non-Abelian Plasma Research Articles

Generalized gluon distribution for quarkonium dynamics in strongly coupled N=4 Yang-Mills theory

We study the generalized gluon distribution that governs the dynamics of quarkonium inside a non-Abelian thermal plasma characterizing its dissociation and recombination rates. This gluon distribution can be written in terms of a correlation function of two chromoelectric fields connected by an adjoint Wilson line. We formulate and calculate this object in N=4 supersymmetric Yang-Mills theory at strong coupling using the AdS/CFT correspondence, allowing for a nonzero center-of-mass velocity v of the heavy quark pair relative to the medium. The effect of a moving medium on the dynamics of the heavy quark pair is described by the simple substitution T→γT in agreement with previous calculations of other observables at strong coupling, where T is the temperature of the plasma in its rest frame, and γ=(1−v2)−1/2 is the Lorentz boost factor. Such a velocity dependence can be important when the quarkonium momentum is larger than its mass. Contrary to general expectations for open quantum systems weakly coupled with large thermal environments, the contributions to the transition rates that are usually thought of as the leading ones in Markovian descriptions vanish in this strongly coupled plasma. This calls for new theoretical developments to assess the effects of strongly coupled non-Abelian plasmas on in-medium quarkonium dynamics. Finally, we compare our results with those from weakly coupled QCD, and find that the QCD result moves toward the N=4 strongly coupled result as the coupling constant is increased within the regime of applicability of perturbation theory. This behavior makes it even more pressing to develop a non-Markovian description of quarkonium in-medium dynamics. Published by the American Physical Society 2024

Debye screening of non-Abelian plasmas in curved spacetimes

Debye screening of non-Abelian plasmas in curved spacetimes

Spacetime dynamics of chiral magnetic currents in a hot non-Abelian plasma

Spacetime dynamics of chiral magnetic currents in a hot non-Abelian plasma

Chiral Instabilities and the Fate of Chirality Imbalance in Non-Abelian Plasmas.

We present a microscopic study of chiral plasma instabilities and axial charge transfer in non-Abelian plasmas with a strong gauge-matter coupling g^{2}N_{f}=64, by performing 3+1D real-time classical-statistical lattice simulation with dynamical fermions. We explicitly demonstrate for the first time that-unlike in an Abelian plasma-the transfer of chirality from the matter sector to the gauge fields occurs predominantly due to topological sphaleron transitions. We elaborate on the similiarities and differences of the axial charge dynamics in cold Abelian U(1) and non-Abelian SU(2) plasmas, and comment on the implications of our findings for the study of anomalous transport phenomena, such as the chiral magnetic effect in QCD matter.

Thermalization of Weakly Coupled non-Abelian Plasmas at Next-to-leading Order

We employ the QCD kinetic theory, including next-to-leading(NLO) order corrections in coupling constant, to study the evolution of weakly coupled non-Abelian plasmas towards thermal equilibrium. For two characteristic far-from-equilibrium systems with either under- or over-occupied initial conditions, the NLO corrections remain well under control for a wide range of couplings, and the overall effect of NLO corrections is a reduction in the time required for thermalization.

Implications of the isobar-run results for the chiral magnetic effect in heavy-ion collisions

Chiral magnetic effect (CME) is a macroscopic transport phenomenon induced by quantum anomaly in the presence of chiral imbalance and an external magnetic field. Relativistic heavy ion collisions provide the unique opportunity to look for CME in a non-Abelian plasma, where the chiral imbalance is created by topological transitions similar to those occurring in the Early Universe. The isobar run at Relativistic Heavy Ion Collider was proposed as a way to separate the possible CME signal driven by magnetic field from the background. The first blind analysis results from this important experiment have been recently released by the STAR Collaboration. Under the pre-defined assumption of identical background in RuRu and ZrZr, the results are inconsistent with the presence of CME, as well as with all existing theoretical models (whether including CME or not). However the observed difference of backgrounds must be taken into account before any physical conclusion is drawn. In this paper, we show that once the observed difference in hadron multiplicity and collective flow are quantitatively taken into account, the STAR results could be consistent with a finite CME signal contribution of about $(6.8\pm2.6)\%$.

Stability analysis of nonthermal fixed points in longitudinally expanding kinetic theory

We use the Hamiltonian formulation of kinetic theory to perform a stability analysis of nonthermal fixed points in a non-Abelian plasma. We construct a perturbative expansion of the Fokker-Planck collision kernel in an adiabatic approximation and show that the (next-to-)leading order solutions reproduce the known nonthermal fixed point scaling exponents. Working at next-to-leading order, we derive the stability equations for scaling exponents and find the relaxation rate to the nonthermal fixed point. This approach provides the basis for an understanding of the prescaling phenomena observed in QCD kinetic theory and nonrelativistic Bose gas systems.

Spectral function of fermions in a highly occupied non-Abelian plasma

We develop a method to obtain fermion spectral functions non-perturbatively in a non-Abelian gauge theory with high occupation numbers of gauge fields. After recovering the free field case, we extract the spectral function of fermions in a highly occupied non-Abelian plasma close to its non-thermal fixed point, i.e., in a self-similar regime of the non-equilibrium dynamics. We find good agreement with hard loop perturbation theory for medium-induced masses, dispersion relations and quasiparticle residues. We also extract the full momentum dependence of the damping rate of the collective excitations.

Fermion and gluon spectral functions far from equilibrium

Motivated by the quark-gluon plasma, we develop a simulation method to obtain the spectral function of (Wilson) fermions non-perturbatively in a non-Abelian gauge theory with large gluon occupation numbers [1]. We apply our method to a non-Abelian plasma close to its non-thermal fixed point, i.e., in a far-from-equilibrium self-similar regime, and find mostly very good agreement with perturbative hard loop (HTL) calculations. For the first time, we extract the full momentum dependence of the damping rate of fermionic collective excitations and compare our results to recent non-perturbative extractions of gluonic spectral functions in two and three spatial dimensions [2, 3].

Heavy quark diffusion in an overoccupied gluon plasma

We extract the heavy-quark diffusion coefficient κ and the resulting momentum broadening 〈p2〉 in a far-from-equilibrium non-Abelian plasma. We find several features in the time dependence of the momentum broadening: a short initial rapid growth of 〈p2〉, followed by linear growth with time due to Langevin-type dynamics and damped oscillations around this growth at the plasmon frequency. We show that these novel oscillations are not easily explained using perturbative techniques but result from an excess of gluons at low momenta. These oscillation are therefore a gauge invariant confirmation of the infrared enhancement we had previously observed in gauge-fixed correlation functions. We argue that the kinetic theory description of such systems becomes less reliable in the presence of this IR enhancement.

The First fm/c of Heavy-Ion Collisions

We present an introductory review of the early-time dynamics of high-energy heavy-ion collisions and the kinetics of high-temperature quantum chromodynamic matter. The equilibration mechanisms in the quark–gluon plasma uniquely reflect the nonabelian and ultrarelativistic character of the many-body system. Starting with a brief exposé of the key theoretical and experimental questions, we provide an overview of the theoretical tools employed in weak coupling studies of the early-time nonequilibrium dynamics. We highlight theoretical progress in understanding different thermalization mechanisms in weakly coupled nonabelian plasmas, and discuss their relevance in describing the approach to local thermal equilibrium during the first fm/ c of a heavy-ion collision. We also briefly discuss some important connections to the phenomenology of heavy-ion collisions.

Spectral function for overoccupied gluodynamics from real-time lattice simulations

We study the spectral properties of a highly occupied non-Abelian non-equilibrium plasma appearing ubiquitously in weak coupling descriptions of QCD matter. The spectral function of this far-from-equilibrium plasma is measured by employing linear response theory in classical-statistical real-time lattice Yang-Mills simulations. We establish the existence of transversely and longitudinally polarized quasiparticles and obtain their dispersion relations, effective mass, plasmon frequency, damping rate and further structures in the spectral and statistical functions. Our new method can be interpreted as a non-perturbative generalization of hard thermal loop (HTL) effective theory. We see indications that our results approach leading order HTL in the appropriate limit. The method can also be employed beyond the range of validity of HTL.

Quarkonium dissociation in a far-from-equilibrium holographic setup

The real-time dissociation of the heavy quarkonium in a strongly coupled boost-invariant non-Abelian plasma relaxing towards equilibrium is analyzed in a holographic framework. The effects driving the plasma out of equilibrium are described by boundary quenching, impulsive variations of the boundary metric. Quarkonium is represented by a classical string with endpoints kept close to the boundary. The evolution of the string profile is computed in the time-dependent geometry, and the dissociation time is evaluated for different configurations with respect to the direction of the plasma expansion. Dissociation occurs fastly for the quarkonium placed in the transverse plane.

Nonlocal wave turbulence in non-Abelian plasmas

We investigate driven wave turbulence in non-Abelian plasmas, in the framework of kinetic theory where both elastic and inelastic processes are considered in the small angle approximation. The gluon spectrum, that forms in the presence of a steady source, is shown to be controlled by nonlocal interactions in momentum space, in contrast to the universal Kolmogorov–Zakharov spectra. Assuming strongly nonlocal interactions, we show that inelastic processes are dominant in the IR and cause a thermal bath to form below the forcing scale, as a result of a detailed balance between radiation and absorption of soft gluons by the hard ones. Above the forcing scale, the inelastic collision term reduces to an inhomogeneous diffusion-like equation yielding a spectrum that spreads to the UV as t1/2, similarly to elastic processes. Due to nonlocal interactions the non-universal turbulent spectrum is not steady and flattens when time goes on toward the thermal distribution. This analysis is complemented by numerical simulations, where we observe that in the explored time interval the spectral exponent of the nonlocal turbulent cascade is close to −2 in agreement with simulations of classical Yang–Mills equations.

Universal Self-Similar Scaling of Spatial Wilson Loops Out of Equilibrium.

We investigate strongly correlated non-Abelian plasmas out of equilibrium. Based on numerical simulations, we establish a self-similar scaling property for the time evolution of spatial Wilson loops that characterizes a universal state of matter far from equilibrium. Most remarkably, it exhibits a generalized area law which holds for a sufficiently large ratio of spatial area and fractional power of time. Performing calculations also for the perturbative regime at higher momenta, we are able to characterize the full nonthermal scaling properties of SU(2) and SU(3) symmetric plasmas from short to large distance scales in terms of two independent universal exponents and associated scaling functions.

Holographic isotropisation in Gauss-Bonnet gravity

We study holographic isotropisation of homogeneous, strongly coupled, non-Abelian plasmas in Gauss-Bonnet gravity with a negative cosmological constant. We focus on small values of the Gauss-Bonnet coupling parameter λGB and linearise the equations of motion around a time-dependent background solution with λGB = 0. We numerically solve the linearised equations and show that the entire time evolution of the pressure anisotropy can be well approximated by the linear in λGB corrections to the quasinormal mode expansion, even in the cases of high anisotropy. We finally show that, quite generally, the time evolution of the pressure anisotropy with the Gauss-Bonnet term is approximately shifted with respect to the evolution without it, with the sign of the shift being directly related to the sign of the λGB parameter. Combined with the observation that negative λGB captures qualitative features of positive gauge coupling corrections, this suggests that the latter generically increase the isotropisation time of strongly coupled plasmas.

Momentum transport in strongly coupled anisotropic plasmas in the presence of strong magnetic fields

We present a holographic perspective on momentum transport in strongly coupled, anisotropic non-Abelian plasmas in the presence of strong magnetic fields. We compute the anisotropic heavy quark drag forces and Langevin diffusion coefficients and also the anisotropic shear viscosities for two different holographic models, namely, a top-down deformation of strongly coupled $\mathcal{N} = 4$ Super-Yang-Mills (SYM) theory triggered by an external Abelian magnetic field, and a bottom-up Einstein-Maxwell-dilaton (EMD) model which is able to provide a quantitative description of lattice QCD thermodynamics with $(2+1)$-flavors at both zero and nonzero magnetic fields. We find that, in general, energy loss and momentum diffusion through strongly coupled anisotropic plasmas are enhanced by a magnetic field being larger in transverse directions than in the direction parallel to the magnetic field. Moreover, the anisotropic shear viscosity coefficient is smaller in the direction of the magnetic field than in the plane perpendicular to the field, which indicates that strongly coupled anisotropic plasmas become closer to the perfect fluid limit along the magnetic field. We also present, in the context of the EMD model, holographic predictions for the entropy density and the crossover critical temperature in a wider region of the $(T,B)$ phase diagram that has not yet been covered by lattice simulations. Our results for the transport coefficients in the phenomenologically realistic magnetic EMD model could be readily used as inputs in numerical codes for magnetohydrodynamics.

Role of nonlocal probes of thermalization for a strongly interacting non-Abelian plasma

We use a holographic method to investigate thermalization of a boost-invariant strongly interacting non-Abelian plasma. Boundary sourcing, a distorsion of the boundary metric, is employed to drive the system far from equilibrium. Thermalization is analyzed through nonlocal probes: the equal-time two-point correlation function of large conformal dimension operators in the boundary theory, and Wilson loops of different shapes. We study the dependence of the thermalization time on the size of the probes, and compare the results to the ones obtained using local observables: the onset of thermalization is first observed at short distances.

Boson-Jet Correlations in a Hybrid Strong/Weak Coupling Model for Jet Quenching in Heavy Ion Collisions

We confront a hybrid strong/weak coupling model for jet quenching to data from LHC heavy ion collisions. The model combines the perturbative QCD physics at high momentum transfer and the strongly coupled dynamics of nonabelian gauge theories plasmas in a phenomenological way. By performing a full Monte Carlo simulation, and after fitting one single parameter, we successfully describe several jet observables at the LHC, including dijet and photon jet measurements. Within current theoretical and experimental uncertainties, we find that such observables show little sensitivity to the specifics of the microscopic energy loss mechanism. We also present a new observable, the ratio of the fragmentation function of inclusive jets to that of the associated jets in dijet pairs, which can discriminate among different medium models. Finally, we discuss the importance of plasma response to jet passage in jet shapes.

Off-equilibrium sphaleron transitions in the glasma

We perform first classical-statistical real time lattice simulations of topological transitions in the non-equilibrium Glasma of weakly coupled but highly occupied gauge fields created immediately after the collision of ultra-relativistic nuclei. Simplifying our description by employing SU(2) gauge fields, and neglecting their longitudinal expansion, we find that the rate of topological transitions is initially strongly enhanced relative to the thermal sphaleron transition rate and decays with time during the thermalization process. Qualitative features of the time dependence of this non-equilibrium transition rate can be understood when expressed in terms of the magnetic screening length, which we also extract non-perturbatively. A detailed investigation of auto-correlation functions of the Chern-Simons number ($N_{CS}$) reveals non-Markovian features of the evolution distinct from previous simulations of non-Abelian plasmas in thermal equilibrium.