Transport properties of a meson gas

We present recent results on a systematic method to calculate transport coefficients for a meson gas (in particular, we analyze a pion gas) at low temperatures in the context of Chiral Perturbation Theory. Our method is based on the study of Feynman diagrams with a power counting which takes into account collisions in the plasma by means of a non-zero particle width. In this way, we obtain results compatible with analysis of Kinetic Theory with just the leading order diagram. We show the behavior with temperature of electrical and thermal conductivities and shear and bulk viscosities, and we discuss the fundamental role played by unitarity. We obtain that bulk viscosity is negligible against shear viscosity near the chiral phase transition. Relations between the different transport coefficients and bounds on them based on different theoretical approximations are also discussed. We also comment on some applications to heavy-ion collisions.Comment: 4 pages, 4 figures, IJMPE style. Contribution to the International Workshop X Hadron Physics (2007), Florianopolis, Brazil. Accepted for publication in IJMPE; 1 typo correcte

QCD Shear Viscosity at (almost) NLO

We compute the shear viscosity of QCD with matter, including almost all next-to-leading order corrections -- that is, corrections suppressed by one power of $g$ relative to leading order. We argue that the still missing terms are small. The next-to-leading order corrections are large and bring $\eta/s$ down by more than a factor of 3 at physically relevant couplings. The perturbative expansion is problematic even at $T \simeq 100$ GeV. The largest next-to-leading order correction to $\eta/s$ arises from modifications to the qhat parameter, which determines the rate of transverse momentum diffusion. We also explore quark number diffusion, and shear viscosity in pure-glue QCD and in QED.Comment: 36 pages plus appendices, 11 figures. The main results are summarized in the introduction (Fig. 1

Hydro+Cascade, Flow, the Equation of State, Predictions and Data

A Hydro+Cascade model has been used to describe radial and elliptic flow at the SPS and successfully predicted the radial and elliptic flow measured by the both STAR and PHENIX collaborations . Furthermore, a combined description of the radial and elliptic flow for different particle species, restricts the Equation of State(EoS) and points towards an EoS with a phase transition to the Quark Gluon Plasma(QGP) .Comment: Quark Matter 2001 Procedings. Corrected Fig. 3b for all charged. Some typos fixe

Jet-Medium Interactions at NLO in a Weakly-Coupled Quark-Gluon Plasma

We present an extension to next-to-leading order in the strong coupling constant $g$ of the AMY effective kinetic approach to the energy loss of high momentum particles in the quark-gluon plasma. At leading order, the transport of jet-like particles is determined by elastic scattering with the thermal constituents, and by inelastic collinear splittings induced by the medium. We reorganize this description into collinear splittings, high-momentum-transfer scatterings, drag and diffusion, and particle conversions (momentum-preserving identity-changing processes). We show that this reorganized description remains valid to NLO in $g$, and compute the appropriate modifications of the drag, diffusion, particle conversion, and inelastic splitting coefficients. In addition, a new kinematic regime opens at NLO for wider-angle collinear bremsstrahlung. These semi-collinear emissions smoothly interpolate between the leading order high-momentum-transfer scatterings and collinear splittings. To organize the calculation, we introduce a set of Wilson line operators on the light-cone which determine the diffusion and identity changing coefficients, and we show how to evaluate these operators at NLO.Comment: 37 pages plus appendices, 15 figures. A pedagogical review has been presented in arXiv:1502.03730. v2: minor changes, matches journal versio

Correlations and fluctuations measured by the CMS experiment in pp and PbPb

Measurements of charged dihadron angular correlations are presented in proton-proton (pp) and Lead-Lead (PbPb) collisions, over a broad range of pseudorapidity and azimuthal angle, using the CMS detector at the LHC. In very high multiplicity pp events at center-of-mass energy of 7 TeV, a striking "ridge"-like structure emerges in the two-dimensional correlation function for particle pairs with intermediate pt of 1-3 GeVc, in the kinematic region 2.0<|\Delta\eta|<4.8 and small \Delta\phi, which is similar to observations in heavy-ion collisions. Studies of this new effect as a function of particle transverse momentum are discussed. The long-range and short-range dihadron correlations are also studied in PbPb collision at a nucleon-nucleon center-of-mass energy of 2.76 TeV, as a function of transverse momentum and collision centrality. A Fourier analysis of the long-range dihadron correlations is presented and discussed in the context of CMS measurements of higher order flow coefficients.Comment: 8 pages, 8 figures, proceedings for Quark Matter 2011, Annecy, France, May 23-28, 201

The Azimuthal Asymmetry at large p_t seem to be too large for a ``Jet Quenching''

We discuss simple generic model of ``jet quenching'' in which matter absorption is defined by one parameter. We show that as absorption grows, the azimuthal asymmetry v_2 grows as well, reaching the finite limit with a simple geometric interpretation. It turns out, that this limit is still below the experimental values for 6 > p_t > 2 GeV, according to preliminary data from STAR experiment at RHIC. We thus conclude that ``jet quenching'' models alone cannot account for the observed phenomenon, and speculate about alternative scenarios.Comment: 3 pages, 2 figs, 1 table. The final version contaning note added in proofs for PRC, which reflects experimental development which seem to suggest that the geometrical model for v2 is in fact correct description of data at pt=2-10 Ge

Spectral densities for hot QCD plasmas in a leading log approximation

We compute the spectral densities of $T^{\mu\nu}$ and $J^{\mu}$ in high temperature QCD plasmas at small frequency and momentum,\, $\omega,k \sim g^4 T$. The leading log Boltzmann equation is reformulated as a Fokker Planck equation with non-trivial boundary conditions, and the resulting partial differential equation is solved numerically in momentum space. The spectral densities of the current, shear, sound, and bulk channels exhibit a smooth transition from free streaming quasi-particles to ideal hydrodynamics. This transition is analyzed with conformal and non-conformal second order hydrodynamics, and a second order diffusion equation. We determine all of the second order transport coefficients which characterize the linear response in the hydrodynamic regime.Comment: 39 pages, 6 figures. v3 contains an analysis of the bulk channel with non-conformal hydrodynamics. Otherwise no significant change