Mon·Oct 22, 2007Monday·October 22, 2007

1. 01*

   Group Theoretical Construction of Nucleon Operators using All-to-All Quark Propagators

   R.G. Edwards🇺🇸 · G.T. Fleming🇺🇸 · B. Joo🇺🇸 · K.J. Juge🇺🇸 · A. Lichtl🇺🇸 · C.J. Morningstar🇺🇸 · D.G. Richards🇺🇸 · S.J. Wallace🇺🇸

   We describe a method to construct irreducible baryon operators using all-to-all quark propagators. It was demonstrated earlier that a large basis of extended baryon operators on anisotropic, quenched lattices can be used to reliably extract the masses of 5 or more excited states in the nucleon channel. All-to-all quark propagators are expected to be needed when studying these excited states on light, dynamical configurations because contributions from multi-particle states are expected to be significant. The dilution method is used to approximate the all-to-all quark propagators. Low-lying eigenmodes can also be used if necessary. For efficient computation of matrix elements of the interpolating operators, the algorithms should exploit the fact that many extended baryon operators can be obtained from the different linear combinations of three-quark colour-singlet operators. The sparseness of the diluted noise vectors also afford several computation simplifications. Some preliminary results are presented for nucleon effective masses.

   hep-latPoS(2007)·6 citations
2. 02*

   External Momentum, Volume Effects, and the Nucleon Magnetic Moment

   B. C. Tiburzi🇺🇸

   We analyze the determination of volume effects for correlation functions that depend on an external momentum. As a specific example, we consider finite volume nucleon current correlators, and focus on the nucleon magnetic moment. Because the multipole decomposition relies on SO(3) rotational invariance, the structure of such finite volume corrections is unrelated to infinite volume multipole form factors. One can deduce volume corrections to the magnetic moment only when a zero-mode photon coupling vanishes, as occurs at next-to-leading order in heavy baryon chiral perturbation theory. To deduce such finite volume corrections, however, one must assume continuous momentum transfer. In practice, volume corrections with momentum transfer dependence are required to address the extraction of the magnetic moment, or other observables that arise in momentum dependent correlation functions. Additionally we shed some light on a puzzle concerning differences in lattice form factor data at equal values of momentum transfer squared.

   hep-lathep-phnucl-thPRD(2008)·20 citations
3. 03*

   Speeding up HMC with better integrators

   M. A. Clark🇺🇸 · A. D. Kennedy🇬🇧

   We discuss how dynamical fermion computations may be made yet cheaper by using symplectic integrators that conserve energy much more accurately without decreasing the integration step size. We first explain why symplectic integrators exactly conserve a ``shadow'' Hamiltonian close to the desired one, and how this Hamiltonian may be computed in terms of Poisson brackets. We then discuss how classical mechanics may be implemented on Lie groups and derive the form of the Poisson brackets and force terms for some interesting integrators such as those making use of second derivatives of the action (Hessian or force gradient integrators). We hope that these will be seen to greatly improve energy conservation for only a small additional cost and that their use will significantly reduce the cost of dynamical fermion computations.

   hep-latPoS(2007)·11 citations
4. 04*

   Adaptive Multigrid Algorithm for the QCD Dirac-Wilson Operator

   J. Brannick🇺🇸 · R. C. Brower🇺🇸 · M. A. Clark🇺🇸 · J. C. Osborn🇺🇸 · C. Rebbi🇺🇸

   We present a new multigrid solver that is suitable for the Dirac operator in the presence of disordered gauge fields. The key behind the success of the algorithm is an adaptive projection onto the coarse grids that preserves the near null space. The resulting algorithm has weak dependence on the gauge coupling and exhibits mild critical slowing down in the chiral limit. Results are presented for the Wilson Dirac operator of the 2d U(1) Schwinger model.

   hep-latPoS(2007)·12 citations
5. 05*

   Hyperon-nucleon potentials from lattice QCD

   H. Nemura🇯🇵 · N. Ishii🇯🇵 · S. Aoki🇯🇵 · T. Hatsuda🇯🇵

   We calculate $p{\Xi }^0$ potentials from the equal-time Bethe-Salpeter amplitude measured in the quenched QCD simulation with the spatial lattice volume, (4.4 fm)${}^3$. The standard Wilson gauge action with the gauge coupling $\beta =5.7$ on $32^4$ lattice together with the standard Wilson quark action are used. The hopping parameter ${\kappa }_{ud}=0.1678$ is chosen for $u$ and $d$ quarks, which corresponds to $m_{\pi }\simeq 0.37$ GeV. The physical strange quark mass is used by taking the parameter ${\kappa }_s=0.1643$ which is deduced from the physical $K$ meson mass. The lattice spacing $a=0.1420$ fm is determined by the physical $\rho$ meson mass. We find that the $p{\Xi }^0$ potential has strong spin dependence. Strong repulsive core is found in ${}^1{S}_0$ channel while the effective central potential in the ${}^3{S}_1$ channel has relatively weak repulsive core. The potentials also have weak attractive parts in the medium to long distance region (0.6 fm $\text{lsim}r\text{lsim}1.2$ fm) in both of the ${}^1{S}_0$ and ${}^3{S}_1$ channels.

   hep-latPoS(2007)·5 citations
6. 06*

   Lattice calculation of the QGP viscosities - Present results and next project -

   Sunao Sakai Atsushi Nakamura

   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 $1<T/T_c<25$. Using these data the bulk viscosities are also determined in the region where $T/T_c<2$. They are roughly one order of magnitude smaller than the shear viscosities. Our next target is to determine the transport coefficients more precisely by a maximum-entropy method. For this purpose the most effective method may be to adopt an anisotropic lattice. In this report, we study the possible systematic error due to the deformation of the anisotropic lattice at short distances. Near the critical temperature, it is found that the standard action suffers from a large deformation on the anisotropic lattice at short distances, while the deformation is slight for Iwasaki's improved action. To reduce the fluctuation of the Matsubara Green function, the improvement of the energy momentum tensor operator by using clover-type loops is promising. We are also attempting to apply the multi-level algorithm to reduce fluctuation.

   hep-latPoS(2007)·50 citations
7. 07*

   A calculation of the bulk viscosity in SU(3) gluodynamics

   Harvey B. Meyer🇺🇸

   We perform a lattice Monte-Carlo calculation of the trace-anomaly two-point function at finite temperature in the SU(3) gauge theory. We obtain the long-distance properties of the correlator in the continuum limit and extract the bulk viscosity zeta via a Kubo formula. Unlike the tensor correlator relevant to the shear viscosity, the scalar correlator depends strongly on temperature. If s is the entropy density, we find that zeta/s becomes rapidly small at high T, zeta/s<0.15 at 1.65T_c and zeta/s<0.015 at 3.2T_c. However zeta/s rises dramatically just above T_c, with 0.5<zeta/s<2.0 at 1.02T_c.

   hep-lathep-phnucl-thPRL(2008)·355 citations
8. 08*

   Composite Strings in (2+1)-Dimensional Anisotropic Weakly-Coupled Yang-Mills Theory

   Peter Orland🇺🇸

   The small-scale structure of a string connecting a pair of static sources is explored for the weakly-coupled anisotropic SU(2) Yang-Mills theory in (2+1) dimensions. A crucial ingredient in the formulation of the string Hamiltonian is the phenomenon of color smearing of the string constituents. The quark-anti-quark potential is determined. We close with some discussion of the standard, fully Lorentz-invariant Yang-Mills theory.

   ↳ hep-thcond-mat.str-elhep-latmath-ph+1PRD(2008)·16 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.