arXiv subjects

R. Rapp

Publications and source records attributed to R. Rapp.

At least 73 records · Page 4Linked to original sources

arXiv2007

Heavy-Quark Kinetics at RHIC and LHC

In ultrarelativistic nuclear collisions heavy quarks are produced out of thermal equilibrium in the very early stage of the reaction and their thermalization time was expected to be considerably larger than that of light quarks. On the other hand, a strongly-interacting QGP can be envisaged in the heavy quark sector due to the presence of heavy-light hadron-like resonances in the QGP for temperatures up to $\sim$$2 T_{C}$. We investigate the consequences of such states employing a relativistic Langevin simulation. Hadronization is modeled by a coalescence-fragmentation scheme. We present the predictions for the nuclear modification factor and elliptic flow of $D$ and $B$ mesons at LHC energies and compare the results with the successful predictions of this model for RHIC conditions. We find similar heavy-quark thermalization effects at LHC and RHIC.

hep-ph↗

arXiv2007

Low-Energy Thermal Photons from Meson-Meson Bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a U$_{\rm em}$(1) gauge, we reinvestigate photon Bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions $ππ\toππγ$ and $πK \toπKγ$ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. % during the hadronic stage of the fireball. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau-Pomeranchuk-Migdal and in-medium effects on "$σ$" and $ρ$-meson exchanges are briefly discussed.

nucl-th↗

arXiv2007

Thermal Dileptons at LHC

We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb collisions at LHC. Hadronic emission in the low-mass region is calculated using in-medium spectral functions of light vector mesons within hadronic many-body theory. In the intermediate-mass region thermal radiation from the Quark-Gluon Plasma, evaluated perturbatively with hard-thermal loop corrections, takes over. An important source over the entire mass range are decays of correlated open-charm hadrons, rendering the nuclear modification of charm and bottom spectra a critical ingredient.

hep-ph↗

arXiv2007

Quark Coalescence based on a Transport Equation

We employ the Boltzmann equation for describing hadron production from a quark-gluon plasma (QGP) in ultrarelativistic heavy-ion collisions. We propose resonance formation in quark-antiquark scattering as the dominant meson-production channel, which, in particular, ensures that energy is conserved in the recombination process. This, in turn, facilitates a more controlled extension of hadronization to low transverse momenta ($p_T$), and to address the experimentally observed transition from a hydrodynamic regime to constituent quark-number scaling (CQNS). Based on input distributions for strange and charm quarks with azimuthal asymmetries, $v_2(p_T)$, characteristic for RHIC energies, we recover CQNS at sufficiently high $p_T$, while at low $p_T$ a scaling with transverse kinetic energy is found, reminiscent to experiment. The dependence of the transition regime on microscopic QGP properties, i.e. resonance widths and $Q$-values in the $q+\bar q \to M$ process, is elucidated.

hep-ph↗

arXiv2006

T-Matrix Approach to Quarkonium Correlation Functions in the QGP

We study the evolution of heavy quarkonium states with temperature in a Quark Gluon Plasma (QGP) by evaluating the in-medium Q-\bar{Q} T-matrix within a reduced Bethe-Salpeter equation in both S- and P-wave channels. The underlying interaction kernel is extracted from recent finite-temperature QCD lattice calculations of the singlet free energy of a Q-\bar{Q} pair. The bound states are found to gradually move above the Q-\bar{Q} threshold after which they rapidly dissolve in the hot system. The T-matrix approach is particularly suited to investigate these mechanisms as it provides a unified treatment of bound and scattering states including threshold effects and the transition to the (perturbative) continuum. We apply the T-matrix to calculate Q-\bar{Q} spectral functions as well as pertinent Euclidean-time correlation functions which are compared to results from lattice QCD. A detailed analysis reveals large sensitivities to the interplay of bound and scattering states, to temperature dependent threshold energies and to the "reconstructed" correlator used for normalization. We furthermore investigate the impact of finite-width effects on the single-quark propagators in the QGP as estimated from recent applications of heavy-quark rescattering to RHIC data.

hep-ph↗

arXiv2006

Q\bar{Q} modes in the Quark-Gluon Plasma

We study the evolution of heavy quarkonium states with temperature in a Quark-Gluon Plasma by evaluating an in-medium Q\bar{Q} T-matrix within a reduced Bethe-Salpeter equation in S- and P-wave channels. The interaction kernel is extracted from finite-temperature QCD lattice calculations of the singlet free energy of a Q\bar{Q} pair. Quarkonium bound states are found to gradually move across the Q\bar{Q} threshold after which they rapidly dissolve in the hot system. We calculate Euclidean-time correlation functions and compare to results from lattice QCD. We also study finite-width effects in the heavy-quark propagators.

hep-ph↗

arXiv2006

Charm and Charmonium in the Quark-Gluon Plasma

In the first part of the talk, we briefly review the problem of parton-energy loss and thermalization at the Relativistic Heavy-Ion Collider and discuss how heavy quarks (charm and bottom) can help to resolve the existing experimental and theoretical puzzles. The second part of the talk is devoted to the properties of heavy quarkonia in the (strongly interacting) Quark-Gluon Plasma (sQGP) and their consequences for observables in heavy-ion collisions.

nucl-th↗

arXiv2006

Heavy-Quark Diffusion, Flow and Recombination at RHIC

We discuss recent developments in assessing heavy-quark interaction in the Quark-Gluon Plasma (QGP). While induced gluon radiation is expected to be the main energy-loss mechanism for fast-moving quarks, we focus on elastic scattering which prevails toward lower energies, evaluating both perturbative (gluon-exchange) and nonperturbative (resonance formation) interactions in the QGP. The latter are treated within an effective model for D- and B-meson resonances above T_c as motivated by current QCD lattice calculations. Pertinent diffusion and drag constants, following from a Fokker-Planck equation, are implemented into an expanding fireball model for Au-Au collisions at RHIC using relativistic Langevin simulations. Heavy quarks are hadronized in a combined fragmentation and coalescence framework, and resulting electron-decay spectra are compared to recent RHIC data. A reasonable description of both nuclear suppression factors and elliptic flow up to momenta of ~5 GeV supports the notion of a strongly interacting QGP created at RHIC. Consequences and further tests of the proposed resonance interactions are discussed.

hep-ph↗

arXiv2006

Medium Modifications of Vector Mesons and NA60

We confront different models for medium modifications of the electromagnetic correlation function to recent dimuon spectra from the NA60 collaboration for central In-In collisions at the CERN-SPS. In the low-mass region (M \lesssim 1.0 GeV), we evaluate medium modifications of the spectral properties of ρ-, ω- and ϕ-mesons. In the intermediate-mass region (M>1.0 GeV), we employ schematic models for the mixing of vector and axialvector current correlators, which provide a mechanism for chiral-symmetry restoration.

hep-ph↗

arXiv2006

Bottomonium Production at RHIC and LHC

Properties of bottomonia (Upsilon, chi_b and Upsilon') in the Quark-Gluon Plasma (QGP) are investigated by assessing inelastic reaction rates and their interplay with open-bottom states (b-quarks or B-mesons) and color-screening. The latter leads to vanishing quarkonium binding energies at sufficiently high temperatures (close to the dissolution point), which, in particular, renders standard gluo-dissociation, g+Upsilon -> b + b-bar, inefficient due to a substantial reduction in final-state phase space. This problem is overcome by invoking a "quasifree" destruction mechanism, g,q,q-bar + Upsilon -> g,q,q-bar + b + b-bar, as previously introduced for charmonia. The pertinent reaction rates are implemented into a kinetic theory framework to evaluate the time evolution of bottomonia in heavy-ion reactions at RHIC and LHC within an expanding fireball model. While bottom quarks are assumed to be exclusively produced in primordial nucleon-nucleon collisions, their thermal relaxation times in the QGP, which importantly figure into Upsilon-formation rates, are estimated according to a recent Fokker-Planck treatment. Predictions for the centrality dependence of Upsilon production are given for upcoming experiments at RHIC and LHC. At both energies, Upsilon suppression turns out to be the prevalent effect.

hep-ph↗

arXiv2006

Thermalization and Flow of Heavy Quarks in the Quark-Gluon Plasma

Elastic scattering of charm (c) and bottom (b) quarks via D- and B-meson resonance states in an expanding, strongly interacting quark-gluon plasma is investigated. Drag and diffusion coefficients are calculated from an effective model based on chiral symmetry and heavy-quark effective theory, and utilized in a relativistic Langevin simulation to obtain transverse-momentum spectra and elliptic flow (v_2) of c- and b-quarks. The hadronization to D- and B-mesons is described by coalescence and fragmentation, and the resulting decay-electron spectra are compared to recent RHIC data.

hep-ph↗

arXiv2005

Heavy-Quark Spectra at RHIC and Resonances in the QGP

Thermalization and collective flow of charm (c) and bottom (b) quarks are evaluated from elastic parton scattering via "D"- and "B"-meson resonances in an expanding, strongly interacting quark-gluon plasma at RHIC. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal flow asymmetries (v_2) of c- and b-quarks. Upon hadronization (including coalescence and fragmentation) and semileptonic D- and B-decays, the resulting electron spectra (R_{AA} and v_2) are compared to recent RHIC data.

hep-ph↗

arXiv2005

Scattering of Quark-Quasiparticles in the Quark-Gluon Plasma

Employing a Brueckner-type many-body approach, based on a driving potential extracted from lattice QCD, we study light quark properties in a Quark-Gluon Plasma (QGP) at moderate temperatures, $T\simeq$~1-2~$T_c$. The quark-antiquark $T$-matrix is calculated self-consistently with pertinent quark self-energies. While the repulsive octet channel induces quasiparticle masses of up to 150~MeV, the attractive color-singlet part exhibits resonance structures which lead to quasiparticle widths of $\sim$200~MeV.

hep-ph↗

arXiv2005

Hadronic Modes and Quark Properties in the Quark-Gluon Plasma

Based on interaction potentials between a heavy quark and antiquark as extracted from recent QCD lattice calculations, we set up a Brueckner-type many-body scheme to study the properties of light (anti-) quarks in a Quark-Gluon Plasma at moderate temperatures, $T\simeq$ 1-2 $T_c$. The quark-antiquark $T$-matrix, including both color-singlet and -octet channels, and corresponding quark self-energies and spectral functions are calculated self-consistently. The repulsive octet potential induces quasiparticle masses of up to 150 MeV, whereas the attractive color-singlet part generates resonance structures in the $q$-$\bar q$ $T$-matrix, which in turn lead to quasiparticle widths of $\sim$200 MeV. This corresponds to scattering rates of $\sim$1 fm$^{-1}$ and may reflect liquid-like properties of the system.

hep-ph↗

arXiv2005

Heavy Quarkonium Physics

This report is the result of the collaboration and research effort of the Quarkonium Working Group over the last three years. It provides a comprehensive overview of the state of the art in heavy-quarkonium theory and experiment, covering quarkonium spectroscopy, decay, and production, the determination of QCD parameters from quarkonium observables, quarkonia in media, and the effects on quarkonia of physics beyond the Standard Model. An introduction to common theoretical and experimental tools is included. Future opportunities for research in quarkonium physics are also discussed.

hep-ph↗

arXiv2005

Fireballs from Quark Stars in the CFL Phase: Application to Gamma Ray Bursters

Recent studies of photon-generation mechanisms in the color-superconducting Color-Flavor Locked (CFL) phase of dense quark matter have found gamma-ray emissivities in excess of ~ 10^{50} erg cm^{-3} s^{-1} for temperatures in the 10-30 MeV range. We suggest that this property can trigger gamma-ray bursts (GRBs) and associated fireballs at the surface of hypothetical hot (newly born) quark stars with an energy release of up to 10^{48}-10^{50} erg within a fraction of a millisecond. If surrounded by an accretion disk following its formation, the star's bursting activity can last from tens of milliseconds to hundreds of seconds releasing up to 10^{52} erg in total energy. We discuss typical features of observed GRBs within our model and explain how quark stars in the CFL phase might constitute natural candidates for corresponding inner engines.

astro-ph↗

arXiv2005

Thermalization of Heavy Quarks in the Quark-Gluon Plasma

Charm- and bottom-quark rescattering in a Quark-Gluon Plasma (QGP) is investigated with the objective of assessing the approach towards thermalization. Employing a Fokker-Planck equation to approximate the collision integral of the Boltzmann equation we augment earlier studies based on perturbative parton cross sections by introducing resonant heavy-light quark interactions. The latter are motivated by recent QCD lattice calculations which indicate the presence of "hadronic" states in the QGP. We model these states by colorless (pseudo-) scalar and (axial-) vector D- and B-mesons within a heavy-quark effective theory framework. We find that the presence of these states at moderate QGP temperatures substantially accelerates the kinetic equilibration of c-quarks as compared to using perturbative interactions. We also comment on consequences for $D$-meson observables in ultra-relativistic heavy-ion collisions.

nucl-th↗

arXiv2005

The Delta(1232) at RHIC

We investigate properties of the Delta(1232) and nucleon spectral functions at finite temperature and baryon density within a hadronic model. The medium modifications of the Delta consist of a renormalization of its pion-nucleon cloud and resonant pi-Delta scattering. Underlying coupling constants and form factors are determined by the elastic π-N scattering phase shift in the isobar channel, as well as empirical partial decay widths of excited baryon resonances. For cold nuclear matter the model provides reasonable agreement with photoabsorption data on nuclei in the Delta-resonance region. In hot hadronic matter typical for late stages of central Au-Au collisions at RHIC we find the Delta-spectral function to be broadened by $\sim$65 MeV together with a slight upward mass shift of 5-10 MeV, in qualitative agreement with preliminary data from the STAR collaboration.

nucl-th↗