The reweighting method is widely used in numerical studies of QCD, in particular, for the cases in which the conventional Monte-Carlo method cannot be applied directly, e.g., finite density QCD. However, the application range of the reweighing method
is restricted due to several problems. One of the most severe problems here is the overlap problem. To solve it, we examine a multipoint reweighting method in which simulations at several simulation points are combined in the data analyses. We systematically study the applicability and limitation of the multipoint reweighting method in two-flavor QCD at zero density. Measuring histograms of physical quantities at a series of simulation points, we apply the multipoint reweighting method to calculate the meson masses as continuous functions of the gauge coupling $beta$ and the hopping parameters $kappa$. We then determine lines of constant physics and beta functions, which are needed in a calculation of the equation of state at finite temperature.
We study scaling behavior of a chiral order parameter in the low density region, performing a simulation of two-flavor QCD with improved Wilson quarks. The scaling behavior of the chiral order parameter defined by a Ward-Takahashi identity agrees wit
h the scaling function of the three-dimensional O(4) spin model at zero chemical potential. We extend the scaling study to finite density QCD. Applying the reweighting method and calculating derivatives of the chiral order parameter with respect to the chemical potential, the scaling properties of the chiral phase transition are discussed in the low density region. We moreover calculate the curvature of the phase boundary of the chiral phase transition in the temperature and chemical potential plane assuming the O(4) scaling relation.
We extend our previous study of the QCD phase structure in the heavy quark region to non-zero chemical potentials. To identify the critical point where the first order deconfining transition terminates, we study an effective potential defined by the
probability distribution function of the plaquette and the Polyakov loop. The reweighting technique is shown to be powerful in evaluating the effective potential in a wide range of the plaquette and Polyakov loop expectation values. We adopt the cumulant expansion to overcome the sign problem in the calculation of complex phase of the quark determinant. We find that the method provides us with an intuitive and powerful way to study the phase structure. We estimate the location of the critical point at finite chemical potential in the heavy quark region.
We present the status of our study on the equation of state in 2+1 flavor QCD with non-perturbatively improved Wilson quarks coupled with the RG improved glue. We apply the T-integration method to non-perturbatively calculate the equation of state by the fixed-scale approach.
The WHOT-QCD Collaboration is pushing forward lattice studies of QCD at finite temperatures and densities using improved Wilson quarks. We first present results on QCD at zero and finite densities with two flavors of degenerate quarks (N_F=2 QCD) ado
pting the conventional fixed-Nt approach. We then report on the status of a study of N_F=2+1 QCD adopting a fixed-scale approach armed with the T-integration method which we have developed.
We report on the status of our study towards the equation of state in 2+1 flavor QCD with improved Wilson quarks. To reduce the computational cost which is quite demanding for Wilson-type quarks, we adopt the fixed scale approach, i.e. the temperatur
e T is varied by N_t at fixed lattice spacing. Since the conventional integral method to obtain the pressure is inapplicable at a fixed scale, we adopt the T-integral method, to calculate the pressure non-perturbatively. Reduction of the computational cost of T=0 simulations thus achieved is indispensable to study EOS in QCD with dynamical quarks.
Dissociation temperatures of J/psi, psi, and chi_c states play key roles in the sequential J/psi suppression scenario for high energy heavy ion collisions. We report on a study of charmonium dissociation temperatures in quenched lattice QCD. On aniso
tropic lattices, we first subtract the effects of the constant mode in finite temperature meson correlators, which have lead to unphysical results in previous studies. We then extract ground and first exited state masses by diagonalizing correlation functions among different source and sink operators. To distinguish bound states from scattering states, we first compare the charmonium mass spectra under different spatial boundary conditions, and examine the shape and the volume-dependence of their Bethe-Salpeter wave functions. From these studies, we found so far no sign of scattering states up to about 2.3T_c.
