy-ion collisions at CERN-SpS energies can be understood if (contrary to most sequential scattering approaches) the backward direction in the process $p\bar p \leftrightarrow \bar{n}π$ (with $\bar{n}$=5-6) is consistently accounted for within a thermal framework. Employing the standard picture of subsequent chemical and thermal freezeout, which induces an over-saturation of pion number with associated chemical potentials of $μ_π\simeq$~60-80 MeV, enhances the backward reaction substantially. The resulting rates and corresponding cross sections turn out to be large enough to maintain the abundance of antiprotons at chemical freezeout until the decoupling temperature, in accord with the measured $\bar{p}/p$ ratio in Pb(158AGeV)+Pb collisions.

hep-ph↗

arXiv2000

Signatures of Thermal Dilepton Radiation at RHIC

The properties of thermal dilepton production from heavy-ion collisions in the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3 GeV. Using an expanding thermal fireball to model the evolution through both quark-gluon and hadronic phases various features of the spectra are addressed. In the low-mass region, due to an expected large background, the focus is on possible medium modifications of the narrow resonance structures from $ω$ and $ϕ$ mesons, whereas in the intermediate-mass region the old idea of identifying QGP radiation is reiterated including effects of chemical under-saturation in the early stages of central Au+Au collisions.

hep-ph↗

arXiv2000

A chiral crystal in cold QCD matter at intermediate densities?

The analogue of Overhauser (particle-hole) pairing in electronic systems (spin-density waves with non-zero total momentum $Q$) is analyzed in finite-density QCD for 3 colors and 2 flavors, and compared to the color-superconducting BCS ground state (particle-particle pairing, $Q$=0). The calculations are based on effective nonperturbative four-fermion interactions acting in both the scalar diquark as well as the scalar-isoscalar quark-hole ('$σ$') channel. Within the Nambu-Gorkov formalism we set up the coupled channel problem including multiple chiral density wave formation, and evaluate the resulting gaps and free energies. Employing medium-modified instanton-induced 't Hooft interactions, as applicable around $μ_q\simeq 0.4$ GeV (or 4 times nuclear saturation density), we find the 'chiral crystal phase' to be competitive with the color superconductor.

hep-ph↗

arXiv2000

A Schematic Model for $ρ$-$a_1$ Mixing at Finite Density and In-Medium Effective Lagrangian

Based on schematic two-level models extended to $a_1$-meson degrees of freedom, we investigate possible mechanisms of chiral restoration in the vector/axialvector channels in cold nuclear matter. In the first part of this article we employ the massive Yang-Mills framework to construct an effective chiral Lagrangian based on low-energy mesonic modes at finite density. The latter are identified through nuclear collective excitations of `meson'-sobar type such as $π\leftrightarrow [Δ(1232)N^{-1}]\equiv\hatπ$, $ρ\leftrightarrow [N^* (1520)N^{-1}]\equiv\hatρ$, etc.. In a mean-field type treatment the in-medium gauge coupling $\hat g$, the (axial-) vector meson masses and $\hat f_π$ are found to decrease with density indicating the approach towards chiral restoration in the language of in-medium effective fields. In the second part of our analysis we evaluate the (first) in-medium Weinberg sum rule which relates vector and axialvector correlators to the pion decay constant. Using in-medium $ρ$/$a_1$ spectral functions (computed in the two-level model) also leads to a substantial reduction of the pion decay constant with increasing density.

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arXiv2000

Vector Mesons in Medium and Dileptons in Heavy-Ion Collisions

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.

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arXiv1999

Modifications of the Rho Meson from the Virtual Pion Cloud in Hot and Dense Matter

The modification of the rho-meson self-energy due to the coupling to in-medium pions is calculated consistently at finite baryon density and temperature, keeping the full 3-momentum dependence in a gauge invariant way. As a function of nucleon density, the rho-meson spectral function is strongly enhanced in the invariant mass region M < 650 MeV, while the maximum, i.e. the pole mass, is slightly shifted upwards. As a function of temperature, for fixed nucleon density, the imaginary part of the self-energy increases further due to Bose-enhancement. At the same time the mass shift from the real part becomes very large. As a consequence of these medium effects, the dilepton rate in the low-mass region M < 650 MeV increases strongly, while the peak at M = 770 MeV disappears.

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arXiv1999

Last Call for RHIC Predictions

This paper contains the individual contributions of all speakers of the session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary by the convenor.

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arXiv1999

Duality and Chiral Restoration from Dilepton Production in Relativistic Heavy-Ion Collisions

We discuss the recent status in the theoretical understanding of dilepton production in central heavy-ion reactions with the Pb-beam at the full CERN-SpS energy of 158 AGeV. In the low-mass region ($M\le$~1 GeV) a strong broadening of the vector meson resonances in hot and dense matter (especially for the $ρ$ meson) entails thermal dilepton rates very reminiscent to perturbative $q\bar q$ annihilation close to the expected phase boundary of the chiral symmetry restoring transition. A consistent description of the experimentally observed enhancement at both low and intermediate masses (1.5 GeV~$\le M \le$~3 GeV) in terms of thermal radiation from an expanding fireball can be obtained.

hep-ph↗

arXiv1999

Thermal Dilepton Radiation at Intermediate Masses at the CERN-SpS

We investigate the significance of thermal dilepton radiation in the intermediate-mass region in heavy-ion reactions at CERN-SpS energies. Within a thermal fireball model for the space-time evolution, the radiation from hot matter is found to dominate over hard 'background' processes (Drell-Yan and open charm) up to invariant masses of about 2 GeV, with a rather moderate fraction emerging from early stages with temperatures $T\simeq 175-200$ MeV associated with deconfined matter. Further including a schematic acceptance for the NA50 experiment we find good agreement with the observed enhancement in the region 1.5 GeV~$<M_{μμ}<$~3 GeV. In particular, there is no need to invoke any anomalous open charm enhancement.

hep-ph↗

arXiv1999

Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

The current theoretical status in the analysis and interpretation of low-mass dilepton measurements in (ultra-) relativistic heavy-ion experiments is reviewed. Special emphasis is put on potential signals of (partial) restoration of dynamically broken chiral symmetry in a hot and dense hadronic medium. It follows from chiral symmetry alone that parity partners of hadronic correlation functions must become identical when the symmetry is restored. The assessment of medium effects in the vector channel, which governs the dilepton production, thus necessitates a simultaneous treatment of the vector and axialvector degrees of freedom. While significant progress in this respect has been made some open questions remain in establishing a rigorous link in the mass region below 1 GeV. From the present calculations a suggestive 'quark-hadron duality' emerges near the phase boundary. It implies substantial medium effects in the dilepton signal from the hadronic phase which smoothly matches a perturbative description within the plasma phase.

hep-ph↗

arXiv1999

High Density QCD and Instantons

Instantons generate strong non-perturbative interactions between quarks. In the vacuum, these interactions lead to chiral symmetry breaking and generate constituent quark masses on the order of 300-400 MeV. The observation that the same forces also provide attraction in the scalar diquark channel leads to the prediction that cold quark matter is a color superconductor, with gaps as large as $\sim$~100 MeV. We provide a systematic treatment of color superconductivity in the instanton model. We show that the structure of the superconductor depends on the number of flavors. In the case of two flavors, we verify the standard scenario and provide an improved calculation of the mass gap. For three flavors, we show that the ground state is color-flavor locked and calculate the chiral condensate in the high density phase. We show that as a function of the strange quark mass, there is a sharp transition between the two phases. Finally, we go beyond the mean field approximation and investigate the role of instanton-anti-instanton molecules, which -- besides superconducting gap formation -- provide a competitive mechanism for chiral restoration at finite density.

hep-ph↗

arXiv1999

Low-Mass Dileptons at the CERN-SpS: Evidence for Chiral Restoration?

Using a rather complete description of the in-medium $ρ$ spectral function - being constrained by various independent experimental information - we calculate pertinent dilepton production rates from hot and dense hadronic matter. The strong broadening of the $ρ$ resonance entails a reminiscence to perturbative $q\bar q$ annihilation rates in the vicinity of the phase boundary. The application to dilepton observables in Pb(158AGeV)+Au collisions - incorporating recent information on the hadro-chemical composition at CERN-SpS energies - essentially supports the broadening scenario. Possible implications for the nature of chiral symmetry restoration are outlined.

hep-ph↗

arXiv1999

Duality and Chiral Restoration from Low-Mass Dileptons at the CERN-SpS

We review recent theoretical progress in low-mass dilepton production at CERN-SpS energies. Various hadronic approaches to calculate the vector correlator in hot/dense hadronic matter are discussed and confronted with each other. Possible consequences for the nature of chiral restoration are indicated.

hep-ph↗

arXiv1999

Rho properties in a hot meson gas

Using effective meson Lagrangians we study the interaction of rho mesons in a hot baryon-free system. Various mesonic resonances in direct $s$-channel reactions are investigated employing standard self-energy techniques, including new reactions that have up to now not been considered in a self-consistent approach at finite temperature. The importance of subthreshold resonances, which are readily accounted for through off-shell effects within our framework, is emphasized. Special care is taken in reproducing radiative decay widths, as they provide valuable constraints on the evaluation of dilepton spectra. In particular, we compare our results for dilepton production rates to earlier calculations based on an incoherent summation of individual processes.

hep-ph↗

arXiv1999

A Schematic Model For Density-Dependent Vector Meson Masses

A schematic two-level model consisting of a "collective" bosonic state and an "elementary" meson is constructed that provides interpolation from a hadronic description (a la Rapp/Wambach) to B/R scaling for the description of properties of vector mesons in dense medium. The development is based on a close analogy to the degenerate schematic model of Brown for giant resonances in nuclei.

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arXiv1998

Enhancement of $πA \to ππA$ Threshold Cross Sections by In-Medium $ππ$ Final State Interactions

We address the problem of pion production in low energy $π$-nucleus collisions. For the production mechanism we assume a simple model consisting of a coherent sum of single pion exchange and the excitation---followed by the decay into two pions and a nucleon---of the $N^*(1440)$ resonance. The production amplitude is modified by the final state interaction between the pions calculated using the chirally improved Jülich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed $πA \to ππA$ cross sections, especially the enhancement with increasing $A$ of the $π^+π^-$ mass distribution in the threshold region.

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arXiv1998

Momentum Dependence of the Pion Cloud for Rho Mesons in Nuclear Matter

We extend hadronic models for rho-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.

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