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R. Rapp

Publications and source records attributed to R. Rapp.

At least 91 records · Page 5Linked to original sources
arXiv2004

Quark Coalescence for Charmed Mesons in Ultrarelativistic Heavy-Ion Collisions

We investigate effects of charm-quark interactions in a Quark-Gluon Plasma on the production of $D$ and $J/ψ$ mesons in high-energy heavy-ion collisions. Employing a previously constructed coalescence model that successfully reproduces the transverse momentum ($p_T$) spectra and elliptic flow ($v_2(p_T)$) of light hadrons at RHIC from underlying light-quark distributions at the phase transition temperature $T_c$, $D$-meson and $J/ψ$ $p_T$ spectra are evaluated. For the charm-quark distributions, we consider two limiting scenarios: (i) {\em no} rescattering, corresponding to perturbative QCD spectra and (ii) {\em complete} thermalization including transverse expansion. With the $D$-meson spectra acquiring a minimal $v_2$ inherited from their light-quark content, the corresponding semileptonic decay spectra of single electrons are found to practically preserve the $v_2$ of the parent particles, exhibiting marked differences between the pQCD and thermal scenarios for $p_T\ge 1$ GeV. Likewise, the $p_T$-spectra and yields of $J/ψ$'s differ appreciably in the two scenarios.

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arXiv2004

Photon Physics in Heavy Ion Collisions at the LHC

Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed.

hep-ph↗
arXiv2004

Medium Modifications of Charm and Charmonium in High-Energy Heavy-Ion Collisions

The production of charmonia in heavy-ion collisions is investigated within a kinetic theory framework simultaneously accounting for dissociation and regeneration processes in both quark-gluon plasma (QGP) and hadron-gas phases of the reaction. In-medium modifications of open-charm states (c-quarks, D-mesons) and the survival of J/psi mesons in the QGP are included as inferred from lattice QCD. Pertinent consequences on equilibrium charmonium abundances are evaluated and found to be especially relevant to explain the measured centrality dependence of the psi'/psi ratio at SPS. Predictions for recent In-In experiments, as well as comparisons to current Au-Au data from RHIC, are provided.

hep-ph↗
arXiv2004

Photon Emission from Dense Quark Matter

Thermal emission rates and mean free paths of photons in a color-flavor locked (CFL) phase of quark matter at high densities and moderate temperatures are evaluated. Our calculations are based on a low-energy effective theory for CFL matter describing Goldstone boson excitations and their electromagnetic as well as strong interactions. In-medium coupling strengths of vector mesons are found to be smaller than in vacuum. As a consequence of in-medium modified pion dispersion relations, novel processes such as pi+ pi- -> gamma and gamma -> pi+ pi- become possible. The total photon emissivity is found to be very large, exceeding contributions from thermal e+ e- annihilation above temperatures of about 5 MeV. At the same time, the corresponding mean free paths become very small. Our results imply that the photon flux from the surface of a (hypothetical) CFL star in its early hot stages saturates the black-body limit. Estimates for the early thermal evolution of the star are also presented.

hep-ph↗
arXiv2003

In-Medium Effects on Charmonium Production in Heavy-Ion Collisions

Charmonium production in heavy-ion collisions is investigated within a kinetic theory framework incorporating in-medium properties of open- and hidden-charm states in line with recent QCD lattice calculations. A continuously decreasing open-charm threshold across the phase boundary of hadronic and quark-gluon matter is found to have important implications for the equilibrium abundance of charmonium states. The survival of $J/ψ$ resonance states above the transition temperature enables their recreation also in the Quark-Gluon Plasma. Including effects of chemical and thermal off-equilibrium, we compare our model results to available experimental data at CERN-SPS and BNL-RHIC energies. In particular, earlier found discrepancies in the $ψ'/ψ$ ratio can be resolved.

hep-ph↗
arXiv2003

Theoretical Overview on (Hidden) Charm in High-Energy Heavy-Ion Collisions

Recent developments in the theoretical evaluation of charmonium production in ultrarelativistic heavy-ion collisions (URHIC's) are discussed. In particular, the consequences of equilibrium properties of open and hidden charm states -- accessible, {\it e.g.}, in QCD lattice gauge calculations -- are assessed. These include abundances as well as formation and dissociation rates of charmonia in both hadronic and quark-gluon matter.

hep-ph↗
arXiv2003

Pi+ Pi- Emission in High-Energy Nuclear Collisions

Realistic vacuum $ππ$ interactions are employed to investigate thermal $π^+π^-$ emission spectra from the late stages of heavy-ion reactions at ultrarelativistic energies. Hadronic in-medium effects, including many-body $ρ$-meson spectral functions used earlier to describe the dilepton excess observed at CERN-SPS energies, are implemented to assess resulting modifications in relation to recent measurements of $π^+π^-$ invariant-mass spectra by the STAR collaboration in $p$-$p$ and $Au$-$Au$ collisions at $\sqrt{s_{NN}}$=200 GeV. Statistical model estimates for the $ρ^0/π^-$ and $K^*/K$ ratios close to the expected thermal freezeout are also given.

hep-ph↗
arXiv2003

D-Meson Production from Recombination in Hadronic Collisions

Nonperturbative effects in $D$-meson production in pion-nucleon and proton-nucleon collisions are investigated within the recombination model. The coalescence of perturbatively created charm quarks with sea- and valence-quarks from projectile and target fragments is shown to be competitive in magnitude with standard fragmentation calculations at both central (small $x_F$) and forward rapidities. Corresponding flavor asymmetries for inclusive $D$-meson production are thus mostly generated on the (light-) parton distribution level, and turn out to be in reasonable overall agreement with available fixed-target data. Predictions for upcoming measurements at RHIC are given.

hep-ph↗
arXiv2002

Hadrons in Hot and Dense Matter

The description of excitations in hot and dense (hadronic) matter is discussed with emphasis on the use of correlation functions as a common framework for comparing different model (and QCD lattice) calculations with each other. Typical regimes of applicability of hadronic approaches are assessed, together with possibilities to confront them with experiment. We also elaborate on recent developments to relate baryonic in-medium effects to chiral symmetry restoration.

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arXiv2002

Two-Component Approach to $J/Ψ$ Production in High-Energy Heavy-Ion Collisions

The production of charmonia in ultrarelativistic heavy-ion collisions is investigated including two sources. These are a primordial contribution coupled with various phases of dissociation, and a statistical coalescence of $c$ and $\bar{c}$ quarks at the hadronization phase transition. Within a schematic fireball evolution, SPS data on $J/Ψ$ production can be reasonably well reproduced. Remaining discrepancies in the $Ψ'/Ψ$ ratio are discussed. Predictions for the $J/Ψ$ centrality dependence at RHIC energies are confronted with first data from PHENIX. The pertinent excitation function of the $N_{J/Ψ}/N_{c\bar{c}}$ ratio exhibits a characteristic minimum structure signaling the transition from the standard $J/Ψ$ suppression scenario (SPS) to predominantly statistical production (RHIC).

hep-ph↗
arXiv2002

Charmonium Suppression and Regeneration from SPS to RHIC

The production of charmonia is investigated for heavy-ion collisions from SPS to RHIC energies. Our approach incorporates two sources of $J/Ψ$ yield: (i) a direct contribution arising from early (hard) parton-parton collisions, subject to subsequent nuclear absorption, quark-gluon plasma and hadronic dissociation, and (ii) statistical production at the hadronization transition by coalescence of $c$ and $\bar{c}$ quarks. Within an expanding thermal fireball framework, the model reproduces $J/Ψ$ centrality dependencies observed at the SPS in Pb-Pb and S-U collisions reasonably well. The study of the $Ψ'/Ψ$ ratio at SPS points at the importance of the hadronic phase for $Ψ'$ interactions, possibly related to effects of chiral symmetry restoration. Predictions are given for the centrality dependence of the $N_{J/Ψ}/N_{c\bar{c}}$ ratio at full RHIC energy. We also calculate the excitation function of this ratio. The latter exhibits a characteristic minimum structure signalling the transition from the standard $J/Ψ$ suppression scenario prevailing at SPS to dominantly thermal regeneration at collider energies.

hep-ph↗
arXiv2002

Hadro-Chemistry and Evolution of (Anti-) Baryon Densities at RHIC

The consequences of hadro-chemical freezeout for the subsequent hadron gas evolution in central heavy-ion collisions at RHIC and LHC energies are discussed with special emphasis on effects due to antibaryons. Contrary to naive expectations, their individual conservation, as implied by experimental data, has significant impact on the chemical off-equilibrium composition of hadronic matter at collider energies. This may reflect on a variety of observables including source sizes and dilepton spectra.

hep-ph↗
arXiv2002

Thermal Lepton Production in Heavy-Ion Collisions

The current status of evaluating thermal production of lepton pairs in high-energy collisions of heavy nuclei is discussed. After a brief survey of emission rates from hot and dense matter, we address applications to (and interpretations of) recent SPS data including 40 AGeV CERES results, as well as prospects for NA60. Emphasis is put on predictions for RHIC. In particular, implications of hadronic observables and first single-electron measurements at $\sqrt{s}=130$ AGeV for upcoming pair spectra from PHENIX at $\sqrt{s}=200$ AGeV are assessed.

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arXiv2002

Anti-Baryon Puzzle in Ultra-Relativistic Heavy-Ion Collisions

The evolution of (non-strange) antibaryon abundances in the hadronic phase of central heavy-ion collisions is studied within a thermal equilibrium framework, based on the well-established picture of subsequent chemical and thermal freezeout. Due to large annihilation cross sections, antiprotons are, a priori, not expected to comply with this scheme. However, we show that a significant regeneration of their abundance occurs upon inclusion of the inverse reaction of multipion fusion, $n_ππ\to p\bar p$ (with $n_π$=5-6), necessary to ensure detailed balance. Especially at SPS energies, the build-up of large pion-chemical potentials between chemical and thermal freezeout reinforces this mechanism, rendering the $\bar p/p$ ratio in reasonable agreement with the observed one (reflecting chemical freezeout). Explicit solutions of the pertinent rate equation, which account for chemical off-equilibrium effects, corroborate this explanation.

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arXiv2002

Dileptons in High-Energy Heavy-Ion Collisions

The current status of our understanding of dilepton production in ultrarelativistic heavy-ion collisions is discussed with special emphasis on signals from the (approach towards) chirally restored and deconfined phases. In particular, recent results of the CERN-SPS low-energy runs are compared to model predictions and interpreted. Prospects for RHIC experiments are given.

hep-ph↗
arXiv2001

Thermal versus Direct $J/Ψ$ Production in Ultrarelativistic Heavy-Ion Collisions

The production of $J/Ψ$ mesons in central collisions of heavy nuclei is investigated as a function of collision energy. Two contributions are considered simultaneously: early (hard) production coupled with subsequent suppression in a Quark-Gluon Plasma, as well as thermal recombination of primordially produced $c$ and $\bar c$ quarks at the hadronization transition. Whereas the former still constitutes the major fraction of the observed $J/Ψ$ abundance at SpS energies, the latter dominates the yield at RHIC. The resulting excitation function for the number of $J/Ψ$'s over open charm pairs exhibits nontrivial structure around $\sqrt{s} \simeq 30$ AGeV, evolving into a significant rise towards maximal RHIC energy. We study this feature within different (thermal) scenarios for $J/Ψ$ suppression, including parton-induced quasifree destruction as a novel mechanism.

hep-ph↗
arXiv2001

Isospin Fluctuations in QCD and Relativistic Heavy-Ion Collisions

We address the role of fluctuations in strongly interacting matter during the dense stages of a heavy-ion collision through its electromagnetic emission. Fluctuations of isospin charge are considered in a thermal system at rest as well as in a moving hadronic fluid at fixed proper time within a finite bin of pseudo-rapidity. In the former case, we use general thermodynamic relations to establish a connection between fluctuations and the space-like screening limit of the retarded photon self-energy, which directly relates to the emissivities of dileptons and photons. Effects of hadronic interactions are highlighted through two illustrative calculations. In the latter case, we show that a finite time scale $τ$ inherent in the evolution of a heavy-ion collision implies that equilibrium fluctuations involve both space-like and time-like components of the photon self-energy in the system. Our study of non-thermal effects, explored here through a stochastic treatment, shows that an early and large fluctuation in isospin survives only if it is accompanied by a large temperature fluctuation at freeze-out, an unlikely scenario in hadronic phases with large heat capacity. We point out prospects for the future which include: (1) A determination of the Debye mass of the system at the dilute freeze-out stage of a heavy-ion collision, and (2) A delineation of the role of charge fluctuations during the dense stages of the collision through a study of electromagnetic emissivities.

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