Jets in QCD media: From color coherence to decoherence

We investigate soft gluon radiation off a quark-antiquark antenna in both color singlet and octet configurations traversing a dense medium. We demonstrate that, in both cases, multiple scatterings lead to a gradual decoherence of the antenna radiation as a function of the medium density. In particular, in the limit of a completely opaque medium, total decoherence is obtained, i.e., the quark and the antiquark radiate as independent emitters in vacuum, thus losing memory of their origin.     

“q-hat” and jet quenching

We discuss bremsstrahlung rates for a weakly coupled fast jet propagating in dense nuclear matter. A time-dependent radiation rate which smoothly merges with previous limits is introduced, exhibiting a clear transition between two regimes after a traversed length of order one to several fermis. The different physical processes probed in the two regimes are discussed.     

The realistic QCD equation of state in relativistic heavy-ion collisions and the early Universe

The realistic equation of state of strongly interacting matter, that has been successfully applied in the recent hydrodynamic studies of hadron production in relativistic heavy-ion collisions at RHIC, is used in the Friedmann equation to determine the precise time evolution of thermodynamic parameters in the early Universe. A comparison with the results obtained with simple ideal-gas equations of state is made. The realistic equation of state describes a crossover rather than the first-order phase transition between the quark–gluon plasma and hadronic matter. Our numerical calculations show that small inhomogeneities of strongly interacting matter in the early Universe are moderately damped during such crossover.     

Jet quenching parameter from a soft wall AdS/QCD model

We study the effect of chemical potential and nonconformality on the jet quenching parameter in a holographic QCD model with conformal invariance broken by background dilaton. The presence of chemical potential and nonconformality both increase the jet quenching parameter, thus enhancing the energy loss, consistently with the findings of the drag force.     

Parton dynamics and hadronization from the sQGP

The hadronization of an expanding partonic fireball is studied within the Parton-Hadron-String Dynamics (PHSD) approach which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. Apart from strong parton interactions the expansion and development of collective flow is found to be driven by strong gradients in the parton mean-fields. An analysis of the elliptic flow v₂ demonstrates a linear correlation with the spatial eccentricity ? as in the case of ideal hydrodynamics. The hadronization occurs by quark-antiquark fusion or 3 quark/3 antiquark recombination which is described by covariant transition rates. Since the dynamical quarks become very massive, the formed resonant ‘pre-hadronic’ color-dipole states ( or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. This solves the entropy problem in hadronization in a natural way. Hadronic particle ratios turn out to be in line with those from a grand-canonical partition function at temperature T≈170 MeV.     

Finiteness of the Coulomb gauge QCD perturbative effective action

At 2-loop order in the Coulomb gauge, individual Feynman graphs contributing to the effective action have energy divergences. It is proved that these cancel in suitable combinations of graphs. This has previously been shown only for transverse external fields. The calculation results in a generalization of the Christ–Lee term which was inserted into the Hamiltonian.     

Asymptotics of QCD traveling waves with fluctuations and running coupling effects

Extending the Balitsky–Kovchegov (BK) equation independently to running coupling or to fluctuation effects due to pomeron loops is known to lead in both cases to qualitative changes of the traveling-wave asymptotic solutions. In this paper we study the extension of the forward BK equation, including both running coupling and fluctuations effects, extending the method developed for the fixed coupling case [E. Brunet, B. Derrida, A.H. Mueller, S. Munier, Phys. Rev. E 73 (2006) 056126, cond-mat/0512021]. We derive the exact asymptotic behavior in rapidity of the probabilistic distribution of the saturation scale.     

Collimation of average multiplicity in QCD jets

The collimation of average multiplicity inside quark and gluon jets is investigated in perturbative QCD in the modified leading logarithmic approximation (MLLA). The role of higher order corrections accounting for energy conservation and the running of the coupling constant leads to smaller multiplicity collimation as compared to leading logarithmic approximation (LLA) results. The collimation of jets produced in heavy-ion collisions has also been explored by using medium-modified splitting functions enhanced in the infrared sector. As compared to elementary collisions, the angular distribution of the jet multiplicity is found to broaden in QCD media at all energy scales.     

Effects of the formation time of parton shower on jet quenching in heavy-ion collisions

Jet quenching has successfully served as a hard probe to study the properties of Quark-Gluon Plasma (QGP). As a multi-particle system, jets require time to develop from a highly virtual parton to a group of partons close to mass shells. In this study, we present a systematical analysis on the effects of this formation time on jet quenching in relativistic nuclear collisions. Jets from initial hard scatterings were simulated with Pythia, and their interactions with QGP were described using a Linear Boltzmann Transport (LBT) model that incorporates both elastic and inelastic scatterings between jet partons and the thermal medium. Three different estimations of the jet formation time were implemented and compared, including instantaneous formation, formation from single splitting, and formation from sequential splittings, before which no jet-medium interaction was assumed. We found that deferring the jet-medium interaction with a longer formation time not only affects the overall magnitude of the nuclear modification factor of jets but also its dependence on the jet transverse momentum.     

Azimuthal angle and rapidity dependence of di-hadron correlations in QCD

I discuss novel QCD phenomena recently observed in p+p, p+A and A+A collisions, that result from the non-linear dynamics of small-x gluons. I focus on di-hadron correlation measurements, as opposed to single-hadron observables often too inclusive to distinguish possible new effects from established mechanisms. Specifically, I discuss angular correlations of forward di-hadrons in d+Au collisions and long-range rapidity correlations in high-multiplicity p+p and Au+Au collisions.     

The QCD critical point: an exciting Odyssey in the Femto-world

Strongly interacting matter, which makes up the nuclei of atoms, is described by a theory called quantum chromodynamics (QCD). A critical point in the phase diagram of QCD, if established either theoretically or experimentally, would be as profound a discovery as the familiar gas–liquid critical point discovered in the nineteenth century. Due to the extremely short-lived nature of the concerned phases, novel experimental techniques are needed to search for it. The Relativistic Heavy Ion Collider (RHIC) in USA has an experimental programme which can fit the bill to do so. Theoretical techniques of Lattice QCD, which is QCD defined on a discrete space-time lattice, have provided glimpses into where the QCD critical point may be, and how to search for it in the experimental data. A brief overview of the theoretical and experimental attempts is provided.     

Physics of strongly coupled quark-gluon plasma

This review covers our current understanding of strongly coupled Quark-Gluon Plasma (sQGP), especially theoretical progress in: (i) explaining the RHIC data by hydrodynamics; (ii) describing lattice data using electric-magnetic duality; (iii) understanding of gauge-string duality known as AdS/CFT and its application for “conformal” plasma. In view of the interdisciplinary nature of the subject, we include a brief introduction into several topics “for pedestrians”. Some fundamental questions addressed are: Why is sQGP such a good liquid? What is the nature of (de)confinement and what do we know about “magnetic” objects creating it? Do they play any important role in sQGP physics? Can we understand the AdS/CFT predictions, from the gauge theory side? Can they be tested experimentally? Can AdS/CFT duality help us understand rapid equilibration/entropy production? Can we work out a complete dynamical “gravity dual” to heavy ion collisions?     

Instability of the plasma oscillations in finite temperature perturbative QCD

We use linear-response dielectric theory to show that the baryon-poor QCD plasma based on the perturbative vacuum is unstable, even at a high temperature. If deconfinement occurs in nuclear collisions or the early universe, it is not accompanied by the restoration of the perturbative vacuum.     

Full jet tomography of high-energy nuclear collisions

Parton energy loss in the hot QCD medium will manifest itself not only in the leading hadron spectra but also in the reconstructed jet productions in high-energy nucleus-nucleus collisions. In this paper we report on recent theoretical efforts in studying full jet observables in relativistic heavy-ion collisions by discussing the modifications of jet shapes, inclusive jet cross section and the vector boson accompanied jet production in the presence of the QGP-induced jet quenching.     

Evaluating results from the Relativistic Heavy Ion Collider with perturbative QCD and hydrodynamics

We review the basic concepts of perturbative quantum chromodynamics (QCD) and relativistic hydrodynamics, and their applications to hadron production in high energy nuclear collisions. We discuss results from the Relativistic Heavy Ion Collider (RHIC) in light of these theoretical approaches. Perturbative QCD and hydrodynamics together explain a large amount of experimental data gathered during the first decade of RHIC running, although some questions remain open. We focus primarily on practical aspects of the calculations, covering basic topics like perturbation theory, initial state nuclear effects, jet quenching models, ideal hydrodynamics, dissipative corrections, freeze-out and initial conditions. We conclude by comparing key results from RHIC to calculations.     

The dimensionally reduced effective theory for quarks in high-temperature QCD

We show that QCD undergoes a partial dimensional reduction at high temperatures also in the quark sector. In the kinematic region relevant to screening physics, where the lowest Matsubara modes are close to their “mass shells”, all static Green functions involving both quarks and gluons are reproducible in the high-T limit by a renormalizable three-dimensional Lagrangian up to order ²(T) 1/1nT. This three-dimensional theory only contains explicitly the lightest bosonic and fermionic Matsubara modes, while the heavier modes correct the tree-level couplings and generate extra local vertices. We also find that the quark degrees of freedom that have been retained in the reduced theory are non-relativistic in the high-T limit. We then improve our result to order ⁴(T) through an explicit non-relativistic expansion, in the spirit of the heavy-quark effective theory. This effective theory is relevant for studying QCD screening phenomena with observables made from quarks, e.g. mesonic and baryonic currents, already at temperatures not much higher than the chiral transition temperature T_c.     

The dynamical holographic QCD method for hadron physics and QCD matter

In this paper we present a short overview on the dynamical holographic QCD (DhQCD) method for hadron physics and QCD matter. The five-dimensional DhQCD model is constructed in the graviton-dilaton-scalar framework with the dilaton background field Φ and the scalar field X dual to the gluon condensate and the chiral condensate operator thus can represent the gluodynamics (linear confinement) and chiral dynamics (chiral symmetry breaking), respectively. The dilaton background field and the scalar field are a function of the 5th dimension, which plays the role of the energy scale, in this way, the DhQCD model can resemble the renormalization group from ultraviolet (UV) to infrared (IR). By solving the Einstein equation, the metric structure at IR is automatically deformed by the nonperturbative gluon condensation and chiral condensation in the vacuum. We review the results on the hadron spectra including the glueball spectra, the light/heavy meson spectra, as well as on QCD phase transitions, and thermodynamical as well as transport properties in the framework of the DhQCD model.