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Search Results: 1 - 10 of 3294 matches for " Sunao Nakamura "
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Lattice calculation of the QGP viscosities - Present results and next project -
Sunao Sakai Atsushi Nakamura
Physics , 2007, DOI: 10.1063/1.2729742
Abstract: The shear and bulk viscosities of gluon plasma are calculated by accumulating a large amount of data for the Matsubara Green function ($G_{\beta}(t_m)$) on isotropic $24^3 \times 8$ and $16^3 \times 8$ lattices. In the case of Iwasaki's improved action, the calculations of $G_{\beta}(t_m)$ are carried out on roughly 6 million configurations, while for the standard action the calculations are done on more than 16 million configurations. The shear viscosities increase roughly with $T^3$, and $\eta/s$ ratios are close to the KSS lower bound in the region where $1
Transport Coefficient of Gluon Plasma from Lattice QCD
Sunao Sakai,Atsushi Nakamura
Physics , 2005,
Abstract: In this report we present our calculation of the transport coefficient of gluon system on $24^3\times 8$ lattice in the quench approximation. Simulations are carried out in the range, $1.4 \le T/T_c \le 24$. In the temperature region slightly above the transition, where the perturbative calculation is not applicable, the shear viscosity($\eta$) is smaller than typical hadron masses. The bulk viscosity is consistent with zero within the range of error bars in $1.4 \le T/T_c \le 24$. We compare our results with the perturbative calculations in large $T/T_c$ region. It is found that the lattice and perturbative results are consistent with each other there. The ratio $\eta/s$ is around $0.1-0.4$ in $T/T_c < 3$ region and satisfies the KSS bound\cite{KSS}. In order to estimate the contribution from high frequency part of the spectral function, we study the effects of a term $\rho^{high}$ proposed by Aarts and Resco\cite{Aarts}. It is found that until the threshold mass becomes small, its effect is quite small, and that viscosity decreases as the threshold decreases. From these studies we think that although our result is obtained under an assumptions for the spectral function, it gives a reasonable estimation for $\eta$($=\pi d\rho/d\omega$ at $\omega=0$), and qualitative results will not be changed when the accurate spectral function is obtained.
Improved gauge action on an anisotropic lattice II - Anisotropy parameter in the medium coupling region -
Sunao Sakai,Atsushi Nakamura
Physics , 2003, DOI: 10.1103/PhysRevD.69.114504
Abstract: 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.
Transport Coefficients of Gluon Plasma
Atsushi Nakamura,Sunao Sakai
Physics , 2004, DOI: 10.1103/PhysRevLett.94.072305
Abstract: Transport coefficients of gluon plasma are calculated for a SU(3) pure gauge model by lattice QCD simulations on $16^3 \times 8$ and $24^3 \times 8$ lattices. Simulations are carried out at a slightly above the deconfinement transition temperature $T_c$, where a new state of matter is currently being pursued in RHIC experiments. Our results show that the ratio of the shear viscosity to the entropy is less than one and the bulk viscosity is consistent with zero in the region, $1.4 \leq T/T_c \leq 1.8 $.
Transport Coefficients of Quark Gluon Plasma for Pure Gauge Models
Sunao Sakai,Atsushi Nakamura,Kazuhisa Amemiya
Physics , 1996, DOI: 10.1016/S0920-5632(96)00680-9
Abstract: The transport coefficients of quark gluon plasma are calculated on a lattice 16**3X8, with the pure gauge models. Matsubara Green's functions of energy momentum tensors have very large fluctuations and about a few million MC sweeps are needed to reduce the errors reasonably small in the case of the standard action. They are much suppressed if Iwasaki's improved action is employed. Preliminary results show that the transport coefficients roughly depend on the coupling constant as a**(-3)(g) in the case of SU(2).
Transport Coefficients of Quark Gluon Plasma From Lattice Gauge Theory
Sunao Sakai,Atsushi Nakamura,Takuya Saito
Physics , 1998, DOI: 10.1016/S0375-9474(98)00364-9
Abstract: Numerical results for the transport coefficients of quark gluon plasma are obtained by lattice simulations on on $16^3 \times 8$ lattice with the quench approximation where we apply the gauge action proposed by Iwasaki. The bulk viscosity is consistent with zero, and the shear viscosity is slightly smaller than the typical hadron masses. They are not far from the simple extrapolation on the figure of perturbative calculation in high temperature limit down to $T \sim T_{c}$. The gluon propagator in the confined and deconfined phases are also discussed.
Anisotropic Improved Gauge Actions; --Perturbative and Numerical Studies --
Sunao Sakai,Atsushi Nakamura,Takuya Saito
Physics , 1998, DOI: 10.1016/S0920-5632(99)85091-9
Abstract: The $\Lambda$ parameter on the anisotropic lattice, the spatial and temperature coupling constant $g_{\sigma}$, $g_{\tau}$ and their derivative with respaect to the the anisotropy parameter $\xi$ are studied perturbatively for the class of improved actions, which cover tree level Symanzik's, Iwasaki's and QCDTARO's improved actions. The $\eta(=g_{\tau}/g_{\sigma})$ becomes less than 1 for Iwasaki's and QCDTARO's action, which is confirmed nonperturbatively by numerical simulations. Derivatives of the coupling constants with respect to the anisotropy parameter, $\partial g_{\tau}/\partial \xi$ and $\partial g_{\sigma}/\partial \xi$, change sign for those improved actions.
Numerical Results for Transport Coefficients of Quark Gluon Plasma with Iwasaki's Improved Action
Sunao Sakai,Atsushi Nakamura,Takuya Saito
Physics , 1997, DOI: 10.1016/S0920-5632(97)00790-1
Abstract: Numerical results for the transport coefficients of quark gluon plasma are calculated by lattice simulation of SU(3) pure gauge model. The bulk viscosity is consistent with zero. The shear viscosity is finite and increases with temperature $T$ roughly as $T^{3}$, and around the finite temperature transition points, it is slightly smaller than the typical hadron masses.
Anisotropic Improved Actions
Sunao Sakai,Atsushi Nakamura,Takuya Saito
Physics , 2000, DOI: 10.1016/S0920-5632(00)91686-4
Abstract: The studies of the quantum corrections for the anisotropy parameter,$\eta(=\xi_R/\xi_B)$, for the improved actions, $\beta (C_0 L({Plaq.}) + C_1 L({Rect.}))$, are proceeded in the medium to strong coupling region on anisotropic lattices. The global features for the $\eta$ parameters as a function of $\beta$ and the coefficient $C_{1}$ have been clarified. It has been found by the perturbative analysis that as $C_1$ decreases, the slope of the $\eta(\beta)$ becomes less steep and for the actions whose $C_{1}$ is less than -0.160, $\eta$ decreases as $\beta$ decreases, contrary to the case of the standard action. In the medium to strong coupling region, the $\eta$ parameter begins to increase as $\beta$ decreases for all $C_{1}$. This means that for the actions with $C_{1} < -0.160$, the one-loop perturbative results for $\eta$ break down qualitatively and the $\eta$ parameters have a dip. As a result of this dip structure the $\eta$ for Iwasaki's action remains close to unity in the wide range of $\beta$.
Anisotropic Lattice and Its Application to Quark Gluon Plasma
Sunao Sakai,Atsushi Nakamura,Takuya Saito
Physics , 2001,
Abstract: We have studied the link-integration method for the improved actions. With this method the $\eta$ parameter in the medium to strong coupling regions is obtained. Effects of the self-energy terms for the $\eta$ parameters are small in the regions of $\beta$ and $\eta$ studied. After these investigations, the anisotropic lattice is used for the calculation of transport coefficients of the quark gluon plasma.
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