Thermalization In Heavy-ion Collisions Research Articles

Can Isotropization and Thermalization in Heavy Ion Collisions be Obtained from Summing Feynman Diagrams?

We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic heavy ion collision does not follow from Feynman diagram analysis to all orders in the coupling constant. We conclude that the apparent thermalization of quarks and gluons, leading to success of Bjorken hydrodynamics in describing heavy ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams.

A modified “bottom-up” thermalization in heavy-ion collisions

In the initial stage of the bottom-up picture of thermalization in heavy ion collisions, the gluon distribution is highly anisotropic which can give rise to plasma instability. This has not been taken account in the original paper. It is shown that in the presence of instability there are scaling solutions, which depend on one parameter, that match smoothly onto the late stage of bottom-up when thermalization takes place.

Isotropization and thermalization in heavy-ion collisions

We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic heavy-ion collision does not follow from Feynman diagram analysis to any order in the coupling constant. We conclude that the apparent thermalization of quarks and gluons, leading to success of perfect fluid hydrodynamics in describing heavy-ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams. We proceed by modeling these non-pertrubative thermalization effects using viscous hydrodynamics. We point out that matching Color Glass Condensate inital conditions with viscous hydrodynamics leads to a continuous evolution of all the components of the energy-momentum tensor and, unlike the case of ideal hydrodynamics, does not give rise to a discontinuity in the longitudinal pressure. An important consequence of such a matching is a relationship between the thermalization time and shear viscosity: we observe that small viscosity leads to short thermalization time.

THE STUDY OF PARTICIPANT-SPECTATOR MATTER AND COLLISION DYNAMICS IN HEAVY-ION COLLISIONS

We present the simulations of heavy-ion collisions in terms of participant-spectator matter. We find that this matter depends crucially on the collision dynamics and history of the nucleons. The important changes in the momentum space are due to the binary nucleon-nucleon collisions experienced during the high dense phase. This was otherwise not possible with mean field alone. The collisions push the colliding nucleons into midrapidity region responsible for the formation of participant matter. This ultimately leads to thermalization in heavy-ion collisions.

The impact of QCD plasma instabilities on bottom-up thermalization

QCD plasma instabilities, caused by an anisotropic momentum distributions of the particles in the plasma, are likely to play an important role in thermalization in heavy ion collisions. We consider plasmas with two different components of particles, one strongly anisotropic and one isotropic or nearly isotropic. The isotropic component does not eliminate instabilities but it decreases their growth rates. We investigate the impact of plasma instabilities on the first stage of the ``bottom-up'' thermalization scenario in which such a two-component plasma emerges, and find that even in the case of non-abelian saturation instabilities qualitatively change the bottom-up picture.

Can thermalization in heavy ion collisions be described by QCD diagrams?

The onset of thermalization in heavy ion collisions in the weak coupling framework can be viewed as a transition from the initial state Color Glass Condensate dynamics, characterized by the energy density scaling like ε ∼ 1 / τ with τ the proper time, to the hydrodynamics-driven expansion of the quark–gluon plasma with ε ∼ 1 / τ 4 / 3 (or higher power of 1 / τ for the boost non-invariant case). We argue that, at any order of the perturbative expansion in the QCD coupling constant, the gluon field generated in an ultrarelativistic heavy ion collision leads to energy density scaling as ε ∼ 1 / τ for late times τ ≫ 1 / Q s . Therefore, it is likely that thermalization and hydrodynamic description of the gluon system produced in heavy ion collisions cannot result from perturbative QCD diagrams at these late times. At earlier times with τ ∼ 1 / Q s the subleading corrections to ε in 1 / τ expansion (terms scaling like ∼ 1 / τ 1 + Δ with Δ > 0 ) may become important possibly leading to hydrodynamic-like behavior of the gluon system. Still, we show that such corrections do not contribute to the particle production cross section, and are likely to be irrelevant for physical observables. We generalize our results by including massless quarks into the system. Thus, it appears that the apparent thermalization of quarks and gluons, leading to success of Bjorken hydrodynamics in describing heavy ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects.

Some aspects of the quark gluon plasma physics

Abstract Focusing on two aspects of quark gluon plasma (QGP) physics: thermalization in heavy ion collisions and quenching of particle spectra in dense media, we shall try to give convincing arguments showing that at high enough energies (already at RHIC?) heavy ion collisions provide tests of the theory.

“Bottom-up” thermalization in heavy ion collisions

We describe how thermalization occurs in heavy ion collisions in the framework of perturbative QCD. When the saturation scale Q s is large compared to Λ QCD, thermalization takes place during a time of order α −13/5 Q s −1 and the maximal temperature achieved is α 2/5 Q s.

The collision integral in nuclear matter at zero temperature

The collision rate in an infinite Fermi system having a deformed Fermi surface is computed in the limit of small deformation. For a given amount of internal energy, the damping rate is nearly independent of temperature. The calculation is applied to thermalization in heavy ion collisions. We find that the collisional damping time is comparable to the duration of the collision for medium-weight nuclei. Thus the predictions of mean field theory, such as the presence of a fusion window at small impact parameters, will only have validity for lighter ions.