## 12.38.Mh Quark-gluon plasma (see also 25.75.Nq Quark deconfinement, quark-gluon plasma production and phase transitions in relativistic heavy ion collisions; see also 21.65.Qr Quark matter)

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Within a dynamical quark recombination model, we explore various proposed event-by-event observables sensitive to the microscopic structure of the QCD-matter created at RHIC energies. Charge ratio fluctuations, charge transfer fluctuations and baryon-strangeness correlations are computed from a sample of central Au + Au events at the highest RHIC energy available (sNN=200 GeV). We find that for all explored observables, the calculations yield the values predicted for a quark–gluon plasma only at early times of the evolution, whereas the final state approaches the values expected for a hadronic gas. We argue that the recombination-like hadronization process itself is responsible for the disappearance of the predicted deconfinement signals. This might explain why no fluctuation signatures for the transition between quark and hadronic matter was ever observed in the experimental data up to now.

We solve the coupled Wong Yang–Mills equations for both U(1) and SU(2) gauge groups and anisotropic particle momentum distributions numerically on a lattice. For weak fields with initial energy density much smaller than that of the particles we confirm the existence of plasma instabilities and of exponential growth of the fields which has been discussed previously. Also, the SU(2) case is qualitatively similar to U(1), and we do find significant “abelianization” of the non-Abelian fields during the period of exponential growth. However, the effect nearly disappears when the fields are strong. This is because of the very rapid isotropization of the particle momenta by deflection in a strong field on time scales comparable to that for the development of Yang–Mills instabilities. This mechanism for isotropization may lead to smaller entropy increase than collisions and multiplication of hard gluons, which is interesting for the phenomenology of high-energy heavy-ion collisions.

We propose that the measurement of the transverse momentum dependence of the double ratio of the nuclear modification factors of charm and bottom jets, RAAc(pT)/RAAb(pT), in central nuclear collisions at the LHC will provide an especially robust observable that can be used to differentiate Standard Model perturbative QCD predictions from recently proposed strong coupling string drag models derived using the AdS/CFT conjecture.

We calculate the antibaryon-to-baryon ratios, p̄/p,Λ̄/Λ,Ξ/Ξ, and Ω/Ω for Au+Au collisions at RHIC (sNN=200 GeV). The effects of strong color fields associated with an enhanced strangeness and diquark production probability and with an effective decrease of formation times are investigated. Antibaryon-to-baryon ratios increase with the color field strength. The ratios also increase with the strangeness content |S|. The netbaryon number at midrapidity considerably increases with the color field strength while the netproton number remains roughly the same. This shows that the enhanced baryon transport involves a conversion into the hyperon sector (hyperonization) which can be observed in the (Λ−Λ̄)/(p−p̄) ratio.

Lattice simulation of a center symmetric three dimensional effective theory for SU(2) Yang-Mills
(2010)

We present lattice simulations of a center symmetric dimensionally reduced effective field theory for SU(2) Yang Mills which employ thermal Wilson lines and three-dimensional magnetic fields as fundamental degrees of freedom. The action is composed of a gauge invariant kinetic term, spatial gauge fields and a potential for the Wilson line which includes a "fuzzy" bag term to generate non-perturbative fluctuations between Z(2) degenerate ground states. The model is studied in the limit where the gauge fields are set to zero as well as the full model with gauge fields. We confirm that, at moderately weak coupling, the "fuzzy" bag term leads to eigenvalue repulsion in a finite region above the deconfining phase transition which shrinks in the extreme weak-coupling limit. A non-trivial Z(N) symmetric vacuum arises in the confined phase. The effective potential for the Polyakov loop in the theory with gauge fields is extracted from the simulations including all modes of the loop as well as for cooled configurations where the hard modes have been averaged out. The former is found to exhibit a non-analytic contribution while the latter can be described by a mean-field like ansatz with quadratic and quartic terms, plus a Vandermonde potential which depends upon the location within the phase diagram. Other results include the exact location of the phase boundary in the plane spanned by the coupling parameters, correlation lengths of several operators in the magnetic and electric sectors and the spatial string tension. We also present results from simulations of the full 4D Yang-Mills theory and attempt to make a qualitative comparison to the 3D effective theory.

This thesis investigates the jet-medium interactions in a Quark-Gluon Plasma using a hydrodynamical model. Such a Quark-Gluon Plasma represents a very early stage of our universe and is assumed to be created in heavy-ion collisions. Its properties are subject of current research. Since the comparison of measured data to model calculations suggests that the Quark-Gluon Plasma behaves like a nearly perfect liquid, the medium created in a heavy-ion collision can be described applying hydrodynamical simulations. One of the crucial questions in this context is if highly energetic particles (so-called jets), which are produced at the beginning of the collision and traverse the formed medium, may lead to the creation of a Mach cone. Such a Mach cone is always expected to develop if a jet moves with a velocity larger than the speed of sound relative to the medium. In that case, the measured angular particle distributions are supposed to exhibit a characteristic structure allowing for direct conclusions about the Equation of State and in particular about the speed of sound of the medium. Several different scenarios of jet energy loss are examined (the exact form of which is not known from first principles) and different mechanisms of energy and momentum loss are analyzed, ranging from weak interactions (based on calculations from perturbative Quantum Chromodynamics, pQCD) to strong interactions (formulated using the Anti-de-Sitter/Conformal Field Theory Correspondence, AdS/CFT). Though they result in different angular particle correlations which could in principle allow to distinguish the underlying processes (if it becomes possible to analyze single-jet events), it is shown that the characteristic structure observed in experimental data can be obtained due to the different contributions of several possible jet trajectories through an expanding medium. Such a structure cannot directly be connected to the Equation of State. In this context, the impact of a strong flow created behind the jet is examined which is common to almost all jet deposition scenarios. Besides that, the transport equations for dissipative hydrodynamics are discussed which are fundamental for any numerical computation of viscous effects in a Quark-Gluon Plasma.

In this work data of the NA49 experiment at CERN SPS on the energy dependence of multiplicity fluctuations in central Pb+Pb collisions at 20A, 30A, 40A, 80A and 158A GeV, as well as the system size dependence at 158A GeV, is analysed for positively, negatively and all charged hadrons. Furthermore the rapidity and transverse momentum dependence of multiplicity fluctuations are studied. The experimental results are compared to predictions of statistical hadron-gas and string-hadronic models. It is expected that multiplicity fluctuations are sensitive to the phase transition to quark-gluon-plasma (QGP) and to the critical point of strongly interacting matter. It is predicted that both the onset of deconfinement, the lowest energy where QGP is created, and the critical point are located in the SPS energy range. Furthermore, the predictions for the multiplicity fluctuations of statistical and string-hadronic models are different, the experimental data might allow to distinguish between them. The used measure of multiplicity fluctuations is the scaled variance omega, defined as the ratio of the variance and the mean of the multiplicity distribution. In the NA49 experiment the tracks of charged particles are detected in four large volume time projection chambers (TPCs). In order to remove possible detector effects a detailed study of event and track selection criteria is performed. Naively one would expect Poisson fluctuations in central heavy ion collisions. A suppression of fluctuations compared to a Poisson distribution is observed for positively and negatively charged hadrons at forward rapidity in Pb+Pb collisions. At midrapidity and for all charged hadrons the fluctuations are larger than the Poisson ones. The fluctuations seem to increase with decreasing system size. It is suggested that this is due to increased relative fluctuations in the number of participants. Furthermore, it was discovered that omega increases for decreasing rapidity and transverse momentum. A hadron-gas model predicts different values of omega for different statistical ensembles. In the grand-canonical ensemble, where all conservation laws are fulfilled only on the average, not on an event-by-event basis, the predicted fluctuations are the largest ones. In the canonical ensemble the charges, namely the electrical charge, the baryon number and the strangeness, are conserved for each event. The scaled variance in this ensemble is smaller than for the grand-canonical ensemble. In the micro-canonical ensemble not only the charges, but also the energy and the momentum are conserved in each event, the predicted $omega$ is the smallest one. The grand-canonical and canonical formulations of the hadron-gas model over-predict fluctuations in the forward acceptance. In contrast to the experimental data no dependence of omega on rapidity and transverse momentum is expected. For the micro-canonical formulation, which predicts small fluctuations in the total phase space, no quantitative calculation is available yet for the limited experimental acceptance. The increase of fluctuations for low rapidities and transverse momenta can be qualitatively understood in a micro-canonical ensemble as an effect of energy and momentum conservation. The string-hadronic model UrQMD significantly over-predicts the mean multiplicities but approximately reproduces the scaled variance of the multiplicity distributions at all measured collision energies, systems and phase-space intervals. String-hadronic models predict for Pb+Pb collisions a monotonous increase of omega with collision energy, similar to the observations for p+p interactions. This is in contrast to the predictions of the hadron-gas model, where omega shows no energy dependence at higher energies. At SPS energies the predictions of the string-hadronic and hadron-gas models are in the same order of magnitude, but at RHIC and LHC energies the difference in omega in the full phase space is much larger. Experimental data should be able to distinguish between them rather easily. Narrower than Poissonian (omega < 1) multiplicity fluctuations measured in the forward kinematic region (1<y(pi)<y_{beam}) can be related to the reduced fluctuations predicted for relativistic gases with imposed conservation laws. This general feature of relativistic gases may be preserved also for some non-equilibrium systems as modeled by the string-hadronic approaches. A quantitative estimate shows that the predicted maximum in fluctuations due to a first order phase transition from hadron-gas to QGP is smaller than the experimental errors of the present experiment and can therefore neither be confirmed nor disproved. No sign of increased fluctuations as expected for a freeze-out near the critical point of strongly interacting matter is observed.

In this work we study the non-equilibrium dynamics of a quark-gluon plasma, as created in heavy-ion collisions. We investigate how big of a role plasma instabilities can play in the isotropization and equilibration of a quark-gluon plasma. In particular, we determine, among other things, how much collisions between the particles can reduce the growth rate of unstable modes. This is done both in a model calculation using the hard-loop approximation, as well as in a real-time lattice simulation combining both classical Yang-Mills-fields as well as inter-particle collisions. The new extended version of the simulation is also used to investigate jet transport in isotropic media, leading to a cutoff-independent result for the transport coefficient $hat{q}$. The precise determination of such transport coefficients is essential, since they can provide important information about the medium created in heavy-ion collisions. In anisotropic media, the effect of instabilities on jet transport is studied, leading to a possible explanation for the experimental observation that high-energy jets traversing the plasma perpendicular to the beam axis experience much stronger broadening in rapidity than in azimuth. The investigation of collective modes in the hard-loop limit is extended to fermionic modes, which are shown to be all stable. Finally, we study the possibility of using high energy photon production as a tool to experimentally determine the anisotropy of the created system. Knowledge of the degree of local momentum-space anisotropy reached in a heavy-ion collision is essential for the study of instabilities and their role for isotropization and thermalization, because their growth rate depends strongly on the anisotropy.

This article generalizes Schwinger’s mechanism for particles production in the arbitrary finite field volume. McLerran-Venugopolan(MV) model and iterative solution of DGLAP equation in the double leading log approximation for small x gluon distribution function were used to derive the new formula for initial chromofield energy density. This initial chromofield energy is distributed among color neutral clusters or strings of different length. This strings are stretched by receding nucleus. From the proposed mechanism of string fragmentation or color field decay based on exact solution of Dirac equation in the different finite volume, the new formulae for esimated baryon kinetic energy loss and rapidity spectrum of produced partons were derived.

I investigate some of the inert phases in three-flavor, spin-zero color-superconducting quark matter: the CFL phase (the analogue of the B phase in superfluid 3He), the A and A* phases, and the 2SC and sSC phases. I compute the pressure of these phases with and without the neutrality condition. Without the neutrality condition, after the CFL phase the sSC phase is the dominant phase. However, including the neutrality condition, the CFL phase is again the energetically favored phase except for a small region of intermediate densities where the 2SC/A* phase is favored. It is shown that the 2SC phase is identical to the A* phase up to a color rotation. In addition, I calculate the self-energies and the spectral densities of longitudinal and transverse gluons at zero temperature in color-superconducting quark matter in the CFL phase. I find a collective excitation, a plasmon, at energies smaller than two times the gap parameter and momenta smaller than about eight times the gap. The dispersion relation of this mode exhibits a minimum at some nonzero value of momentum, indicating a van Hove singularity.