We analyze the collective modes of high-temperature QCD in the case when there is an anisotropy in the momentum-space distribution function for the gluons. We perform a tensor decomposition of the gluon self-energy and solve the dispersion relations for both stable and unstable modes. Results are presented for a class of anisotropic distribution functions which can be obtained by stretching or squeezing an isotropic distribution function along one direction in momentum space. We find that there are three stable modes and either one or two unstable modes depending on whether the distribution function is stretched or squeezed. The presence of unstable modes which have exponential growth can lead to a more rapid thermalization and isotropization of the soft modes in a quark gluon plasma and therefore may play an important role in the dynamical evolution of a quark-gluon plasma.
This report reviews the study of open heavy-flavour and quarkonium production in high-energy hadronic collisions, as tools to investigate fundamental aspects of Quantum Chromodynamics, from the proton and nucleus structure at high energy to deconfinement and the properties of the Quark–Gluon Plasma. Emphasis is given to the lessons learnt from LHC Run 1 results, which are reviewed in a global picture with the results from SPS and RHIC at lower energies, as well as to the questions to be addressed in the future. The report covers heavy flavour and quarkonium production in proton–proton, proton–nucleus and nucleus–nucleus collisions. This includes discussion of the effects of hot and cold strongly interacting matter, quarkonium photoproduction in nucleus–nucleus collisions and perspectives on the study of heavy flavour and quarkonium with upgrades of existing experiments and new experiments. The report results from the activity of the SaporeGravis network of the I3 Hadron Physics programme of the European Union 7 Framework Programme.
In this paper we present a method to improve the description of 0+1 dimensional boost invariant dissipative dynamics in the presence of large momentum-space anisotropies. We do this by reorganizing the canonical hydrodynamic expansion of the distribution function around a momentum-space anisotropic ansatz rather than an isotropic equilibrium one. At leading order the result obtained is two coupled ordinary differential equations for the momentum-space anisotropy and typical momentum of the degrees of freedom. We show that this framework can reproduce both the ideal hydrodynamic and free streaming limits. Additionally, we demonstrate that when linearized the differential equations reduce to 2nd order Israel-Stewart viscous hydrodynamics. Finally, we make quantitative comparisons of the evolution of the pressure anisotropy within our approach and 2nd order viscous hydrodynamics in both the strong and weak coupling limits.
We calculate the free energy of a hot gluon plasma to leading order in hard-thermal-loop perturbation theory. Effects associated with screening, gluon quasiparticles, and Landau damping are resummed to all orders. The ultraviolet divergences generated by the hard-thermal-loop propagator corrections can be canceled by a counterterm which depends on the thermal gluon mass. The deviation of the hardthermal-loop free energy from lattice QCD results for T . 2T c has the correct sign and roughly the correct magnitude to be accounted for by next-to-leading order corrections. PACS numbers: 12.38.Mh, 11.10.Wx, 12.38.Cy Relativistic heavy-ion collisions will soon allow the experimental study of hadronic matter at energy densities that should exceed that required to create a quark-gluon plasma. A quantitative understanding of the properties of a quark-gluon plasma is essential in order to determine whether it has been created. Because QCD, the gauge theory that describes strong interactions, is asymptotically free, its running coupling constant a s becomes weak at sufficiently high temperatures. This would seem to make the task of understanding the high-temperature limit of hadronic matter relatively straightforward, because the problem can be attacked using perturbative methods. Unfortunately, the perturbative expansion in powers of a s does not seem to be of any quantitative use even at temperatures that are orders of magnitude higher than those achievable in heavy-ion collisions.The problem is evident in the free energy F of the quark-gluon plasma, whose weak-coupling expansion has been calculated through order a 5͞2 s [1,2]. An optimist might hope to use perturbative methods at temperatures as low as 0.3 GeV, because the running coupling constant a s ͑2pT ͒ at the scale of the lowest Matsubara frequency is about 1͞3. However, the expansion in powers of a 1͞2 s appears to converge only for extremely small values of a s . For example, if N f 6, the a 3͞2 s term is smaller than the a s term only for a s , 0.075, which corresponds to a temperature greater than 10 3 GeV. At temperatures below 1 GeV, the corrections show no sign of converging, although the convergence can be somewhat improved by using Padé approximations [3]. It is clear that a reorganization of the perturbation series is essential if perturbative calculations are to be of any quantitative use at temperatures accessible in heavy-ion collisions.The poor convergence of the perturbation series is puzzling, because lattice gauge theory calculations indicate that the free energy F of the quark-gluon plasma can be approximated by that of an ideal gas unless the temperature T is very close to the critical temperature T c for the phase transition [4,5]. The deviation of F from the free energy of an ideal gas of massless quarks and gluons is less than about 25% if T is greater than 2T c . Furthermore, the lattice results can be described surprisingly well for all T . T c by an ideal gas of quark and gluon quasiparticles with temperature-dependent masses [6].The lar...
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