arXiv subjects

Ralf Rapp

Publications and source records attributed to Ralf Rapp.

At least 109 records · Page 6Linked to original sources

arXiv2011

Quark Recombination and Heavy Quark Diffusion in Hot Nuclear Matter

We discuss resonance recombination for quarks and show that it is compatible with quark and hadron distributions in local thermal equilibrium. We then calculate realistic heavy quark phase space distributions in heavy ion collisions using Langevin simulations with non-perturbative T-matrix interactions in hydrodynamic backgrounds. We hadronize the heavy quarks on the critical hypersurface given by hydrodynamics after constructing a criterion for the relative recombination and fragmentation contributions. We discuss the influence of recombination and flow on the resulting heavy meson and single electron R_AA and elliptic flow. We will also comment on the effect of diffusion of open heavy flavor mesons in the hadronic phase.

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arXiv2011

Heavy-Quark Diffusion and Hadronization in Quark-Gluon Plasma

We calculate diffusion and hadronization of heavy quarks in high-energy heavy-ion collisions implementing the notion of a strongly coupled quark-gluon plasma in both micro- and macroscopic components. The diffusion process is simulated using relativistic Fokker-Planck dynamics for elastic scattering in a hydrodynamic background. The heavy-quark transport coefficients in the medium are obtained from non-perturbative $T$-matrix interactions which build up resonant correlations close to the transition temperature. The latter also form the basis for hadronization of heavy quarks into heavy-flavor mesons via recombination with light quarks from the medium. The pertinent resonance recombination satisfies energy conservation and provides an equilibrium mapping between quark and meson distributions. The recombination probability is derived from the resonant heavy-quark scattering rate. Consequently, recombination dominates at low transverse momentum ($p_T$) and yields to fragmentation at high $p_T$. Our approach thus emphasizes the role of resonance correlations in the diffusion and hadronization processes. We calculate the nuclear modification factor and elliptic flow of $D$- and $B$-mesons for Au-Au collisions at the Relativistic Heavy Ion Collider, and compare their decay-electron spectra to available data. We also find that a realistic description of the medium flow is essential for a quantitative interpretation of the data.

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arXiv2011

Thermal Relaxation of Charm in Hadronic Matter

The thermal relaxation rate of open-charm ($D$) mesons in hot and dense hadronic matter is calculated using empirical elastic scattering amplitudes. $D$-meson interactions with thermal pions are approximated by $D^*$ resonances, while scattering off other hadrons ($K$, $η$, $ρ$, $ω$, $K^*$, $N$, $Δ$) is evaluated using vacuum scattering amplitudes as available in the literature based on effective Lagrangians and constrained by realistic spectroscopy. The thermal relaxation time of $D$-mesons in a hot $π$ gas is found to be around 25-50\,fm/$c$ for temperatures $T$=150-180\,MeV, which reduces to 10-25\,fm/$c$ in a hadron-resonance gas. The latter values, argued to be conservative estimates, imply significant modifications of $D$-meson spectra in heavy-ion collisions. Close to the critical temperature ($T_c$), the spatial diffusion coefficient ($D_s$) is surprisingly similar to recent calculations for charm quarks in the Quark-Gluon Plasma using non-perturbative $T$-matrix interactions. This suggests a possibly continuous minimum structure of $D_s$ around $T_c$.

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arXiv2011

Medium Modifications and Production of Charmonia at LHC

A previously constructed transport approach to calculate the evolution of quarkonium yields and spectra in heavy-ion collisions is applied to Pb-Pb($\sqrt{s}$=2.76\,ATeV) collisions at the Large Hadron Collider (LHC). In this approach spectral properties of charmonia are constrained by euclidean correlators from thermal lattice QCD and subsequently implemented into a Boltzmann equation accounting for both suppression and regeneration reactions. Based on a fair description of SPS and RHIC data, we provide predictions for the centrality dependence of $J/ψ$ yields at LHC. The main uncertainty is associated with the input charm cross section, in particular its hitherto unknown reduction due to shadowing in nuclear collisions. Incomplete charm-quark thermalization and non-equilibrium in charmonium chemistry entail a marked reduction of the regeneration yield compared to the statistical equilibrium limit.

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arXiv2011

The influence of bulk evolution models on heavy-quark phenomenology

We study the impact of different Quark-Gluon Plasma expansion scenarios in heavy-ion collisions on spectra and elliptic flow of heavy quarks. For identical heavy-quark transport coefficients relativistic Langevin simulations with different expansion scenarios can lead to appreciable variations in the calculated suppression and elliptic flow of the heavy-quark spectra, by up to a factor of two. A cross comparison with two sets of transport coefficients supports these findings, illustrating the importance of realistic expansion models for quantitative evaluations of heavy-quark observables in heavy-ion collisions. It also turns out that differences in freeze-out prescriptions and Langevin realizations play a significant role in these variations. Light-quark observables are essential in reducing the uncertainties associated with the bulk-matter evolution, even though uncertainties due to the freeze-out prescription persist.

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arXiv2011

Theory of Heavy Flavor in the Quark-Gluon Plasma

Heavy-quark interactions in the Quark-Gluon Plasma are analyzed in terms of a selfconsistent Brueckner scheme using a thermodynamic $T$-matrix based on a potential model. The interrelations between quarkonium correlators, spectral functions and zero-modes, and open heavy-flavor transport and susceptibilities are elaborated. Independent constraints from thermal lattice QCD can be used to improve predictions for heavy-quark phenomenology in heavy-ion collisions.

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arXiv2010

Selfconsistent Evaluation of Charm and Charmonium in the Quark-Gluon Plasma

A selfconsistent calculation of heavy-quark (HQ) and quarkonium properties in the Quark-Gluon Plasma (QGP) is conducted to quantify flavor transport and color screening in the medium. The main tool is a thermodynamic $T$-matrix approach to compute HQ and quarkonium spectral functions in both scattering and bound-state regimes. The $T$-matrix, in turn, is employed to calculate HQ selfenergies which are implemented into spectral functions beyond the quasiparticle approximation. Charmonium spectral functions are used to evaluate eulcidean-time correlation functions which are compared to results from thermal lattice QCD. The comparisons are performed in various hadronic channels including zero-mode contributions consistently accounting for finite charm-quark width effects. The zero modes are closely related to the charm-quark number susceptibility which is also compared to existing lattice "data". Both the susceptibility and the heavy-light quark $T$-matrix are applied to calculate the thermal charm-quark relaxation rate, or, equivalently, the charm diffusion constant in the QGP. Implications of our findings in the HQ sector for the viscosity-to-entropy-density ratio of the QGP are briefly discussed.

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arXiv2010

In-Medium Vector Mesons, Dileptons and Chiral Restoration

Medium modifications of the electromagnetic spectral function in hadronic and quark-gluon matter are reviewed. A strong broadening of the $ρ$ meson, which dominates the spectral function in the low-mass regime, is quantitatively consistent with dilepton excess spectra measured in photoproduction off cold nuclei (CLAS/JLab) and in fixed-target ultrarelativistic heavy-ion collisions (NA45,NA60/CERN-SPS). The large excess observed by PHENIX at RHIC remains unexplained to date, but is most likely not due to emission from the Quark-Gluon Plasma. Connections to thermal lattice QCD promise progress in the search for chiral symmetry restoration.

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arXiv2010

Charmonium in Medium: From Correlators to Experiment

We set up a framework in which in-medium charmonium properties are constrained by thermal lattice QCD and subsequently implemented into a thermal rate equation enabling the comparison with experimental data in heavy-ion collisions. Specifically, we evaluate phenomenological consequences for charmonium production originating from two different scenarios in which either the free or the internal energy are identified with the in-medium 2-body potential between charm and anti-charm quarks. These two scenarios represent $J/ψ$ "melting temperatures" of approximately 1.25\,$T_c$ ("weak binding") and 2\,$T_c$ ("strong binding"), respectively. Within current uncertainties in dissociation rates and charm-quark momentum spectra, both scenarios can reproduce the centrality dependence of inclusive $J/ψ$ yields in nuclear collisions at SPS and RHIC reasonably well. However, the "strong-binding" scenario associated the the internal energy as the potential tends to better reproduce current data on transverse momentum spectra at both SPS and RHIC.

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arXiv2010

Scaling of Elliptic Flow, Recombination and Sequential Freeze-Out of Hadrons in Heavy-Ion Collisions

The scaling properties of elliptic flow of hadrons produced in ultrarelativistic heavy-ion collisions are investigated at low transverse momenta, $p_T\lsim 2$\,GeV. Utilizing empirical parameterizations of a thermalized fireball with collective-flow fields, Resonance Recombination Model (RRM) is employed to describe hadronization via quark coalescence at the hadronization transition. We reconfirm that RRM converts equilibrium quark distribution functions into equilibrated hadron spectra including the effects of space-momentum correlations on elliptic flow. This provides the basis for a controlled extraction of quark distributions of the bulk matter at hadronization from spectra of multi-strange hadrons which are believed to decouple close to the critical temperature. The resulting elliptic flow from empirical fits at RHIC exhibits transverse kinetic-energy and valence-quark scaling. Utilizing the well-established concept of sequential freeze-out we find that the scaling at low momenta can be extended to bulk hadrons ($π$, $K$, $p$) at thermal freeze-out, and thus result in an overall description compatible with both equilibrium hydrodynamics and quark recombination.

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arXiv2009

Dileptons, Charm and Charmonium at Finite Temperature and Chemical Potential

We discuss how dileptons, open charm and charmonia may be utilized in heavy-ion collisions to extract information specific to hot and dense matter at finite quark chemical potential, $μ_q$. For each observable we briefly discuss underlying theoretical frameworks and the current status in interpreting available heavy-ion data at SPS and RHIC energies. Low-mass dileptons are particularly sensitive to baryonic medium effects in spectral modifications of the $ρ$ meson, and may serve as an accurate measure of the fireball lifetime. In the open-charm sector, observable signals may be generated by a ``critical'' enhancement of scattering rates via $t$-channel exchange of a soft $σ$ mode. For charmonia, finite-$T$ potential models could be used to extrapolate color-screening effects to finite $μ_q$ to facilitate a quantitative evaluation of dissociation rates in the medium.

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arXiv2009

Heavy Quarks in the Quark-Gluon Plasma

Heavy-flavor particles are believed to provide valuable probes of the medium produced in ultrarelativistic collisions of heavy nuclei. In this article we review recent progress in our understanding of the interactions of charm and bottom quarks in the Quark-Gluon Plasma (QGP). For individual heavy quarks, we focus on elastic interactions for which the large quark mass enables a Brownian motion treatment. This opens a unique access to thermalization mechanisms for heavy quarks at low momentum, and thus to their transport coefficients in the quark-gluon fluid. Different approaches to evaluate heavy-quark diffusion are discussed and compared, including perturbative QCD, effective potential models utilizing input from lattice QCD and string-theoretic estimates in conformal field theories. Applications to heavy-quark observables in heavy-ion collisions are realized via relativistic Langevin simulations, where we illustrate the important role of a realistic medium evolution to quantitatively extract the heavy-quark diffusion constant. In the heavy quarkonium sector, we briefly review the current status in potential-model based interpretations of correlation functions computed in lattice QCD, followed by an evaluation of quarkonium dissociation reactions in the QGP. The discussion of the phenomenology in heavy-ion reactions focuses on thermal model frameworks paralleling the open heavy-flavor sector. We also emphasize connections to the heavy-quark diffusion problem in both potential models and quarkonium regeneration processes.

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arXiv2008

Theory and Phenomenology of Heavy Flavor at RHIC

We review the problem of heavy-quark diffusion in the Quark-Gluon Plasma and its ramifications for heavy-quark spectra in heavy-ion collisions at RHIC. In particular, we attempt to reconcile underlying mechanisms of several seemingly different approaches that have been put forward to explain the large suppression and elliptic flow of non-photonic electron spectra. We also emphasize the importance of a quantitative description of the bulk medium evolution to extract reliable values for the heavy-quark diffusion coefficient.

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arXiv2008

Forward and midrapidity charmonium production at RHIC

$J/ψ$ production at forward and midrapidity at the Relativistic Heavy-Ion Collider (RHIC) is calculated within a previously constructed rate-equation approach accounting for both direct production and regeneration from $c$ and $\bar c$. The results are compared to experimental data. The observed stronger suppression at forward rapidity can be qualitatively explained by a smaller statistical regeneration component together with stronger cold nuclear matter induced suppression compared to midrapidity. The $χ_c$ over $J/ψ$ ratio and $ψ'$ over $J/ψ$ ratio are also calculated.

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arXiv2008

T-matrix approach to heavy quark diffusion in the QGP

We assess transport properties of heavy quarks in the Quark-Gluon Plasma (QGP) using static heavy-quark (HQ) potentials from lattice-QCD calculations in a Brueckner many-body T-matrix approach to evaluate elastic heavy-quark-light-quark scattering amplitudes. In the attractive meson and diquark channels resonance states are formed for temperatures up to ~1.5 T_c, increasing pertinent drag and diffusion coefficients for heavy-quark rescattering in the QGP beyond the expectations from perturbative-QCD calculations. We use these transport coefficients, complemented with perturbative elastic HQ gluon scattering, in a relativistic Langevin simulation to obtain HQ p_t distributions and elliptic flow (v_2) under conditions relevant for the hot and dense medium created in ultrarelativistic heavy-ion collisions. The heavy quarks are hadronized to open-charm and -bottom mesons within a combined quark-coalescence fragmentation scheme. The resulting single-electron spectra from their semileptonic decays are confronted with recent data on "non-photonic electrons" in 100 AGeV Au-Au collisions at the Relativistic Heavy-Ion Collider (RHIC).

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arXiv2008

Transverse Momentum Spectra of J/ψin Heavy-Ion Collisions

We investigate J/ψtransverse-momentum (p_t) distributions and their centrality dependence in heavy-ion collisions at SPS and RHIC within the framework of a two-component model, which includes (i) primordial production coupled with various phases of dissociation, (ii) statistical coalescence of c and \bar{c} quarks at the hadronization transition. The suppression of the direct component (i) is calculated by solving a transport equation for J/ψ, χ_c and ψ' in an expanding fireball using momentum dependent dissociation rates in the Quark-Gluon Plasma (QGP). The coalescence component is inferred from a kinetic rate equation with a momentum dependence following from a blast wave approach. At SPS energies, where the direct component dominates, the interplay of Cronin effect and QGP suppression results in fair agreement with NA50 p_t spectra. At RHIC energies, the p_t spectra in central Au+Au collisions are characterized by a transition from regeneration at low p_t to direct production above. At lower centralities, the latter dominates at all p_t.

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arXiv2008

Charmonium Production at High $p_t$ at RHIC

We calculate transverse-momentum ($p_t$) spectra and elliptic flow of $J/ψ$'s in $Au$-$Au$ and $Cu$-$Cu$ collisions at RHIC. We employ an earlier constructed 2-component approach for direct and regenerated charmonia which is fairly consistent with the centrality dependence of inclusive $J/ψ$ yields at SPS and RHIC. Discrepancies with high-$p_t$ data in $Cu$-$Cu$ collisions are addressed by implementing effects of a finite formation time and an estimate of $B$ meson feeddown, which largely resolve the problem. The sensitivity of the spectra to the Cronin effect and different in-medium dissociation rates is studied. The predicted elliptic flow in semicentral $Au$-$Au$ collisions is rather small, $\sim$2-3%.

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arXiv2008

Dilepton Radiation at the CERN Super Proton Synchrotron

A quantitative evaluation of dilepton sources in heavy-ion reactions is performed taking into account both thermal and non-thermal production mechanisms. The hadronic thermal emission rate is based on an electromagnetic current-correlation function with a low-mass region (LMR, M \lsim 1 GeV) dominated by vector mesons (ρ, ω, ϕ) and an intermediate-mass region (IMR, 1 GeV \le M \le 3 GeV) characterized by (the onset of) a multi-meson continuum. A convolution of the emission rates over a thermal fireball expansion results in good agreement with experiment in the low-mass spectra, confirming the predicted broadening of the ρmeson in hadronic matter in connection with the prevalence of baryon-induced medium effects. The absolute magnitude of the LMR excess is mostly controlled by the fireball lifetime, which in turn leads to a consistent explanation of the dilepton excess in the IMR in terms of thermal radiation. The analysis of experimental transverse-momentum (q_T) spectra reveals discrepancies with thermal emission for q_T \gsim 1 GeV in noncentral In-In collisions, which we address by extending our calculations by: (i) a refined treatment of ρdecays at thermal freezeout, (ii) primordially produced ρ's subject to energy-loss, (iii) Drell-Yan annihilation, and (iv) thermal radiation from t-channel meson exchange processes. We investigate the sensitivity of dilepton spectra to the critical temperature and hadro-chemical freezeout of the fireball. The ρbroadening in the LMR turns out to be robust, while in the IMR Quark-Gluon Plasma radiation is moderate unless the critical temperature is rather low.

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