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Register a new userEquation of State and Heavy-Quark Free Energy at Finite Temperature and Density in Two Flavor Lattice QCD with Wilson Quark Action
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We study the equation of state at finite temperature and density in two-flavor QCD with the RG-improved gluon action and the clover-improved Wilson quark action on a $ 16^3 times 4$ lattice. Along the lines of constant physics at $m_{rm PS}/m_{rm V} = 0.65$ and 0.80, we compute the second and forth derivatives of the grand canonical partition function with respect to the quark chemical potential $mu_q = (mu_u+mu_d)/2$ and the isospin chemical potential $mu_I = (mu_u-mu_d)/2$ at vanishing chemical potentials, and study the behaviors of thermodynamic quantities at finite $mu_q$ using these derivatives for the case $mu_I=0$. In particular, we study density fluctuations at none-zero temperature and density by calculating the quark number and isospin susceptibilities and their derivatives with respect to $mu_q$. To suppress statistical fluctuations, we also examine new techniques applicable at low densities. We find a large enhancement in the fluctuation of quark number when the density increased near the pseudo-critical temperature, suggesting a critical point at finite $mu_q$ terminating the first order transition line between hadronic and quark gluon plasma phases. This result agrees with the previous results using staggered-type quark actions qualitatively. Furthermore, we study heavy-quark free energies and Debye screening masses at finite density by measuring the first and second derivatives of these quantities for various color channels of heavy quark-quark and quark-anti-quark pairs. The results suggest that, to the leading order of $mu_q$, the interaction between two quarks becomes stronger at finite densities, while that between quark and anti-quark becomes weaker.
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We study the phase structure of lattice QCD with heavy quarks at finite temperature and density by a histogram method. We determine the location of the critical point at which the first-order deconfining transition in the heavy-quark limit turns into a crossover at intermediate quark masses through a change of the shape of the histogram under variation of coupling parameters. We estimate the effect of the complex phase factor which causes the sign problem at finite density, and show that, in heavy-quark QCD, the effect is small around the critical point. We determine the critical surface in 2+1 flavor QCD in the heavy-quark region at all values of the chemical potential mu including mu=infty.
We study the equation of state in two-flavor QCD at finite temperature and density. Simulations are made with the RG-improved gluon action and the clover-improved Wilson quark action. Along the lines of constant physics for $m_{rm PS}/m_{rm V} = 0.65$ and 0.80, we compute the derivatives of the quark determinant with respect to the quark chemical potential $mu_q$ up to the fourth order at $mu_q=0$. We adopt several improvement techniques in the evaluation. We study thermodynamic quantities and quark number susceptibilities at finite $mu_q$ using these derivatives. We find enhancement of the quark number susceptibility at finite $mu_q$, in accordance with previous observations using staggered-type quarks. This suggests the existence of a nearby critical point.
We calculate the equation of state in 2+1 flavor QCD at finite temperature with physical strange quark mass and almost physical light quark masses using lattices with temporal extent Nt=8. Calculations have been performed with two different improved staggered fermion actions, the asqtad and p4 actions. Overall, we find good agreement between results obtained with these two O(a^2) improved staggered fermion discretization schemes. A comparison with earlier calculations on coarser lattices is performed to quantify systematic errors in current studies of the equation of state. We also present results for observables that are sensitive to deconfining and chiral aspects of the QCD transition on Nt=6 and 8 lattices. We find that deconfinement and chiral symmetry restoration happen in the same narrow temperature interval. In an Appendix we present a simple parametrization of the equation of state that can easily be used in hydrodynamic model calculations. In this parametrization we also incorporated an estimate of current uncertainties in the lattice calculations which arise from cutoff and quark mass effects. We estimate these systematic effects to be about 10 MeV
Meson properties at finite temperature and density are studied in lattice QCD simulations with two-flavor Wilson fermions. For this purpose, we investigate screening masses of mesons in pseudo-scalar (PS) and vector (V) channels. The simulations are performed on $16^3times 4$ lattice along the lines of constant physics at $m_{rm PS}/m_{rm V}|_{T=0}=0.65$ and 0.80, where $m_{rm PS}/m_{rm V}|_{T=0}$ is a ratio of meson masses in PS and V channels at $T=0$. A temperature range is $T/T_{rm pc}=(0.8 - 4.0)$, where $T_{rm pc}$ is the pseudo-critical temperature. We find that the temperature dependence of the screening masses normalized by temperature, $M_0/T$, shows notable structure around $T_{rm pc}$, and approach $2pi$ at high temperature in both channels, which is consistent with twice the thermal mass of a free quark in high temperature limit. The screening masses at low density are also investigated by using the Taylor expansion method with respect to the quark chemical potential. We find that the expansion coefficients in the leading order become positive in the temperature range, and thermal and density effect on the meson screening-masses becomes apparent in the quark-gluon plasma phase. The meson screening-masses are also compared with the gluon (Debye) screening masses at finite temperature and density.
The free energy between a static quark and an antiquark is studied by using the color-singlet Polyakov-line correlation at finite temperature in lattice QCD with 2+1 flavors of improved Wilson quarks. From the simulations on $32^3 times 12$, 10, 8, 6, 4 lattices in the high temperature phase, based on the fixed scale approach, we find that, the heavy-quark free energies at short distance converge to the heavy-quark potential evaluated from the Wilson loop at zero temperature, in accordance with the expected insensitivity of short distance physics to the temperature. At long distance, the heavy-quark free energies approach to twice the single-quark free energies, implying that the interaction between heavy quarks is screened. The Debye screening mass obtained from the long range behavior of the free energy is compared with the results of thermal perturbation theory.
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