arXiv subjects

Atsushi Nakamura

Publications and source records attributed to Atsushi Nakamura.

At least 55 records · Page 3Linked to original sources

arXiv2009

Manifestations of magnetic vortices in equation of state of Yang-Mills plasma

The vacuum of Yang-Mills theory contains singular stringlike objects identified with center (magnetic) vortices. Percolation of magnetic vortices is known to be responsible for the color confinement in the low-temperature phase of the theory. In our work we study properties of the vortices at finite temperature using lattice simulations of SU(2) gauge theory. We show that magnetic vortices provide a numerically large contribution to thermodynamic quantities of the gluon plasma in Yang-Mills theory. In particular, we observe that in the deconfinement phase at temperatures T_c < T < 3 T_c the magnetic component of the gluon plasma produces a negative (ghostlike) contribution to the anomaly of the energy-momentum tensor. In the confinement phase the vortex contribution is positive. The thermodynamical significance of the magnetic objects allows us to suggest that the quark-gluon plasma may contain a developed network of magnetic flux tubes. The existence of the vortex network may lead to observable effects in the quark-gluon plasma because the chromomagnetic field of the vortices should scatter and drag quarks.

hep-lat↗

arXiv2009

Lattice study of K pi scattering in I = 3/2 and 1/2

We report the first lattice QCD results of the scattering amplitudes of the $Kπ$ system for $I = 1/2$ channel together with $I=3/2$ case. We investigate all quark diagrams contributing to these iso-spin states, and find that the scattering amplitudes are expressed as combinations of only three diagrams after setting the masses of $u$-quark and $d$-quark to be the same. The lattice simulations are performed in the quenched approximation at $β= 2.23$ on a 12$^3 \times 24$ lattice with an improved Iwasaki gauge action. We employ a new dilution-type noise method to get accuracy of data with reasonable CPU time. A simple method is proposed and applied to eliminate lattice artefact due to the finite extent of lattice along the time direction. A clear difference in the quark mass dependence between $I=3/2$ and $I=1/2$ channels is observed. Although the chiral extrapolation is subtle, we assume $E_{Kπ}^2 \propto m_{u,d}^2 $, and obtain the $S$-wave scattering lengths as $a_0(I=3/2) m_π =-0.084^{+0.051}_{-0.064}$ and $a_0(I=1/2) m_π = -0.625 \pm0.012$. We show all necessary formulas which make the calculation possible. We argue that $ΛN$ is the most appropriate target of the Lüscher's formula for baryonic system because it has no $π$ exchange diagrams and has a scattering length suitable for a lattice QCD simulation.

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arXiv2009

Deconfinement phase transition in mirror of symmetries

We summarize and extend evidence that the deconfinement phase transition in Yang-Mills theories can be viewed as change of effective non-perturbative degrees of freedom and of symmetries of their interactions. In short, the strings in four dimensions (4d) at temperatures below the critical temperature Tc are replaced by particles, or field theories in 3d at T>Tc. The picture emerges within various approaches based, in particular, on dual models, lattice data and field theoretic models. We concentrate mostly on the lattice data, or on the language of quantum geometry.

hep-ph↗

arXiv2008

Abelian monopoles and center vortices in Yang-Mills plasma

Condensation of the Abelian monopoles and the center vortices leads to confinement of color in low temperature phase of Yang-Mills theory. We stress that these topological magnetic degrees of freedom are also very important in the deconfinement regime: at the point of the deconfinement phase transition both the monopoles and the vortices are released into the thermal vacuum contributing, in particular, to the equation of state and, definitely, to transport properties of the hot gluonic medium. Thus, we argue that a novel, magnetic component plays a crucial role. On the other hand, it was demonstrated that an effective three-dimensional description can be brought, beginning with high temperatures, down to the critical temperature by postulating existence of a system of 3d Higgs fields. We propose to identify the 3d color-singlet Higgs field with the 3d projection of the 4d magnetic vortices. Such identification fits well the 3d properties of the theory and contributes to interpretation of the magnetic component of the Yang-Mills plasma.

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arXiv2007

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.

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arXiv2007

Topological defects and equation of state of gluon plasma

We show that the degrees of freedom associated with magnetic monopole- and vortexlike gluonic configurations make a strong contribution to the anomaly of the energy-momentum tensor of Yang-Mills theory in the deconfinement phase immediately above the critical temperature. As is well known in zero-temperature Yang-Mills theory, the monopoles and vortices are constituents of a generic gluonic object in which the two neighbor monopoles are connected together by a segment of vortex string. Our results provide evidence that the monopole-vortex chains in SU(2) gauge theory and their SU(3) counterparts, the monopole-vortex nets, are thermodynamically relevant degrees of freedom in the gluonic plasma.

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arXiv2007

Lattice Study of Low-lying Nonet Scalar Mesons in Quenched Approximation

Using lattice QCD simulation in the quenched approximation, we study the $κ$ meson, which is ^3P_0 in the quark model, and compare experimental and other lattice data. The $κ$ is the lowest scalar meson with strangeness and constitutes the scalar nonet. The obtained mass is much higher than the recent experimental value, and therefore the $κ(800)$ is difficult to consider as a simple two-body constituent-quark structure, and may have another unconventional structure.

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arXiv2006

Quark propagators at finite temperature with the clover action

We study properties of the finite temperature quark propagator by using the SU(3) quenched lattice simulation in the Landau gauge and report numerical results of the standard Wilson quark case as well as the improved clover one. The mass function in the deconfinement phase is different from that of the confinement phase, especially at low momentum regions.

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arXiv2005

Transport Coefficient of Gluon Plasma from Lattice QCD

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($η$) 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 $η/s$ 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 $ρ^{high}$ 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 $η$($=πdρ/dω$ at $ω=0$), and qualitative results will not be changed when the accurate spectral function is obtained.

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arXiv2004

Contribution of disconnected diagrams to the hyperfine splitting of charmonium

We discuss the contribution of OZI-suppressed diagrams to the hyperfine splitting of charmonium in lattice QCD. We study valence quark mass regions from strange to charm quark masses. No contribution of the disconnected diagram is seen in the vector meson channel. In the pseudo-scalar channel and for valence quark masses around the strange quark, the disconnected contribution induces a considerable increase of the meson mass. This contribution quickly decreases as the quark mass increases. For charmonium the effect is very small although a decrease of the pseudoscalar mass induced by the disconnected contribution cannot be ruled out.

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arXiv2004

Heavy $qq$ interaction at finite temperature

The first lattice QCD numerical study of heavy quark-quark potentials at finite temperature is reported. Using the quenched approximation, we evaluate the color anti-symmetric and symmetric potentials.

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arXiv2004

Transport Coefficients of Gluon Plasma

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 $.

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arXiv2004

Long-distance behavior of $q\bar{q}$ 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 $α$ is changed from 0.6 to 1.3.

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arXiv2003

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

The quantum correction of the anisotropy parameter, $η$, is calculated for $ξ=2$ and 3 in the $β$ 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 $β$ dependences of $η$ for the renormalization group improved actions are quite different from those of the standard and Symanzik actions. In Iwasaki and DBW2 actions, $η$ stays almost constant in a wide range of $β$, which also differs from the one-loop perturbative result, while in the case of Symanzik action, it increases as $β$ decreases, which is qualitatively similar to the perturbative result, but the slope is steeper. In the calculation of the $η$ 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.

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arXiv2003

Scalar Mesons in Lattice QCD

We explore whether "f$_{0}$(600) or $σ$" exists as a pole in QCD, by a full lattice QCD simulation on the $8^{3} \times 16$ lattice using the plaquette action and Wilson fermions. It is shown that a low-mass $σ$ pole exists owing to the disconnected diagram of the meson propagator. A discussion is given on the physical content of the $σ$.

hep-ph↗

arXiv2003

Lattice QCD at Finite Density -- An introductory review

This is a pedagogical review of the lattice study of finite density QCD, which is intended to provide the minimum necessary contents, so that the paper may be used as the first reading for a newcomer to the field and also for those working in nonlattice communities. After a brief introduction to argue why finite density QCD can be a new attractive subject, we describe fundamental formulae which are necessary for the following sections. Then we survey lattice QCD simulations in small chemical potential regions, where several prominent works have been reported recently. Next, two-color QCD calculations are discussed, where we have a chance to glance at many new features of finite density QCD, and indeed recent simulations indicated quark pair condensation and the in-medium effect. Tables of SU(3) and SU(2) lattice simulations at finite baryon density are given. In the next section, we make a survey of several related works which may be a starting point of the new development in the future, although some works do not attract much attention now. Materials are described in a pedagogical manner. Starting from a simple 2-d model, we briefly discuss a lattice analysis of NJL model. We describe a non-perturbative analytical approach, i.e., strong coupling approximation method and some results. Canonical ensemble approach instead of usual grand canonical ensample may be another route to reach high density. We examine the density of state method and show this old idea includes recently proposed factorization method. An alternative method, complex Langevin equation and an interesting model, finite isospin model, are also discussed. In Appendix, we give several technical points which are useful in practical calculations.

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arXiv2002

Behavior of Hadrons at Finite Density -- Lattice Study of Color SU(2) QCD

Using two-color lattice QCD with Wilson fermions, we report a study of the finite baryon number density system with two-flavors. First we investigate the Polyakov line and thermodynamical uantities in the $(κ,μ)$ plane, where $κ$ and $μ$ are the hopping parameter and chemical potential in the fermion action, respectively. Then we calculate propagators of meson ($\bar{q}Γq$) and baryon ($qΓq$) states. We find that the vector meson propagators are strongly modified in large $μ$ regions, indicating the reduction of the mass. This anomalous behavior of the vector meson is observed for the first time in lattice QCD.

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arXiv2002

Lattice study of "f$_{0}$(600) or $σ$"

We investigate the propagator of "f$_{0}$(600) or the $σ$" by the full-QCD simulation with Wilson fermions. We calculate the mesonic correlator in the I=0, $J^P=0^{+}$ channel on the $8^{3} \times 16$ lattice. Plaquet action and Wilson fermion action are adopted. A coupling constant $β$ is set to 4.8 and three kinds of hopping parameter, $κ$=0.1846, 0.1874 and 0.1891 are assayed. The disconnected diagram in the propagator is evaluated through taking average over 500 or 1000 Z2 noise. Simulations with the larger hopping parameter provide us with less noisy results. Though the statistics is not yet enough, our results indicate the existence of a pole with a mass in almost the same order as that of the $ρ$.

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