arXiv subjects
Publications and source records attributed to R. Rapp.
We present a first calculation of the dilepton yield and elliptic flow done with 3+1D viscous hydrodynamical simulations of relativistic heavy ion collisions at the top RHIC energy. A comparison with recent experimental data from the STAR collaboration is made.
We study the production of bottomonium states in heavy-ion reactions at collider energies available at RHIC and LHC. We employ an earlier constructed rate equation approach which accounts for both suppression and regeneration mechanisms in the quark-gluon plasma (QGP) and hadronization phases of the evolving thermal medium. Our previous predictions utilizing two limiting cases of strong and weak bottomonium binding in the QGP are updated by (i) checking the compatibility of the pertinent spectral functions with lattice-QCD results for euclidean correlators, (ii) adapting the initial conditions of the rate equation by updating bottom-related input cross sections and the charged-particle multiplicity of the fireball, and (iii) converting our calculations into observables as recently measured by the STAR and CMS experiments. Our main findings are a preference for strong $Υ$ binding as well as a significant regeneration component at the LHC.
We assess transport properties of heavy quarks in the Quark-Gluon Plasma (QGP) that show a strong non-perturbative behavior. A T-matrix approach based on a potential taken from lattice QCD hints at the presence of heavy-quark (HQ) resonant scattering with an increasing strength as the temperature, $T$, reaches the critical temperature, $T_c \simeq 170 \; \MeV$ for deconfinement from above. The implementation of HQ resonance scattering along with a hadronization via quark coalescence under the conditions of the plasma created in heavy-ion collisions has been shown to correctly describe both the nuclear modification factor, $R_{AA}$, and the elliptic flow, $v_2$, of single electrons at RHIC and have correctly predicted the $R_{AA}$ of D mesons at LHC energy.
We update our calculations of thermal-photon production in nuclear collisions at the Relativistic Heavy-Ion Collider (RHIC). Specifically, we address the recent experimental observation of an elliptic flow of direct photons comparable in magnitude to that of pions, which is at variance with expectations based on quark-gluon plasma (QGP) dominated photon radiation. Our thermal emission rate is based on previous work, i.e., resummed leading-order QGP emission and in-medium hadronic rates in the confined phase. These rates are nearly degenerate at temperatures close to the expected QCD-phase change. The rates are convoluted over an improved elliptic-fireball expansion with transverse- and elliptic-flow fields quantitatively constrained by empirical light- and strange-hadron spectra. The resulting direct-photon spectra in central Au-Au collisions are characterized by hadron-dominated emission up to transverse momenta of ~2-3 GeV. The associated large elliptic flow in the hadronic phase mitigates the discrepancy with the measured photon-v2 compared to scenarios with QGP-dominated emission.
We review the present status in the theoretical and phenomenological understanding of charmonium and bottomonium production in heavy-ion collisions. We start by recapitulating the basic notion of "anomalous quarkonium suppression" in heavy-ion collisions and its recent amendments involving regeneration reactions. We then survey in some detail concepts and ingredients needed for a comprehensive approach to utilize heavy quarkonia as a probe of hot and dense matter. The theoretical discussion encompasses recent lattice QCD computations of quarkonium properties in the Quark-Gluon Plasma, their interpretations using effective potential models, inelastic rate calculations and insights from analyses of electromagnetic plasmas. We illustrate the powerful techniques of thermodynamic Green functions (T-matrices) to provide a general framework for implementing microscopic properties of heavy quarkonia into a kinetic theory of suppression and regeneration reactions. The theoretical concepts are tested in applications to heavy-ion reactions at SPS, RHIC and LHC. We outline perspectives for future experiments on charmonium and bottomonium production in heavy-ion collisions over a large range in energy (FAIR, RHIC-II and LHC). These are expected to provide key insights into hadronic matter under extreme conditions using quarkonium observables.
We extend our recent study of dilepton invariant-mass spectra from the decays of rho mesons produced by photon reactions off nuclei. We specifically focus on experimental spectra as recently measured by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility using carbon and iron nuclei. Building on our earlier work we broaden our description to a larger set of observables in order to identify sensitivities to the medium effects predicted by microscopic calculations of the rho spectral function. We compute mass spectra for several target nuclei and study the spectral shape as a function of the 3-momentum of the outgoing lepton pair. We also compute the so-called nuclear transparency ratio which provides an alternative means (and thus consistency check) of estimating the inelastic rho width in the cold nuclear medium.
A thermodynamic T-matrix approach for elastic 2-body interactions is employed to calculate spectral functions of open and hidden heavy-quark systems in the Quark-Gluon Plasma. This enables the evaluation of quarkonium bound-state properties and heavy-quark diffusion on a common basis and thus to obtain mutual constraints. The two-body interaction kernel is approximated within a potential picture for spacelike momentum transfers. An effective field-theoretical model combining color-Coulomb and confining terms is implemented with relativistic corrections and for different color channels. Four pertinent model parameters, characterizing the coupling strengths and screening, are adjusted to reproduce the color-average heavy-quark free energy as computed in thermal lattice QCD. The approach is tested against vacuum spectroscopy in the open (D, B) and hidden (Psi and Upsilon) flavor sectors, as well as in the high-energy limit of elastic perturbative QCD scattering. Theoretical uncertainties in the static reduction scheme of the 4-dimensional Bethe-Salpeter equation are elucidated. The quarkonium spectral functions are used to calculate Euclidean correlators which are discussed in light of lattice QCD results, while heavy-quark relaxation rates and diffusion coefficients are extracted utilizing a Fokker-Planck equation.
We present a theoretical study of the properties of the a1(1260) axial-vector resonance in a cold nuclear medium. In the vacuum, the a1(1260) resonance is generated dynamically from the interactions of a pseudoscalar and vector meson (rho pi and K barK*) in a coupled channel chiral unitary approach. Medium effects are implemented through the modification of the rho and pi propagators at finite nuclear density from well established microscopic many-body calculations. The in-medium pion spectral function accounts for the coupling to N-hole and Delta-hole excitations including short range correlations, whereas the in-medium rho incorporates modifications of its virtual pion cloud as well as direct resonance-hole excitations. The resulting in-medium a1(1260) exhibits significant broadening with increasing density as reflected in the rho pi scattering amplitude. The possible relation of our results with partial restoration of chiral symmetry in nuclear matter is discussed in terms of in-medium Weinberg sum rules.
We adopt a $T$-matrix approach to study quarkonium properties and heavy-quark transport in a Quark-Gluon Plasma. The $T$-matrix approach is well suited to implement potential scattering and thus provides a common framework for low-momentum transfer interactions in heavy-heavy and heavy-light quark systems. We assume that the underlying potentials can be estimated from the heavy-quark free energy computed in lattice QCD. We discuss constraints from vacuum spectroscopy, uncertainties arising from different choices of the potential, and the role of elastic and inelastic widths which are naturally accounted for in the $T$-matrix formalism.
We investigate the consequences of space-momentum correlations in quark phase-space distributions for coalescence processes at the hadronization transition. Thus far it has been proved difficult to reconcile such correlations with the empirically observed constituent quark number scaling (CQNS) at the Relativistic Heavy-Ion Collider (RHIC). To address this problem we combine our earlier developed quark recombination model with quark phase-space distributions computed from relativistic Langevin simulations in an expanding Quark-Gluon Plasma (QGP). Hadronization is based on resonance formation within a Boltzmann equation which recovers thermal equilibrium and obeys energy conservation in the quark-coalescence process, while the fireball background is adjusted to hydrodynamic simulations of semi-central Au-Au collisions at RHIC. To facilitate the applicability of the Langevin process, we focus on strange and charm quarks. Their interactions in the QGP are modeled using leading-order perturbative QCD augmented by effective Lagrangians with resonances which smoothly merge into hadronic states formed at T_c. The interaction strength is adjusted to reproduce the empirical saturation value for the quark-elliptic flow, v_{2,q}^{sat}~7-8%. The resulting phi and J/ψelliptic flow recover CQNS over a large range in transverse momentum (p_T) within a few percent. As a function of transverse kinetic energy, both the quark spectra from the Langevin simulations and the meson spectra generated via resonance recombination recover CQNS from zero to at least 3 GeV.
We compute dilepton invariant mass spectra from the decays of rho mesons produced by photon reactions off nuclei. Our calculations employ a realistic model for the rho photoproduction amplitude on the nucleon which provides fair agreement with measured cross sections. Medium effects are implemented via an earlier constructed rho propagator based on hadronic many-body theory. At incoming photon energies of 1.5 -3 GeV as used by the CLAS experiment at JLAB, the average density probed for iron targets is estimated at about half saturation density. At the pertinent rho-meson 3-momenta the predicted medium effects on the rho propagator are rather moderate. The resulting dilepton spectra approximately agree with recent CLAS data.
Recent developments in the evaluation of vector-meson spectral functions in hot and dense matter are discussed with emphasis on connections to the chiral phase transition in QCD. Model independent approaches including chiral low-density expansions, lattice QCD, chiral and QCD sum rules are put into context with model predictions for in-medium vector-spectral function utilizing effective Lagrangians. Hadronic many-body calculations predict a strong broadening (and little mass shift) of the $ρ$ spectral function which rapidly increases close to the expected phase boundary of hadronic and quark-gluon matter. Pertinent dilepton rates appear to degenerate with perturbative quark-antiquark annihilation in the Quark-Gluon Plasma, suggestive for chiral symmetry restoration. Applications to low-mass dilepton spectra in heavy-ion collisions result in quantitative agreement with recent high-quality data at the CERN-SPS. Thermal radiation from temperatures around $T_c$ consistently reproduces the experimental dilepton excess observed at masses above 1 GeV as well. The interpretation of dilepton sources at high transverse momentum appears to be more involved.
Due to their penetrating nature, dileptons are a valuable probe for the properties of the hot and/or dense medium created in relativistic heavy-ion collisions. Dilepton invariant-mass spectra provide direct access to the properties of the electromagnetic current-correlation function in strongly interacting matter. In this paper an overview is given of our current theoretical understanding of the dilepton phenomenology in comparison to recent data in heavy-ion collisions at the CERN SPS.
We attempt a unified treatment of heavy quarkonia and heavy-quark diffusion in the Quark-Gluon Plasma. Our approach is based on finite-temperature T-matrices with interaction potentials estimated from the heavy-quark internal energy computed in thermal lattice QCD (lQCD). In the charmonium sector S-wave bound states (J/ψ, η_c) survive up to temperatures of ~2 T_c, not inconsistent with constraints from euclidean correlation functions in lQCD. In the open-heavy flavor sector, the T-matrix interaction reduces heavy-quark diffusion substantially, leading to fair agreement with single-electron spectra at RHIC and suggestive for a small viscosity-to-entropy ratio close to T_c.
We report on recent research on the properties of elementary particle matter governed by the strong force at high temperatures, where QCD predicts hadrons to dissolve into the Quark-Gluon Plasma (QGP). After a short introduction to the basic elements of QCD in the vacuum, most notably quark confinement and mass generation, we discuss how these phenomena relate to phase changes in strongly interacting matter at high temperature. We briefly review the main experimental findings at the Relativistic Heavy Ion Collider (RHIC) which provide strong evidence that a QGP has been produced, with unprecedentedly small viscosity and large opacity. We discuss how heavy quarks (charm and bottom) can be utilized to quantitatively probe the transport properties of a strongly coupled QGP (sQGP). The large heavy-quark (HQ) mass allows to set up a Brownian motion approach, which can serve to evaluate different approaches for HQ interactions in the sQGP. The implementation of lattice QCD based HQ potentials generates pre-hadronic resonance structures in HQ scattering in the medium, leading to large interaction rates and small diffusion coefficients. The resonance correlations are strongest close to the critical temperature (T_c), suggesting an intimate connection to the hadronization of the QGP. The implementation of HQ transport into Langevin simulations of an expanding QGP fireball at RHIC enables quantitative comparisons with experiment. The extracted HQ diffusion coefficients are employed for schematic estimates of the shear viscosity, corroborating the notion of a strongly-coupled QGP in the vicinity of T_c.
We evaluate heavy-quark (HQ) transport properties in a Quark-Gluon Plasma (QGP) employing interaction potentials extracted from thermal lattice QCD. Within a Brueckner many-body scheme we calculate in-medium T-matrices for charm- and bottom-quark scattering off light quarks in the QGP. The interactions are dominated by attractive meson and diquark channels which support bound and resonance states up to temperatures of ~1.5 T_c. We apply pertinent drag and diffusion coefficients (supplemented by perturbative scattering off gluons) in Langevin simulations in an expanding fireball to compute HQ spectra and elliptic flow in \sqrt{s_{NN}}=200 GeV Au-Au collisions. We find good agreement with semileptonic electron-decay spectra which supports our nonperturbative computation of the HQ diffusion coefficient, suggestive for a strongly coupled QGP.
We summarize how future measurements of electromagnetic (e.m.) probes at the Relativistic Heavy Ion Collider (RHIC), in connection with theoretical analysis, can advance our understanding of strongly interacting matter at high energy densities and temperatures. After a brief survey of the important role that e.m. probes data have played at the Super Proton Synchrotron (SPS, CERN) and RHIC to date, we identify key physics objectives and observables that remain to be addressed to characterize the (strongly interacting) Quark-Gluon Plasma (sQGP) and associated transition properties at RHIC. These include medium modifications of vector mesons via low-mass dileptons, a temperature measurement of the hot phases via continuum radiation, as well as gamma-gamma correlations to characterize early source sizes. We outline strategies to establish microscopic matter and transition properties such as the number of degrees of freedom in the sQGP, the origin of the hadron masses and manifestations of chiral symmetry restoration, which will require accompanying but rather well-defined advances in theory. Increased experimental precision, order of magnitude higher statistics than currently achievable, as well as a detailed scan of colliding species and energies are then mandatory to achieve sufficient discrimination power in theoretical interpretations. This increased precision can be achieved with hardware upgrades to the large RHIC detectors (PHENIX and STAR) along with at least a factor of ten as increase in luminosity over the next few years as envisioned for RHIC-II.
The ultra-relativistic heavy-ion program at RHIC has shown that at intermediate transverse momenta ($p_T \simeq 2$-6 GeV) standard (independent) parton fragmentation can neither describe the observed baryon-to-meson ratios nor the empirical scaling of the hadronic elliptic flow ($v_2$) according to the number of valence quarks. Both aspects find instead a natural explanation in a coalescence plus fragmentation approach to hadronization. After a brief review of the main results for light quarks, we focus on heavy quarks showing that a combined fragmentation and quark-coalescence framework is relevant also here. Moreover, within relativistic Langevin simulations we find evidence for the importance of heavy-light resonances in the Quark-Gluon Plasma (QGP) to explain the strong energy loss and collective flow of heavy-quark spectra as inferred from non-photonic electron observables. Such heavy-light resonances can pave the way to a unified understanding of the microscopic structure of the QGP and its subsequent hadronization by coalescence.