Hadronic fluctuations in the QGP

We analyze fluctuations of quark number and electric charge, in 2-flavour QCD at finite temperature and vanishing net baryon number density. In the hadronic phase we find that an enhancement of charge fluctuations arises from contributions of doubly charged hadrons to the thermodynamics. The rapid suppression of fluctuations seen in the high temperature phase suggests that in the QGP quark number and electric charge are predominantly carried by quasi-particles with the quantum numbers of quarks.Comment: 4 pages, 6 EPS-files, talk presented at Quark Matter 2005, Budapes

Correlation functions of the energy-momentum tensor in SU(2) gauge theory at finite temperature

We calculate correlation functions of the energy-momentum tensor in the vicinity of the deconfinement phase transition of (3+1)-dimensional SU(2) gauge theory and discuss their critical behavior in the vicinity of the second order deconfinement transition. We show that correlation functions of the trace of the energy momentum tensor diverge uniformly at the critical point in proportion to the specific heat singularity. Correlation functions of the pressure, on the other hand, stay finite at the critical point. We discuss the consequences of these findings for the analysis of transport coefficients, in particular the bulk viscosity, in the vicinity of a second order phase transition point.Comment: 24 pages, 6 figure

Group projection method in statistical systems

We discuss an application of group theoretical methods to the formulation of the thermodynamics of systems constrained by the conservation laws described by a semi--simple compact Lie group. A general projection method that allows to construct a partition function for a given irreducible representation of the Lie group is outlined. Applications of the method in Lattice Gauge Theory (LGT) for non--zero baryon number and in the phenomenological description of particle production in ultrarelativistic heavy ion collisions are also indicated.Comment: Invited talk presented at the XXIV International Colloquium on Group Theoritical Methods in Physic

Heavy ion collisions and lattice QCD at finite baryon density

We discuss a relation between the QCD thermodynamics obtained from a statistical analysis of particle production in heavy ion collisions at SPS and RHIC energies and recent LGT results at finite chemical potential. We show that basic thermodynamic properties obtained from the phenomenological statistical operator of a hadron resonance gas that describes particle yields in heavy ion collisions are consistent with recent LGT results. We argue that for $T\leq T_c$ the equation of state derived from Monte--Carlo simulations of two quark--flavor QCD at finite chemical potential can be well described by a hadron resonance gas when using the same set of approximations as used in LGT calculations. We examine the influence of a finite quark mass on the position of the deconfinement transition in temperature and chemical potential plane.Comment: To appear in the proceedings of 17in International Conference on Ultra Relativistic Nucleus-Nucleus Collisions (Quark Matter 2004), Oakland, California, 11-17 Jan 200

The Ising model universality of the electroweak theory

Lattice simulations have shown that the first order electroweak phase transition turns into a regular cross-over at a critical Higgs mass m_{H,c}. We have developed a method which enables us to make a detailed investigation of the critical properties of the electroweak theory at m_{H,c}. We find that the transition falls into the 3d Ising universality class. The continuum limit extrapolation of the critical Higgs mass is m_{H,c} = 72(2) GeV, which implies that there is no electroweak phase transition in the Standard Model.Comment: 3 pages, contribution to LATTICE98(electroweak

Fluctuations in the vicinity of the phase transition line for two flavor QCD

We study the susceptibilities of quark number, isospin number and electric charge in numerical simulations of lattice QCD at high temperature and density. We discuss the equation of state for 2 flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $(\mu_q)$ are calculated up to sixth order. From this Taylor series, the susceptibilities are estimated as functions of temperature and $\mu_q$. Moreover, we comment on the hadron resonance gas model, which explains well our simulation results below $T_c$.Comment: 3 pages, 5 figures, Talk presented at Lattice2004(non-zero

Thermodynamics at Non-Zero Baryon Number Density: A Comparison of Lattice and Hadron Resonance Gas Model Calculations

We compare recent lattice studies of QCD thermodynamics at non-zero quark chemical potential with the thermodynamics of a hadron resonance gas. We argue that for T < Tc the equation of state derived from Monte--Carlo simulations of two flavour QCD at non-zero chemical potential can be well described by a hadron resonance gas when using the same set of approximations as used in current lattice calculations. We estimate the importance of truncation errors arising from the use of a Taylor expansion in terms of the quark chemical potential and examine the influence of unphysically large quark masses on the equation of state and the critical conditions for deconfinement.Comment: 15 pages, LaTex2e, 6 EPS-figures, minor corrections, reference adde

Free energies of static three quark systems

We study the behaviour of free energies of baryonic systems composed of three heavy quarks on the lattice in SU(3) pure gauge theory at finite temperature. For all temperatures above $T_c$ we find that the connected part of the singlet (decuplet) free energy of the three quark system is given by the sum of the connected parts of the free energies of $qq$-triplets (-sextets). Using renormalized free energies we can compare free energies in different colour channels as well as those of $qq$- and $qqq$-systems on an unique energy scale.Comment: 5 pages, 6 figures, Contribution to Strong and Electroweak Matter 2004 (SEWM04), Helsinki, Finland 16-19 June 200

Charge fluctuations in chiral models and the QCD phase transition

We consider the Polyakov loop-extended two flavor chiral quark--meson model and discuss critical phenomena related with the spontaneous breaking of the chiral symmetry. The model is explored beyond the mean-field approximation in the framework of the functional renormalisation group. We discuss properties of the net-quark number density fluctuations as well as their higher cumulants. We show that with the increasing net-quark number density, the higher order cumulants exhibit a strong sensitivity to the chiral crossover transition. We discuss their role as probes of the chiral phase transition in heavy-ion collisions at RHIC and LHC.Comment: 4 pages, 3 figures, to appear in the proceedings of Quark Matter 2011, 23-28 May 2011, Annecy, Franc

Comparing the drag force on heavy quarks in N=4 super-Yang-Mills theory and QCD

Computations of the drag force on a heavy quark moving through a thermal state of strongly coupled N=4 super-Yang-Mills theory have appeared recently in hep-th/0605158, hep-ph/0605199, and hep-th/0605182. I compare the strength of this effect between N=4 gauge theory and QCD, using the static force between external quarks to normalize the 't Hooft coupling. Comparing N=4 and QCD at fixed energy density then leads to a relaxation time of roughly 2 fm/c for charm quarks moving through a quark-gluon plasma at T=250 MeV. This estimate should be regarded as preliminary because of the difficulties of comparing two such different theories.Comment: 16 pages, 1 figure. v2: corrected estimate of eta/s, refs adde