Hendrik van Hees

Heavy-quark probes of the quark-gluon plasma and interpretation of recent data taken at the BNL Relativistic Heavy Ion Collider

Thermalization and collective flow of charm (c) and bottom (b) quarks in ultra-relativistic heavyion 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 parton coalescence, can play an essential role in the interpretation of recent data from the Relativistic HeavyIon Collider (RHIC), and thus illuminate the nature of the sQGP and its hadronization. Our main assumption, motivated by recent findings in lattice Quantum Chromodynamics, is the existence of Dand B-meson states in the sQGP, providing resonant cross sections for heavy quarks. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal asymmetries (v2) of band c-quarks in Au-Au collisions at RHIC. After hadronization into Dand B-mesons using quark coalescence and fragmentation, associated electron-decay spectra and v2 are compared to recent RHIC data. Our results suggest a reevaluation of radiative and elastic quark energy-loss mechanisms in the sQGP.

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 toward 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 that 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 resonances 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 ultrarelativistic heavy-ion collisions.

Dilepton radiation at the CERN super-proton synchrotron

Abstract 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 ≲ 1 GeV ) dominated by vector mesons ( ρ , ω , ϕ ) and an intermediate-mass region (IMR, 1 GeV ⩽ M ⩽ 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 ρ meson 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 ≳ 1 GeV in non-central In–In collisions, which we address by extending our calculations by: (i) a refined treatment of ρ decays at thermal freeze-out, (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 freeze-out of the fireball. The ρ broadening 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.

Renormalization in self-consistent approximation schemes at finite temperature: Theory

Within finite temperature field theory, we show that truncated non-perturbative selfconsistent Dyson resummation schemes can be renormalized with local counter terms defined at the vacuum level. The requirements are that the underlying theory is renormalizable and that the self-consistent scheme follows Baym’s �-derivable concept. The scheme generates both, the renormalized self-consistent equations of motion and the closed equations for the infinite set of counter terms. At the same time the corresponding 2PI-generating functional and the thermodynamical potential can be renormalized, in consistency with the equations of motion. This guarantees the standard �-derivable properties like thermodynamic consistency and exact conservation laws also for the renormalized approximation schemes to hold. The proof uses the techniques of BPHZ-renormalization to cope with the explicit and the hidden overlapping vacuum divergences.

Thermal photons and collective flow at energies available at the BNL Relativistic Heavy-Ion Collider

Cyclotron Institute and Department of Physics&Astronomy,Texas A&M University, College Station, Texas 77843-3366, USA(Dated: August 11, 2011)We update our calculations of thermal-photon production in nuclear collisions at the Relativis-tic Heavy-Ion Collider (RHIC). Specifically, we address the recent experimental observation of anelliptic flow of direct photons comparable in magnitude to that of pions, which is at variance withexpectations based on quark-gluon plasma (QGP) dominated photon radiation. Our thermal emis-sion rate is based on previous work, i.e., resummed leading-order QGP emission and in-mediumhadronic rates in the confined phase. These rates are nearly degenerate at temperatures close to theexpected QCD-phase change. The rates are convoluted over an improved elliptic-fireball expansionwith 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 byhadron-dominated emission up to transverse momenta of ∼ 2-3 GeV. The associated large ellipticflow in the hadronic phase mitigates the discrepancy with the measured photon-v

Comprehensive interpretation of thermal dileptons measured at the CERN super proton synchrotron

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≃0.9 GeV, with good sensitivity to details of the predicted ρ-meson line shape. This, in particular, identifies baryoninduced effects as the prevalent ones. We show that a reliable description of the ρ contribution opens the possibility to study further medium effects: at higher masses (M≃0.9-1.5 GeV) 4-pion type annihilation is required to account for the experimentally observed excess (possibly augmented by effects of “chiral mixing”), while predictions for thermal emission from modified ω and φ line shapes may be tested in the future.

Renormalization of self-consistent approximation schemes at finite temperature. II. Applications to the sunset diagram

The theoretical concepts for the renormalization of self-consistent Dyson resummations, deviced in the first paper of this series, are applied to first example cases for the

HEAVY QUARKS IN THE QUARK-GLUON PLASMA

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Pseudo-critical enhancement of thermal photons in relativistic heavy-ion collisions?

-theory. Besides the tadpole (Hartree) approximation as a novel part the numerical solutions are presented which includes the sunset self-energy diagram into the self-consistent scheme based on the

Heavy quark transport in heavy ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider within the UrQMD hybrid model

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