Long-distance behavior of qqˉq\bar{q} color dependent potentials at finite temperature

SU(3) heavy quark potentials at finite temperature are studied with quenched lattice QCD using the stochastic gauge-fixing method. In addition to the standard color average channel, we investigate $q\bar{q}$ potentials in singlet and octet channels. % We obtain clear signals in all cases. The singlet $q\bar{q}$ channel yields an attractive force, while the octet $q\bar{q}$ potential is repulsive; the corresponding color average channel also results in an attractive force. %As the temperature increases, these forces are weakened and As the temperature increases, these forces become weak and their variations are very small; at $T/T_c=1.8-5.6$, % the singlet attractive force is not so weakened even for long distances. the singlet attractive force survives over $R \sim 1/T$. The singlet and octet potentials calculated with this algorithm have a small gauge dependence when the gauge parameter $\alpha$ is changed from 0.6 to 1.3.Comment: 12 pages, 9 figures, typos corrected, to appear in Prog. Theor. Phy

Imaginary Chemical Potential Approach for the Pseudo-Critical Line in the QCD Phase Diagram with Clover-Improved Wilson Fermions

The QCD phase diagram is studied in the lattice QCD simulation with the imaginary chemical potential approach. We employ a clover-improved Wilson fermion action of two-flavors and a renormalization-group improved gauge action, and perform the simulation at an intermediate quark mass on a $8^3\times 4$ lattice. The QCD phase diagram in the imaginary chemical potential $\mu_I$ region is investigated by performing the simulation for more than 150 points on the $(\beta,\mu_I)$ plane. We find that the Roberge-Weiss phase transition at $\mu_I/T=\pi/3$ is first order and its endpoint is second order, which are identified by the phase of the Polyakov loop. We determine the pseudo-critical line from the susceptibility of the Polyakov loop modulus. We find a clear deviation from a linear dependence of the pseudo-critical line on $\mu_I^2$.Comment: 10 pages, 20 figures, 3 tables. Revtex4. References are added and, discussions are sharpene

Simulations of one-flavor QCD at finite temperature by RHMC

We simulate one-flavor QCD with standard Wilson fermions at finite temperature by the rational hybrid Monte Carlo algorithm. In the heavy quark region when we decrease the quark mass there is an endpoint which terminates the first order phase transition. We try to locate it by calculating the Binder cumulant of the Polyakov loop norm. We estimate the end-point to be kappa_c \sim 0.07-0.08.Comment: 7 pages, Presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, German

Improved gauge action on an anisotropic lattice II - Anisotropy parameter in the medium coupling region -

The quantum correction of the anisotropy parameter, $\eta$, is calculated for $\xi=2$ and 3 in the $\beta$ region where numerical simulations such as hadron spectroscopy are currently carried out, for the improved actions composed of plaquette and rectangular 6-link loops. The $\beta$ dependences of $\eta$ for the renormalization group improved actions are quite different from those of the standard and Symanzik actions. In Iwasaki and DBW2 actions, $\eta$ stays almost constant in a wide range of $\beta$, which also differs from the one-loop perturbative result, while in the case of Symanzik action, it increases as $\beta$ decreases, which is qualitatively similar to the perturbative result, but the slope is steeper. In the calculation of the $\eta$ parameter close to and in the confined phase, we have applied the link integration method to suppress the fluctuation of the gauge fields. Some technical details are summarized.Comment: 21 pages, 10 figure

Measuring Speed of Gravitational Waves by Observations of Photons and Neutrinos from Compact Binary Mergers and Supernovae

Detection of gravitational waves (GW) provides us an opportunity to test general relativity in strong and dynamical regimes of gravity. One of the tests is checking whether GW propagates with the speed of light or not. This test is crucial because the velocity of GW has not ever been directly measured. Propagation speed of a GW can deviate from the speed of light due to the modification of gravity, graviton mass, and the nontrivial spacetime structure such as extra dimensions and quantum gravity effects. Here we report a simple method to measure the propagation speed of a GW by directly comparing arrival times between gravitational waves, and neutrinos from supernovae or photons from short gamma-ray bursts. As a result, we found that the future multimessenger observations of a GW, neutrinos, and photons can test the GW propagation speed with the precision of ~10^{-16} improving the previous suggestions by 8-10 orders of magnitude. We also propose a novel method that distinguishes the true signal due to the deviation of GW propagation speed from the speed of light and the intrinsic time delay of the emission at a source by looking at the redshift dependence.Comment: 8 pages, 6 figure

Wilson Fermion Determinant in Lattice QCD

We present a formula for reducing the rank of Wilson fermions from $4 N_c N_x N_y N_z N_t$ to $4 N_c N_x N_y N_z$ keeping the value of its determinant. We analyse eigenvalues of a reduced matrix and coefficients $C_n$ in the fugacity expansion of the fermion determinant $\sum_n C_n (\exp(\mu/T))^n$, which play an important role in the canonical formulation, using lattice QCD configurations on a $4^4$ lattice. Numerically, $\log |C_n|$ varies as $N_x N_y N_z$, and goes easily over the standard numerical range; We give a simple cure for that. The phase of $C_n$ correlates with the distribution of the Polyakov loop in the complex plain. These results lay the groundwork for future finite density calculations in lattice QCD.Comment: 20 pages, 2 tables, 32 figure