Dense Quark-gluon Plasma Research Articles

Vacuumlike Jet Fragmentation in a Dense QCD Medium.

We study the fragmentation of a jet propagating in a dense quark-gluon plasma. Using a leading, double-logarithmic approximation in perturbative QCD, we compute for the first time the effects of the medium on multiple vacuumlike emissions. We show that, due to the scatterings off the plasma, the in-medium parton showers differ from the vacuum ones in two crucial aspects: their phase-space is reduced and the first emission outside the medium can violate angular ordering. We compute the jet fragmentation function and find results in qualitative agreement with LHC measurements.

Enhancement of Elliptic Flow of π− under Intense Magnetic Field in √ sNN = 200 GeV Au+Au Collisions: A (2 + 1)-Dimensional Reduced-MHD Model Study

We investigate the effect of intense magnetic fields on the ( 2 + 1 ) -dimensional reduced- magnetohydrodynamical (MHD) expansion of hot and dense quark–gluon plasma (QGP) produced in s NN = 200 GeV Au+Au collisions. For the sake of simplicity, we consider the case in which the magnetic field points in the direction perpendicular to the reaction plane. We also consider this field to be external, with energy density parametrized as a two-dimensional Gaussian. The width of the Gaussian along the directions orthogonal to the beam axis varies with the centrality of the collision. The dependence of the magnetic field on proper time ( τ ) is parametrized for the case of zero and finite electrical conductivity of the QGP. We solve the equations of motion of ideal hydrodynamics for such an external magnetic field. For collisions with a non-zero impact parameter we observe a considerable increase in the elliptic-flow coefficient v 2 of π − in the presence of an external magnetic field, and the increment in v 2 is found to depend on the evolution and the initial magnitude of the magnetic field.

Jet modifications in event-by-event hydrodynamically evolving media

The nearly perfect fluid-like nature of the Quark Gluon Plasma may be understood through two key experimental signatures: collective flow and jet suppression. Event-by-event relativistic viscous hydrodynamics (with an extremely small shear viscosity to entropy density ratio) has been very successful at describing collective flow observables for the last 7 years. More recently, the effects of event-by-event fluctuations have been studied in the context of high pT particles that lose energy as they pass through the dense Quark Gluon Plasma liquid. In this summary of the corresponding plenary talk at Quark Matter 2017, the recent developments on the effects of event-by-event fluctuations on jet suppression are summarized.

Event-by-event picture for the medium-induced jet evolution

We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-democratic, with offspring gluons carrying sizable fractions of the energy of their parent parton. We find large fluctuations in the energy loss and in the multiplicity of soft gluons. The multiplicity distribution is predicted to exhibit KNO (Koba-Nielsen-Olesen) scaling. These predictions can be tested in Pb+Pb collisions at the LHC, via event-by-event measurements of the di-jet asymmetry.Based on [1, 2].

Event-by-event picture for the medium-induced jet evolution

We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-democratic, with offspring gluons carrying sizable fractions of the energy of their parent parton. We find large fluctuations in the energy loss and in the multiplicity of soft gluons. The multiplicity distribution is predicted to exhibit KNO (Koba-Nielsen-Olesen) scaling. These predictions can be tested in Pb+Pb collisions at the LHC, via event-by-event measurements of the di-jet asymmetry. Based on JHEP 1605 (2016) 008 and arXiv:1609.06104

Energy loss, equilibration, and thermodynamics of a baryon rich strongly coupled quark-gluon plasma

Lattice data for the QCD equation of state and the baryon susceptibility near the crossover phase transition (at zero baryon density) are used to determine the input parameters of a 5-dimensional Einstein-Maxwell-Dilaton holographic model that provides a consistent holographic framework to study both equilibrium and out-of-equilibrium properties of a hot and {\it baryon rich} strongly coupled quark-gluon plasma (QGP). We compare our holographic equation of state computed at nonzero baryon chemical potential, $\mu_B$, with recent lattice calculations and find quantitative agreement for the pressure and the speed of sound for $\mu_B \leq 400$ MeV. This holographic model is used to obtain holographic predictions for the temperature and $\mu_B$ dependence of the drag force and the Langevin diffusion coefficients associated with heavy quark jet propagation as well as the jet quenching parameter $\hat{q}$ and the shooting string energy loss of light quarks in the baryon dense plasma. We find that the energy loss of heavy and light quarks generally displays a nontrivial, fast-varying behavior as a function of the temperature near the crossover. Moreover, energy loss is also found to generally increase due to nonzero baryon density effects even though this strongly coupled liquid cannot be described in terms of well defined quasiparticle excitations. Furthermore, to get a glimpse of how thermalization occurs in a hot and baryon dense QGP, we study how the lowest quasinormal mode of an external massless scalar disturbance in the bulk is affected by a nonzero baryon charge. We find that the equilibration time associated with the lowest quasinormal mode decreases in a dense medium.

Jet quenching in high-energy heavy-ion collisions

Jet quenching in high-energy heavy-ion collisions can be used to probe properties of hot and dense quark–gluon plasma. We provide a brief introduction to the concept and framework for the study of jet quenching. Different approaches and implementation of multiple scattering and parton energy loss are discussed. Recent progresses in the theoretical and phenomenological studies of jet quenching in heavy-ion collisions at RHIC and LHC are reviewed.

Estimation of the shear viscosity at finite net-baryon density fromA+Acollision data atsNN=7.7–200GeV

Hybrid approaches based on relativistic hydrodynamics and transport theory have been successfully applied for many years for the dynamical description of heavy-ion collisions at ultrarelativistic energies. In this work a new viscous hybrid model employing the hadron transport approach UrQMD for the early and late nonequilibrium stages of the reaction, and 3+1 dimensional viscous hydrodynamics for the hot and dense quark-gluon plasma stage, is introduced. This approach includes the equation of motion for finite baryon number and employs an equation of state with finite net-baryon density to allow for calculations in a large range of beam energies. The parameter space of the model is explored and constrained by comparison with the experimental data for bulk observables from Super Proton Synchrotron and the phase I beam energy scan at Relativistic Heavy Ion Collider. The favored parameter values depend on energy but allow extraction of the effective value of the shear viscosity coefficient over entropy density ratio $\ensuremath{\eta}/s$ in the fluid phase for the whole energy region under investigation. The estimated value of $\ensuremath{\eta}/s$ increases with decreasing collision energy, which may indicate that $\ensuremath{\eta}/s$ of the quark-gluon plasma depends on baryochemical potential ${\ensuremath{\mu}}_{B}$.

Equation of state of hot and dense QCD: resummed perturbation theory confronts lattice data

We perform a detailed analysis of the predictions of resummed perturbation theory for the pressure and the second-, fourth-, and sixth-order diagonal quark number susceptibilities in a hot and dense quark-gluon plasma. First, we present an exact one-loop calculation of the equation of state within hard-thermal-loop perturbation theory (HTLpt) and compare it to a previous one-loop HTLpt calculation that employed an expansion in the ratios of thermal masses and the temperature. We find that this expansion converges reasonably fast. We then perform a resummation of the existing four-loop weak coupling expression for the pressure, motivated by dimensional reduction. Finally, we compare the exact one-loop HTLpt and resummed dimensional reduction results with state-of-the-art lattice calculations and a recent mass-expanded three-loop HTLpt calculation.

Possibility of event shape selection in relativistic heavy ion collisions

We investigate the possibility of selecting heavy ion collision events with certain features in the initial state (event engineering). Anisotropic flow measurements in heavy ion reactions have confirmed the almost ideal fluid dynamical behavior of the hot and dense quark-gluon plasma state. As a consequence, it is intriguing to pursue the idea of selecting collisions with a certain special initial geometry, e.g., a large ellipsoidal or triangular deformation, by classifying events by the value of their final observed flow coefficients. This procedure could be especially interesting for azimuthally dependent jet energy loss studies. We investigate the correlation between initial state features and final state momentum space anisotropies within an event-by-event hybrid approach. We find that the finite particle number and hadronic rescattering of the final state leads to large event-by-event fluctuations in the observables that could be used to characterize the features of the initial state. This makes event engineering by final state selection difficult.

Quantifying Initial State Fluctuations in Heavy Ion Collisions

Author(s): Petersen, H; Coleman-Smith, CE; Wolpert, RL | Abstract: Quantum fluctuations induce interesting structures in the initial state profiles for hydrodynamic calculations of the hot and dense quark-gluon plasma phase of heavy ion collisions. Especially, after the discovery of non-zero odd higher flow coefficients, there has been a lot of theoretical effort to quantify the correlation lengths of energy density fluctuations. A new method to characterize initial state profiles based on a 2D Fourier analysis is presented and an outlook on how this analysis can lead to a better quantitative understanding of the properties of hot and dense nuclear matter is given.

Structure of the ground state of a nonsuperfluid dense quark-gluon plasma

The single-particle spectrum and momentum distribution of quasiparticles in a cold dense quark-gluon plasma are calculated within the Fermi liquid approach. It is shown that this system does not behave as a standard Fermi liquid: at zero temperature, the single-particle spectrum has a plateau at the Fermi surface, while the Fermi surface itself has a nonzero volume in momentum space.

A systematic study of direct photon production in heavy ion collisions

A theoretical derivation of photon bremsstrahlung, induced by the interactions of an energetic quark in a hot and dense quark–gluon plasma, is given in the framework of the reaction operator approach. For the physically relevant case of hard jet production, followed by few in-medium interactions, we find that the Landau–Pomeranchuk–Migdal suppression of the bremsstrahlung photon intensity is much stronger than in the previously discussed limit of on-shell quarks and a large number of soft scatterings. This result is incorporated in the first systematic study of direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion Collider at center of mass energies s=62.4 GeV and 200 GeV. We find that the contribution of the photons created via final-state interactions is limited to 35% for 2 GeV<pT<5 GeV and at high transverse momenta the modification of the direct photon cross section is dominated by initial-state cold nuclear matter effects.

How a quark–gluon plasma phase modifies the bounds on extra dimensions from SN1987a

The shape of the neutrino pulse from the supernova SN1987a provides one of the most stringent constraints on the size of large, compact, “gravity-only” extra dimensions. Previously, calculations have been carried out for a newly-born proto-neutron star with a temperature of about 50 MeV at nuclear matter density. It is arguable that, due to the extreme conditions in the interior of the star, matter might be a quark–gluon plasma, where the relevant degrees of freedom are quarks and gluons rather than nucleons. We consider an energy-loss scenario where seconds after rebounce the core of the star consists of a hot and dense quark–gluon plasma. Adopting a simplified model of the plasma we derive the necessary energy-loss formulae in the soft-radiation limit. The emissivity is found to be comparable to the one for nuclear matter and bounds on the radius of extra dimensions are similar to those found previously from nuclear matter calculations.

Non-Fermi liquid behavior, the Becchi-Rouet-Stora-Tyutin identity in the dense quark-gluon plasma, and color superconductivity

At sufficiently high baryon densities, the physics of a dense quark-gluon plasma may be investigated through the tools of perturbative QCD. This approach has recently been successfully applied to the study of color superconductivity, where the dominant di-quark pairing interaction arises from one gluon exchange. Screening in the plasma leads to novel behaviour, including a remarkable non-BCS scaling of T_C, the transition temperature to the color superconducting phase. Radiative corrections to one gluon exchange were previously considered and found to affect T_C. In particular, the quark self-energy in a plasma leads to non-Fermi liquid behaviour and suppresses T_C. However, at the same time, the quark-gluon vertex was shown not to modify the result at leading order. This dichotomy between the effects of the radiative corrections at first appears rather surprising, as the BRST identity connects the self-energy to the vertex corrections. Nevertheless, as we demonstrate, there is in fact no contradiction with the BRST identity, at least to leading log order. This clarifies some of the previous statements on the importance of the higher order corrections to the determination of T_C and the zero temperature gap in color superconductivity.

Effect of the gluon damping rate on the viscosity coefficient of the quark-gluon plasma at finite chemical potential

By considering the Debye screening and the damping rate of gluons, the viscosity coefficient of the quark-gluon plasma was evaluated via real-time finite temperature QCD in the relaxation time approximation at finite temperature and chemical potential. The results show that both the damping rate and the quark chemical potential cause considerable enhancements to the viscosity coefficient of the hot dense quark-gluon plasma.

Fermi Excitations in Hot and Dense Quark–Gluon Plasma

The Fermi excitations in hot and dense quark–gluon plasma are studied in the Feynman gauge using the temperature Green function technique. We find four well-separated spectrum branches for all |q| in the case m=0 and establish the additional splitting between them (four different effective masses) when m≠0. The long wavelength limit of these excitations is found in the general case of the massive fermions at finite temperature and baryon densities to give the exact one-loop spectrum. This spectrum exhibits many known results as its different limits.