Collective thermalization in quark gluon plasma

A very important question in ultrarelativistic heavy ion collisions is that of thermalization of the high energy density quark gluon plasma forud in the central rapidity region. Different approaches have been adopted by various authors to study this thermalization problem. These include phenomenological string and capacitor plate models, perturbative QCD based parton cascade models and the classical non-perturbative approach. In this paper we briefly review the earlier studies and discuss our work which emphasizes the role of non-perturbative collective effects (classical chaos) in the thermalization of the plasma. In particular, using classical equations of motion of a coloured parton in self-consistent colour fields, we have carried out a 1+1 dimensional simulation of coloured partonic matter. We find that in certain parameter domains, the system exhibits chaotic behaviour in non-abelian plasma oscillations, which then leads to thermalization of the plasma.     

Electromagnetic signals of quark gluon plasma

Successive equilibration of quark degrees of freedom and its effects on electromagnetic signals of quark gluon plasma are discussed. The effects of the variation of vector meson masses and decay widths on photon production from hot strongly interacting matter formed after Pb + Pb and S + Au collisions at CERN SPS energies are considered. It has been shown that the present photon spectra measured by WA80 and WA98 Collaborations can not distinguish between the formation of quark matter and hadronic matter in the initial state.     

Strangeness production and evolution in quark gluon plasma

The formation and evolution of strange quarks in quark gluon plasma is studied assuming perturbative QCD and qualitative models of plasma phase expansion. Chemical equilibrium abundance characteristic of the hottest and densest phase of nuclear collisions is proven to survive the process of expansion and cooling of the plasma.     

Status of the quark gluon plasma search

A selection of results are discussed that support the conclusion that strongly interacting quark gluon plasma is produced in heavy-ion collisions at the Relativistic Heavy Ion Collider at BNL.     

Strong interacting probes for quark gluon plasma

We discuss hadronic signals for quark gluon plasma formation in relativistic nuclear collisions utilizing a non-equilibrium model for hadronisation of the plasma state. In particular, we find that several (non-)strange antibaryon to baryon ratios may serve as a signal for a baryon-rich plasma state. The? toη ratio turns out to be not a direct signal for plasma formation due to its dependence on details of the collision dynamics and hadronisation mechanism. Finally we argue that the η′ to η ratio might be useful as a “gluonometer” in measuring the gluonic content of the matter formed in the initial stage of the collision process.     

On diquark clustering in quark—gluon plasma

The possibility that pairs of quarks will form diquark clusters in the regime above deconfinement transition for hadron matter at finite density is revisited. Here we present the results on the diquark-diquark (dq-dq) interaction in the framework of constituent quark model taking account of spin, isospin and color degrees of freedom in the spirit of generalized Pauli principle. By constructing the appropriate spin and color states of the dq-dq clusters we compute the expectation values of the interaction Hamiltonian involving pairwise quark—quark interaction. We find that the effective interaction between two diquark clusters is quite sensitive to different configurations characterized by color and spin states, obtained after the coupling of two diquark states. The value of the coupling parameter for a particular color—spin state, i.e., -3, 1 is compared to the one obtained earlier by Donoghue and Sateesh,Phys. Rev. D38, 360 (1988) based on the effective Φ⁴-theory. This new value of λ derived for different color-spin dq-dq states, may lead to several important implications in the studies of diquark star and diquark gas.     

Stability of quark gluon plasma to nielsen-olesen mode

Nielsen and Olesen showed that perturbative vacuum with uniform chromomagnetic field in one space and one color direction is unstable. This instability is called Nielsen-Olesen instability (NOI), and leads to formation of a ‘spaghetti of flux tubes’ as a model for non-perturbative vacuum and confinement. We re-examine this instability in presence of color sources, quarks and gluons, at a finite temperature and find that at sufficiently high temperature NOI is stabilized due to an ‘effective mass’ of gluons arising through plasma effects. This explains how a QGP with no confinement effects may exist at high temperature. As the temperature is lowered, NOI reappears at a valueT=T _c, which is very close to confinement-deconfinement transition from hadrons to QGP..     

The extent of strangeness equilibration in quark gluon plasma

The evolution and production of strangeness from chemically equilibrating and transversely expanding quark gluon plasma which may be formed in the wake of relativistic heavy-ion collisions is studied with initial conditions obtained from the self screened parton cascade (SSPC) model. The extent of partonic equilibration increases almost linearly with the square of the initial energy density, which can then be scaled with the number of participants.     

Properties of quark gluon plasma from lattice calculations

I discuss lattice QCD calculations of the properties of strongly interacting matter at finite temperature, including the determination of the transition temperature T_c, equation of state, different static screening lengths and quarkonium spectral functions. The lattice data suggest that at temperatures above 2.0T_c many properties of the quark gluon plasma can be understood using weak coupling approach, although non-perturbative effects due to static magnetic fields are significant in some quantities.     

Photons from quark gluon plasma and hot hadronic matter

The productions of real photons from quark gluon plasma and hot hadronic matter formed after the nucleus-nucleus collisions at ultra-relativistic energies are discussed. The effects of the spectral shift of the hadrons at finite temperature on the production of photons are investigated. On the basis of the present analysis it is shown that the photon spectra measured by WA98 collaboration in Pb + Pb collisions at CERN SPS energies can be explained by both QGP as well as hadronic initial states if the spectral shift of hadrons at finite temperature is taken into account. Several other works on the analysis of WA98 photon data have also been briefly discussed.     

Comments on quasiparticle models of quark–gluon plasma

Here we comment on the thermodynamic inconsistency problem and the reformulation of statistical mechanics of widely studied quasiparticle models of quark–gluon plasma. Their starting relation, the expression for pressure itself is a wrong choice and lead to thermodynamic inconsistency and the requirements of the reformulation of statistical mechanics. We propose a new approach to the problem using the standard statistical mechanics and is thermodynamically consistent. We also show that the other quasiparticle models may be obtained from our general formalism as a special case under certain restrictive condition. Further, as an example, we have applied our model to explain the nonideal behaviour of gluon plasma and obtained a remarkable good fit to the lattice results by adjusting just a single parameter.     

Photon production from quark gluon plasma at finite baryon density

The photon yield from a baryon-rich quark gluon plasma (QGP) at SPS energy has been estimated. In the QGP phase, rate of photon production is evaluated up to two-loop level. In the hadron phase, dominant contribution from π,ρ, ω mesons has been considered. The evolution of the plasma has been studied with appropriate equation of state in both QGP and hadron phase for a baryon-rich system. At SPS energy, the total photon yield is found to increase marginally in the presence of baryon density.     

Revisiting the quasi-particle model of the quark–gluon plasma

The quasi-particle model of the quark–gluon plasma (QGP) is revisited here with a new method, different from earlier studies, one without the need of a temperature dependent bag constant and other effects such as confinement, effective degrees of freedom etc. Our model has only one system dependent parameter and shows a surprisingly good fit to the lattice results for the gluon plasma, and for 2-flavor, 3-flavor and (2+1)-flavor QGP. The basic idea is first to evaluate the energy density ε from the grand partition function of quasi-particle QGP, and then derive all other thermodynamic functions from ε. Quasi-particles are assumed to have a temperature dependent mass equal to the plasma frequency. Energy density, pressure and speed of sound at zero chemical potential are evaluated and compared with the available lattice data. We further extend the model to a finite chemical potential, without any new parameters, to obtain the quark density, quark susceptibility etc., and the model fits very well with the lattice results on 2-flavor QGP. PACS 12.38.Mh; 12.38.Gc; 05.70.Ce; 52.25.Kn     

Diphoton production from quark–gluon plasma and hadronic matter

The rate of diphoton production is calculated from a quark–gluon plasma and compared with the rate from hadronic matter. Background diphotons are identified and quantified, including effects from hadronic form factors. Robust thermometry and spectroscopy are shown to be possible owing to the structure of the expected invariant mass spectrum. Charmonium could also be probed with diphotons. PACS 25.75.-q; 13.85.Qk     

Jet quenching from soft QCD scattering in the quark–gluon plasma

We show that partons traversing a quark–gluon plasma can lose substantial amounts of energy also by scatterings, and not only through medium-induced radiation as mainly considered previously. Results from Monte Carlo simulations of soft interactions of partons, emerging from a hard scattering, through multiple elastic scatterings on gluons in an expanding relativistic plasma show a sizeable jet quenching which can account for a substantial part of the effect observed in RHIC data.     

The quark gluon plasma: Lattice computations put to experimental test

I describe how lattice computations are being used to extract experimentally relevant features of the quark gluon plasma. I deal specifically with relaxation times, photon emissivity, strangeness yields, event-by-event fluctuations of conserved quantities and hydrodynamic flow. Finally I give evidence that the plasma is rather liquid-like in some ways.     

Time dependence of theJ/Ψ suppression in the quark gluon plasma

The time dependent survival probability of theJ/Ψ, embedded in the medium of quark gluon plasma has been studied. The screening as well as the dissipative effects in the dissolution ofJ/Ψ in QGP have been described through an optical model potential.