Thermalized Quark Gluon Plasma Research Articles

Revisiting transverse momentum broadening in dense QCD media

We reconsider the problem of transverse momentum broadening of a highly-energetic parton suffering multiple scatterings in dense colored media, such as the thermal Quark-Gluon plasma or large nuclei. In the framework of Moli\`ere's theory of multiple scattering we re-derive a simple analytic formula, to be used in jet quenching phenomenology, that accounts for both the multiple soft and hard Rutherford scattering regimes. Further, we discuss the sensitivity of momentum broadening to modeling of the non-perturbative infrared sector by presenting a detailed analytic and numerical comparison between the two widely used models in phenomenology: the Hard Thermal Loop and the Gyulassy-Wang potentials. We show that for the relevant values of the parameters the non-universal, model dependent contributions are negligible, at LHC, RHIC and EIC energies thus consolidating the predictive power of jet quenching theory.

Open charm and dileptons from relativistic heavy-ion collisions

Dileptons are considered as one of the cleanest signals of the quark-gluon plasma (QGP), however, the QGP radiation is masked by many 'background' sources from either hadronic decays or semileptonic decays from correlated charm pairs. In this study we investigate the relative contribution of these channels in heavy-ion collisions from $\sqrt{s_{\rm NN}}=$ 8 GeV to 5 TeV with a focus on the competition between the thermal QGP radiation and the semileptonic decays from correlated $D-$meson pairs. As a 'tool' we employ the parton-hadron-string dynamics (PHSD) transport approach to study dilepton spectra in Pb+Pb (Au+Au) collisions in a wide energy range incorporating for the first time a fully microscopic treatment of the charm dynamics and their semileptonic decays. We find that the dileptons from correlated $D-$meson decays dominate the 'thermal' radiation from the QGP in central Pb+Pb collisions at the intermediate masses (1.2 GeV $< M <$ 3 GeV) for $\sqrt{s_{\rm NN}} > $ 40 GeV, while for $\sqrt{s_{\rm NN}}=$ 8 to 20 GeV the contribution from $D,{\bar D}$ decays to the intermediate mass dilepton spectra is subleading such that one should observe a rather clear signal from the QGP radiation. We, furthermore, study the $p_T$-spectra and the $R_{AA}(p_T)$ of single electrons at different energies as well as the excitation function of the inverse slope of the $m_T$- spectra for intermediate-mass dileptons from the QGP and from charm decays. We find moderate but characteristic changes in the inverse slope parameter for $\sqrt{s_{\rm NN}} > $ 20 GeV which can be observed experimentally in high statistics data. Additionally, we provide detailed predictions for dilepton spectra from Pb+Pb collisions at $\sqrt{s_{\rm NN}} = $ 5.02 TeV.

How brightly does the Glasma shine? Photon production off-equilibrium

We present a parametric estimate of photon production at early times in heavy-ion collisions based on a consistent weak coupling thermalization scenario. We quantify the contribution of the off-equilibrium Glasma phase relative to that of a thermalized Quark-Gluon Plasma. Taking into account the constraints from charged hadron multiplicity data, the Glasma contribution is found to be significant especially for large values of the saturation scale.

Effect of pre-equilibrium phase on RAA and v2 of heavy quarks in heavy ion collisions

Heavy quark RAA and v2 has been calculated at Relativistic Heavy Ion Collider energy considering initial conditions with and without pre-equilibrium phase to highlight the effect of the latter on heavy quark observables. The momentum evolution of the heavy quark has been studied by means of the Boltzmann transport equation. To model the pre-equilibrium phase we have used the KLN initial condition. We found that the pre-equilibrium phase impacts the RAA by about 20%–25% whereas the impact on v2 is very negligible. We also calculated heavy quark RAA and v2 in the case of initializations with early thermalized quark–gluon plasma (QGP). We checked that the particular form of the initial spectrum is not very important for RAA, the larger energy density being more important. In fact, comparing the results obtained within the two initializations we found that one can mimic the impact of the pre-equilibrium phase using an early thermalized QGP medium.

Parametric estimate of the relative photon yields from the glasma and the quark-gluon plasma in heavy-ion collisions

Recent classical-statistical numerical simulations have established the "bottom-up" thermalization scenario of Baier et al. as the correct weak coupling effective theory for thermalization in ultrarelativistic heavy-ion collisions. We perform a parametric study of photon production in the various stages of this bottom-up framework to ascertain the relative contribution of the off-equilibrium "Glasma" relative to that of a thermalized Quark-Gluon Plasma. Taking into account the constraints imposed by the measured charged hadron multiplicities at RHIC and the LHC, we find that Glasma contributions are important especially for large values of the saturation scale at both energies. These non-equilibrium effects should therefore be taken into account in studies where weak coupling methods are employed to compute photon yields.

Coarse-graining approach for dilepton production at energies available at the CERN Super Proton Synchrotron

Coarse-grained output from transport calculations is used to determine thermal dilepton emission rates by applying medium-modified spectral functions from thermal quantum field theoretical models. By averaging over an ensemble of events generated with the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model, we extract the local thermodynamic properties at each time step of the calculation. With an equation of state the temperature $T$ and chemical potential ${\ensuremath{\mu}}_{\mathrm{B}}$ can be determined. The approach goes beyond simplified fireball models of the bulk-medium evolution by treating the full (3+1)-dimensional expansion of the system with realistic time and density profiles. For the calculation of thermal dilepton rates we use the in-medium spectral function of the $\ensuremath{\rho}$ meson developed by Rapp and Wambach and consider thermal quark-gluon plasma (QGP) and multipion contributions as well. The approach is applied to the evaluation of dimuon production in In+In collisions at top CERN Super Proton Synchrotron (SPS) energy. Comparison to the experimental results of the NA60 experiment shows good agreement of this ansatz. We find that the experimentally observed low-mass dilepton excess in the mass region from 0.2 to 0.6 GeV can be explained by a broadening of the $\ensuremath{\rho}$ spectral function with a small mass shift. In contrast, the intermediate-mass region $(M&gt;1.5$ GeV) is dominated by a contribution from the quark-gluon plasma. These findings agree with previous calculations with fireball parametrizations. This agreement, in spite of differences in the reaction dynamics between both approaches, indicates that the time-integrated dilepton spectra are not very sensitive to details of the space-time evolution of the collision.

The Glasma, photons and the implications of anisotropy

We introduce distribution functions for quarks and gluons in the Glasma and discuss how they satisfy various relationships of statistical physics. We use these distributions to compute photon production in the early stages of heavy ion collisions. Photon rates satisfy geometric scaling, that is, the emission rate per unit area scales as a function of the saturation momenta divided by the transverse momentum of the photon. Photon distributions from the Glasma are steeper than those computed in the Thermalized Quark Gluon Plasma (TQGP). Both the delayed equilibration of the Glasma and a possible anisotropy in the pressure lead to slower expansion and mean times of photon emission of fixed energy are increased. This delayed emission might allow for larger photon elliptic flow.

Viscosity and Jet Quenching from a Holographic Model

We consider the backreaction of the fundamental flavor degrees of freedom on the AdS5-Schwarz background, and calculate their contributions to the shear viscosity and jet-quenching parameter of the thermal quark-gluon plasma.

A first look at Bottomonium melting via a stochastic potential

We investigate the phenomenon of Bottomonium melting in a thermal quark-gluon plasma using three-dimensional stochastic simulations based on the concept of open-quantum systems. In this non-relativistic framework, introduced in [Phys.Rev. D85 (2012) 105011], which makes close contact to the potentials derived in effective field theory, the $b\bar{b}$ system evolves unitarily under the incessant kicks by the constituents of the surrounding heat bath. In particular thermal fluctuations and the presence of a complex potential in the EFT are naturally related. An intricate interplay between state mixing and thermal excitations emerges as we show how non-thermal initial conditions of Bottomonium states evolve over time. We emphasize that the dynamics of these states gives us access to information beyond what is encoded in the thermal Bottomonium spectral functions. Assumptions underlying our approach and their limitations, as well as the refinements necessary to connect to experimental measurements under more realistic conditions are discussed.

Initial state, parton saturation and thermalization

Abstract The early time behavior of heavy ion collisions involves new forms of matter such as the Color Glass Condensate and the Glasma. An area of much recent work involves attempts to understand the development of the Glasma into a thermalized Quark Gluon Plasma. The evolution of the Glasma involves turbulent Yang–Mills fields, and might also have condensates and non-trivial topological excitations. The proton–nucleus collision experiments at the LHC accelerator may provide unique insight into these forms of matter and the role they play at various times in hadronic collisions. I introduce and review aspects of these forms of matter as related to experiments at the LHC and RHIC accelerators.

Kinetic evolution of the glasma and thermalization in heavy-ion collisions

In relativistic heavy-ion collisions, a highly occupied gluonic matter is created shortly after initial impact, which is in a nonthermal state and often referred to as the Glasma. Successful phenomenology suggests that the glasma evolves rather quickly toward the thermal quark–gluon plasma (QGP) and a hydrodynamic behavior emerges at a very early time ~ô(1) fm/c. Exactly how such "apparent thermalization" occurs and connects the initial conditions to the hydrodynamic onset, remains a significant challenge for theory as well as phenomenology. We briefly review various ideas and recent progress in understanding the approach of the glasma to the thermalized QGP, with an emphasis on the kinetic theory description for the evolution of such far-from-equilibrium and highly overpopulated, thus weakly-coupled yet strongly interacting glasma.

Are the angular correlations in pA collisions due to a Glasmion or Bose condensation?

Experiments at the LHC have recently reported results on the angular asymmetry coefficients v n [ m ] , for various angular moments n and orders of cumulants m , in high multiplicity p + Pb collisions. These coefficients are large, and have both even and odd moments. We discuss here some of the implications of these results for our understanding of the initial state of the collision (Color Glass Condensate) and for the evolution in the final state (Glasma and thermalized Quark Gluon Plasma). We show the Color Glass Condensate predicts large even moments, v n with n an even integer. Odd moments are generated by final state interactions or fragmentation. For a multi-particle determination of v 2 [ m ] , where m is the number of particles used to determine the correlation, we argue that if these coefficients approach equality for large m in high multiplicity events, this may imply the existence of either solitonic solutions or Bose condensation either for the JIMWLK action that describes the CGC, or for the Glasma that might be produced in such a collision.

The Color Glass Condensate, Glasma and the Quark Gluon Plasma in the Context of Recent pPb Results from LHC

This paper describes some of the implications of the LHC pPb experimental results for understanding the Color Glass Condensate, the Glasma and the thermalized Quark Gluon Plasma. I attempt to outline the scientific issues, various possible explanations about what has been seen, and present ideas on how some of the alternative explanations might be resolved.

Production of photons and dileptons in the Glasma

We study the production of photons and dileptons during the pre-equilibrium Glasma stage in heavy ion collisions and discuss the implications in light of the PHENIX data. We find that the measured distributions of such electromagnetic emissions, while having some features not well understood if hypothesized to entirely arise from a thermalized Quark–Gluon Plasma, have some qualitative features that might be described after including effects from a thermalizing Glasma. The shape and centrality dependence of the transverse momentum spectra of the so-called “thermal photons” are well described. The mass and transverse momentum dependence of intermediate mass dileptons also agree with our estimates. The low transverse momenta from which the excessive dileptons (in low to intermediate mass region) arise is suggestive of emissions from a Bose condensate. We also predict the centrality dependence of dilepton production. Uncertainties in the current approach and improvements in the future are discussed.

Bjorken flow, plasma instabilities, and thermalization

At asymptotically high energies, thermalization in heavy ion collisions can be described via weak-coupling QCD. We present a complete treatment of how thermalization proceeds, at the parametric weak-coupling level. We show that plasma instabilities dominate the dynamics, from immediately after the collision until well after the plasma becomes nearly in equilibrium. Initially they drive the system close to isotropy, but Bjorken expansion and increasing diluteness makes the system again become more anisotropic. At time $ \tau \sim {\alpha^{{\frac{{ - 12}}{5}}}}Q_s^{{ - 1}} $ the dynamics become dominated by a nearly-thermal bath; and at time $ \tau \sim {\alpha^{{\frac{{ - 5}}{2}}}}Q_s^{{ - 1}} $ the bath comes to dominate the energy density, completing thermalization. After this time there is a nearly isotropic and thermal Quark-Gluon Plasma.

Heavy-quark production from relativistic heavy-ion collisions

We study the production of heavy quarks, charm at BNL-RHIC ( = 200 GeV/nucleon) and CERN-LHC ( = 5.5 TeV/nucleon) and bottom at CERN-LHC, from heavy ions colliding at relativistic energies. We consider initial fusion of gluons (and quark–anti-quark annihilation), pre-thermal parton interactions and interactions in a thermalized quark–gluon plasma. We also consider free-streaming partons as another extreme and compare the results with those from a thermalized plasma of partons. The pre-thermal contribution is calculated by considering the interaction among partons having large transverse momenta (jet–partons) after the initial interaction and from the passage of these partons through a thermalized quark–gluon plasma. Charm production from pre-thermal processes is found to be comparable to that from prompt (initial) interactions at LHC. It is suggested that this may have important implications for the study of the nuclear modification factor, RAA, as well as for the back-to-back correlation of heavy quarks and the production of dileptons having a large mass.

Photon production from a thermalized quark–gluon plasma: quantum kinetics and non-perturbative aspects

We study the production of photons from a quark–gluon plasma in local thermal equilibrium by introducing a non-perturbative formulation of the real time evolution of the density matrix. The main ingredient is the real time effective action for the electromagnetic field to O ( α em ) and to all orders in α s . The real time evolution is completely determined by the solution of a classical stochastic non-local Langevin equation which provides a Dyson-like resummation of the perturbative expansion. The Langevin equation is solved in closed form by Laplace transform in terms of the thermal photon polarization. A quantum kinetic description emerges directly from this formulation. We find that photons with k ≲ 200 MeV thermalize as plasmon quasiparticles in the plasma on time scales t ∼ 10 – 20 fm / c which is of the order of the lifetime of the QGP expected at RHIC and LHC. We then obtain the direct photon yield to lowest order in α em and to leading logarithmic order in α s in a uniform expansion valid at all time. The yield during a QGP lifetime t ∼ 10 fm / c is systematically larger than that obtained with the equilibrium formulation and the spectrum features a distinct flattening for k ≳ 2.5 GeV . We discuss the window of reliability of our results, the theoretical uncertainties in any treatment of photon emission from a QGP in LTE and the shortcomings of the customary S-matrix approach.

Entropy production at RHIC

For central heavy ion collisions at the RHIC energy, the entropy per unit rapidity dS/dy at freeze-out is extracted with minimal model dependence from available experimental measurements of particle yields, spectra, and source sizes estimated from two-particle interferometry. The extracted entropy rapidity density is consistent with lattice gauge theory results for a thermalized quark–gluon plasma with an energy density estimated from transverse energy production at RHIC.

(Strange) meson interferometry at RHIC

We make predictions for the kaon interferometry measurements in Au+Au collisions at the relativistic heavy ion collider (RHIC). A first-order phase transition from a thermalized quark–gluon plasma (QGP) to a gas of hadrons is assumed for the transport calculations. The fraction of kaons that are directly emitted from the phase boundary is considerably enhanced at large transverse momenta KT ∼ 1 GeV/c. In this kinematic region, the sensitivity of the Rout/Rside ratio to the QGP properties is enlarged. The results of the one-dimensional correlation analysis are presented. The extracted interferometry radii, depending on KT, are not unusually large and are strongly affected by finite momentum resolution effects.

Equilibrium distribution of heavy quarks in fokker-planck dynamics

We obtain an explicit generalization, within Fokker-Planck dynamics, of Einstein's relation between drag, diffusion, and the equilibrium distribution for a spatially homogeneous system, considering both the transverse and longitudinal diffusion for dimension n>1. We provide a complete characterization of the equilibrium distribution in terms of the drag and diffusion transport coefficients. We apply this analysis to charm quark dynamics in a thermal quark-gluon plasma for the case of collisional equilibration.