arXiv subjects

Ralf Rapp

Publications and source records attributed to Ralf Rapp.

124 records · Page 7Linked to original sources

arXiv2007

Interpretation of Recent SPS Dilepton Data

We summarize our current theoretical understanding of in-medium properties of the electromagnetic current correlator in view of recent dimuon data from the NA60 experiment in In(158 AGeV)-In collisions at the CERN-SPS. We discuss the sensitivity of the results to space-time evolution models for the hot and dense partonic and hadronic medium created in relativistic heavy-ion collisions and the contributions from different sources to the dilepton-excess spectra.

hep-ph↗

arXiv2007

Theory and Phenomenology of Vector Mesons in Medium

Electromagnetic probes promise to be direct messengers of (spectral properties of) hot and dense matter formed in heavy-ion collisions, even at soft momentum transfers essential for characterizing possible phase transitions. We examine how far we have progressed toward this goal by highlighting recent developments, and trying to establish connections between lattice QCD, effective hadronic models and phenomenology of dilepton production.

nucl-th↗

arXiv2006

Melting rho Meson and Thermal Dileptons

We give a brief survey of theoretical evaluations of light vector mesons in hadronic matter, focusing on results from hadronic many-body theory. We emphasize the importance of imposing model constraints in obtaining reliable results for the in-medium spectral densities. The latter are subsequently applied to the calculation of dilepton spectra in high-energy heavy-ion collisions, with comparisons to recent NA60 data at the CERN-SPS. We discuss aspects of space-time evolution models and the decomposition of the excess spectra into different emission sources.

hep-ph↗

arXiv2006

Comprehensive Interpretation of Thermal Dileptons at the SPS

Employing thermal dilepton rates based on medium-modified electromagnetic correlation functions we show that recent dimuon spectra of the NA60 collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated ρ-meson spectral functions, as following from hadronic many-body theory, provide an accurate description of the data up to dimuon invariant masses of about M\simeq 0.9 GeV, with good sensitivity to details of the predicted ρ-meson line shape. This, in particular, identifies baryon-induced effects as the prevalent ones. We show that a reliable description of the ρcontribution opens the possibility to study further medium effects: at higher masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account for the experimentally observed excess indicating precursor effects of chiral symmetry restoration (``chiral mixing''), while remaining structures in the ωand ϕregion are suggestive for modifications in their line shapes as well.

hep-ph↗

arXiv2006

Dileptons and Medium Effects in Heavy-Ion Collisions

We discuss the status of calculating in-medium modifications of vector-meson spectral functions in hot and dense matter, their application to dilepton spectra in ultrarelativistic heavy-ion collisions, and possible relations to chiral symmetry restoration. We emphasize the importance of constraining in-medium spectral functions by empirical information from scattering data, QCD sum rules, and lattice QCD. This is a mandatory prerequisite to arrive at reliable predictions for low-mass dileptons in heavy-ion collisions.

nucl-th↗

arXiv2006

Heavy-Quark Probes of the Quark-Gluon Plasma at RHIC

Thermalization and collective flow of charm (c) and bottom (b) quarks in ultra-relativistic heavy-ion collisions are evaluated based on elastic parton rescattering in an expanding quark-gluon plasma (QGP). We show that resonant interactions in a strongly interacting QGP (sQGP), as well as the effects of parton coalescence, can play an essential role in the interpretation of recent data from the Relativistic Heavy-Ion Collider (RHIC), and thus illuminate the nature of the sQGP and its hadronization. Our main assumption, motivated by recent findings in lattice computations of Quantum Chromodynamics, is the existence of D- and B-meson states in the sQGP, providing resonant cross sections for heavy quarks up to temperatures of sim 2 T_c. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal asymmetries (v_2) of b- and c-quarks in Au-Au collisions at RHIC. Hadronization into D- and B-mesons is calculated from a combination of coalescence with light quarks and fragmentation, and associated electron-decay spectra and v_2 are compared to recent RHIC data. We also comment on the relative importance of radiative and elastic energy loss of heavy quarks in the QGP.

nucl-th↗

arXiv2005

Quark Coalescence and Charm(onium) in QGP

The potential of heavy quarks as probes of the environment produced in hadronic and heavy-ion reactions is discussed. A key role is played by coalescence processes and/or resonance formation which are promising candidates to provide a comprehensive understanding of phenomena associated with reinteractions of both open and hidden heavy-quark states.

hep-ph↗

arXiv2005

Electromagnetic Radiation and in-Medium Effects

The theory of thermal photon and dilepton emission from a hot and dense hadronic gas, as well as from the Quark-Gluon Plasma, is reviewed in the context of extracting in-medium properties of the matter constituents. In phenomenological applications to ultrarelativistic heavy-ion collisions we focus on recent photon and dilepton spectra as measured by WA98 and CERES/NA45, respectively, at CERN-SPS energies.

nucl-th↗

arXiv2004

Delta(1232) and Nucleon Spectral Functions in Hot Hadronic Matter

Modifications of Delta(1232) and nucleon spectral functions at finite temperature and baryon density are evaluated in terms of resonant scattering off thermal pions, as well as a renormalization of the vacuum Delta-width including vertex corrections. The interactions are based on effective Lagrangians of pions and baryon resonances, with underlying parameters (coupling constants and form factors) determined by the elastic pi N scattering phase shift in the isobar channel, and by empirical decay branchings of excited resonances. Pion modifications are included via interactions in a pion gas, as well as standard Delta- and nucleon-hole excitations in nuclear matter. In hot hadronic matter, under conditions resembling thermal freezeout at the Relativistic Heavy-Ion Collider (RHIC), the Delta exhibits a significant broadening of ~65 MeV together with a slight upward peak shift of 5-10 MeV, in qualitative agreement with preliminary data from the STAR collaboration.

nucl-th↗

arXiv2004

The Vector Probe in Heavy-Ion Reactions

We review essential elements in using the $J^P=1^-$ channel as a probe for hot and dense matter as produced in (ultra-) relativistic collisions of heavy nuclei. The uniqueness of the vector channel resides in the fact that it directly couples to photons, both real and virtual (dileptons), enabling the study of thermal radiation and in-medium effects on both light ($ρ, ω, ϕ$) and heavy ($Ψ, Υ$) vector mesons. We emphasize the importance of interrelations between photons and dileptons, and characterize relevant energy/mass regimes through connections to Quark-Gluon-Plasma emission and chiral symmetry restoration. Based on critical analysis of our current understanding of data from fixed-target energies, we identify open key questions to be addressed.

nucl-th↗

arXiv2004

Photon production in relativistic nuclear collisions at SPS and RHIC energies

Chiral Lagrangians are used to compute the production rate of photons from the hadronic phase of relativistic nuclear collisions. Special attention is paid to the role of the pseudovector a_1 meson. Calculations that include reactions with strange mesons, hadronic form factors and vector spectral densities consistent with dilepton production, as well as the emission from a quark-gluon plasma and primordial nucleon-nucleon collisions, reproduce the photon spectra measured at the Super Proton Synchrotron (SPS). Predictions for the Relativistic Heavy Ion Collider (RHIC) are made.

hep-ph↗

arXiv2004

Thermal Photons in Strong Interactions

A brief survey is given on the current status of evaluating thermal production of photons from a strongly interacting medium. Emphasis is put on recent progress in assessing equilibrium emission rates in both hadronic and quark-gluon matter. We also give an update on the status of comparing theoretical calculations with experimental data from heavy-ion collisions at the SPS, as well as prospects for RHIC. Finally, applications of photon rate calculations to colorsuperconducting quark matter are discussed.

nucl-th↗

arXiv2003

Hadronic Production of Thermal Photons

We study the thermal emission of photons from hot and dense strongly interacting hadronic matter at temperatures close to the expected phase transition to the Quark-Gluon Plasma (QGP). Earlier calculations of photon radiation from ensembles of interacting mesons are re-examined with additional constraints, including new production channels as well as an assessment of hadronic form factor effects. Whereas strangeness-induced photon yields turn out to be moderate, the hitherto not considered t-channel exchange of omega mesons is found to contribute appreciably for photon energies above ~1.5 GeV. The role of baryonic effects is assessed using existing many-body calculations of lepton pair production. We argue that our combined results constitute a rather realistic emission rate, appropriate for applications in relativistic heavy-ion collisions. Supplemented with recent evaluations of QGP emission, and an estimate for primordial (hard) production, we compute photon spectra at SPS, RHIC and LHC energies.

hep-ph↗

arXiv2003

Transverse flow and hadro-chemistry in Au+Au collisions at \sqrt{s_{NN}}=200 GeV

We present a hydrodynamic assessment of preliminary particle spectra observed in Au+Au collisions at \sqrt{s_{NN}}=200 GeV. The hadronic part of the underlying equation of state is based on explicit conservation of (measured) particle ratios throughout the resonance gas stage after chemical freezeout by employing chemical potentials for stable mesons, nucleons and anti-nucleons. We find that under these conditions the data (in particular the proton spectra) favor a low freeze-out temperature of around 100 MeV. Furthermore we show that through inclusion of a moderate pre-hydrodynamic transverse flow field the shape of the spectra improves with respect to the data. The effect of the initial transverse boost on elliptic flow and the freeze-out geometry of the system is also elucidated.

hep-ph↗

arXiv2002

Photon and dilepton emission rates from high density quark matter

We compute the rates of real and virtual photon (dilepton) emission from dense QCD matter in the color-flavor locked (CFL) phase, focusing on results at moderate densities (3-5 times the nuclear saturation density) and temperatures $T\simeq80$ MeV. We pursue two approaches to evaluate the electromagnetic (e.m.) response of the CFL ground state: (i) a direct evaluation of the photon self energy using quark particle/-hole degrees of freedom, and (ii) a Hidden Local Symmetry (HLS) framework based on generalized mesonic excitations where the $ρ$ meson is introduced as a gauge boson of a local SU(3) color-flavor group. The $ρ$ coupling to generalized two-pion states induces a finite width and allows to address the issue of vector meson dominance (VMD) in the CFL phase. We compare the calculated emissivities (dilepton rates) to those arising from standard hadronic approaches including in-medium effects. For rather large superconducting gaps (several tens of MeV at moderate densities), as suggested by both perturbative and nonperturbative estimates, the dilepton rates from CFL quark matter turn out to be very similar to those obtained in hadronic many-body calculations, especially for invariant masses above $M\simeq0.3$ GeV. A similar observation holds for (real) photon production.

hep-ph↗

arXiv2000

Intermediate-Mass Dileptons at the CERN-SpS and RHIC

The significance of thermal dilepton radiation at intermediate invariant masses (1GeV<M<3GeV) in ultrarelativistic heavy-ion collisions is investigated. At CERN-SpS energies, a consistent explanation of the excess observed by NA50 can be given. At RHIC energies the thermal signal is dominated by early emission indicative for QGP formation. Chemical under-saturation effects and the competition with open-charm contributions are addressed.

hep-ph↗