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Register a new userColor screening potential at finite density in two-flavor lattice QCD with Wilson fermions
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We investigate chemical-potential (mu) dependence of static-quark free energies in both the real and imaginary mu regions, performing lattice QCD simulations at imaginary mu and extrapolating the results to the real mu region with analytic continuation. Lattice QCD calculations are done on a 16^{3}times 4 lattice with the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. Static-quark potential is evaluated from the Polyakov-loop correlation functions in the deconfinement phase. As the analytic continuation, the potential calculated at imaginary mu=imu_{rm I} is expanded into a Taylor-expansion series of imu_{rm I}/T up to 4th order and the pure imaginary variable imu_{rm I}/T is replaced by the real one mu_{rm R}/T. At real mu, the 4th-order term weakens mu dependence of the potential sizably. At long distance, all of the color singlet and non-singlet potentials tend to twice the single-quark free energy, indicating that the interactions between heavy quarks are fully color-screened for finite mu. For both real and imaginary mu, the color-singlet q{bar q} and the color-antitriplet qq interaction are attractive, whereas the color-octet q{bar q} and the color-sextet qq interaction are repulsive. The attractive interactions have stronger mu/T dependence than the repulsive interactions. The color-Debye screening mass is extracted from the color-singlet potential at imaginary mu, and the mass is extrapolated to real mu by analytic continuation. The screening mass thus obtained has stronger mu dependence than the prediction of the leading-order thermal perturbation theory at both real and imaginary mu.
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We investigate the real and imaginary chemical-potential dependence of pion and $rho$-meson screening masses in both the confinement and the deconfinement region by using two-flavor lattice QCD. The spatial meson correlators are calculated in the imaginary chemical potential region with lattice QCD simulations. We extract pion and $rho$-meson screening masses from the correlators. The obtained meson screening masses are extrapolated to the real chemical potential region by assuming some analytic function. In the real chemical potential region, the resulting pion and $rho$-meson screening masses monotonically increase as real chemical potential becomes large.
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.
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.
As computing resources are limited, choosing the parameters for a full Lattice QCD simulation always amounts to a compromise between the competing objectives of a lattice spacing as small, quarks as light, and a volume as large as possible. Aiming to push unquenched simulations with the Wilson action towards the computationally expensive regime of small quark masses we address the question whether one can possibly save computing time by extrapolating results from small lattices to the infinite volume, prior to the usual chiral and continuum extrapolations. In the present work the systematic volume dependence of simulated pion and nucleon masses is investigated and compared with a long-standing analytic formula by Luescher and with results from Chiral Perturbation Theory. We analyze data from Hybrid Monte Carlo simulations with the standard (unimproved) two-flavor Wilson action at two different lattice spacings of a=0.08fm and 0.13fm. The quark masses considered correspond to approximately 85 and 50% (at the smaller a) and 36% (at the larger a) of the strange quark mass. At each quark mass we study at least three different lattices with L/a=10 to 24 sites in the spatial directions (L=0.85-2.08fm).
We study the curvature of the chiral transition/crossover line between the low-temperature hadronic phase and the high-temperature quark-gluon-plasma phase at low densities, performing simulations of two-flavor QCD with improved Wilson quarks. After confirming that the chiral order parameter defined by a Ward-Takahashi identity is consistent with the scaling of the O(4) universality class at zero chemical potential, we extend the scaling analysis to finite chemical potential to determine the curvature of the chiral transition/crossover line at low densities assuming the O(4) universality. To convert the curvature in lattice units to that of the $T_c(mu_B)$ in physical units, we evaluate the lattice scale applying a gradient flow method. We find $kappa=0.0006(1)$ in the chiral limit, which is much smaller than that obtained in (2+1)-flavor QCD with improved staggered quarks.
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