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arXiv2009

Mesonic and baryonic correlation functions at fine lattice spacings

We report on our on-going project to compute mesonic and baryonic two- and three-point correlation functions in simulations using Nf=2 flavours of O(a) improved Wilson quarks and the Wilson plaquette action. We present performance figures for the DD-HMC algorithm on commodity cluster hardware and discuss the issue of critical slowing down, which is particularly pronounced for the topological charge. The effectiveness of stochastic noise sources and Jacobi smearing are investigated. Our preliminary results obtained at three quark masses on 96x48^3 at beta=5.5 imply that the lattice spacing is about 0.06 fm, while the smallest pion mass in the current runs is around 360 MeV, which corresponds to m_pi*L=5.3.

hep-lat↗

arXiv2009

Determination of $B_K$ using improved staggered fermions (III) Finite volume effects

We study the finite-volume effects in our calculation of $B_K$ using HYP-smeared improved staggered valence fermions. We calculate the predictions of both SU(3) and SU(2) staggered chiral perturbation theory at one-loop order. We compare these to the results of a direct calculation, using MILC coarse lattices with two different volumes: $20^3$ and $28^3$. From the direct calculation, we find that the finite volume effect is $\approx 2%$ for the SU(3) analysis and $\approx 0.9%$ for the SU(2) analysis. We also show how the statistical error depends on the number of measurements made per configuration, and make a first study of autocorrelations.

hep-lat↗

arXiv2009

One loop matching factors for staggered bilinear operators with improved glue

We present results for matching factors for bilinear operators composed of HYP-smeared staggered fermions and constructed using HYP-smeared fat links. The matching factors are calculated perturbatively at one-loop order. The new feature of our calculation compared to previous work on HYP-smeared staggered fermions is the use of the Symanzik-improved gluon propagator, which allows our results to be applied to our ongoing simulations based on configurations generated by the MILC collaboration. We address the issue of the relative efficiency of various improvement schemes in reducing one-loop corrections to the matching factors.

hep-lat↗

arXiv2009

Spectrum to all orders of Polchinski-Strominger {Effective} String Theory of Polyakov-Liouville Type

The spectrum of a Polchinski-Strominger type effective string theory, extended to all orders, herein called an effective string theory of the \emph{Polyakov-Liouville Type} (for obvious reasons) is investigated to all orders in the small parameter $R^{-1}$. Here $R$ is the length of the \emph{closed} string. It is established that to \emph{all orders} the spectrum of this theory is \emph{identical} to that of the free bosonic string theory. While the latter is consistent only in the critical dimension $D_c=26$, the PS- type effective string theories are by construction consistent in \emph{all} dimensions. This work extends earlier results by Drummond, and, by Hari Dass and Matlock to order $R^{-3}$. When combined with Drummond's results about absence of candidate actions at orders $R^{-4},R^{-5}$, our results imply that the spectrum of \emph{all} effective string theories coincides with that of free bosonic string theories to order $R^{-5}$. This agrees with the recent results by Aharony and Karzbrun. Our work is the first all order analysis of any effective string theory.

hep-th↗

arXiv2009

N to Delta transition form factors with Nf=2+1 domain wall fermions

The electromagnetic, axial and pseudoscalar nucleon to Delta form factors are calculated using dynamical domain wall fermions at a lattice spacing of a = 0.114 fm on a lattice of spatial size 2.74 fm and pion mass of 331 MeV. Pion pole dominance and the Goldberger-Treiman relations are examined.

hep-lat↗

arXiv2009

Spectrum to all orders of Polchinski-Strominger Effective String Theories of the Drummond Type

Drummond had proposed four actions for Polchinski-Strominger effective string theories at order $R^{-6}$, where $2πR$ is the length of the (closed) string >. In \cite{covariant} it had been shown, based on covariance arguments, that only two of them are independent. We analyse the spectral content of effective string theories with these two actions. We show that the inclusion of these actions does not yield corrections to the spectrum of Nambu-Goto theory \cite{Arv}.

hep-th↗

arXiv2009

Dyson's Instability in Lattice Gauge Theory

We discuss Dyson's argument that the vacuum is unstable under a change g^2 -> - g^2, in the context of lattice gauge theory. For compact gauge groups, the partition function is well defined at negative g^2, but the average plaquette P has a discontinuity when g^2 changes sign. This reflects a change of vacuum rather than a loss of vacuum. In addition, P has poles in the complex g^2 plane, located at the complex zeros of the partition function (Fisher's zeros). We discuss the relevance of these singularities for lattice perturbation theory. We present new methods to locate Fisher's zeros using numerical values for the density of state in SU(2) and U(1) pure gauge theory. We briefly discuss similar issues for O(N) nonlinear sigma models where the local integrals are also over compact spaces.

hep-lat↗

arXiv2009

Comparing the vacuum structure of quenched and dynamical configurations

We systematically compare filtering methods used to extract topological structures on SU(3) lattice configurations. We show that there is a strong correlation of the topological charge densities obtained by APE and Stout smearing. To get rid of artifacts of these methods, we analyze structures that are also seen by Laplace filtering. This combined analysis shows that the topological charge density is more fragmented in the presence of dynamical quarks.

hep-lat↗

arXiv2009

Quark number fluctuations at high temperatures

We calculate the second, fourth and sixth order quark number fluctuations in the deconfined phase of 2+1 flavor QCD using lattices with temporal extent Nt=4, 6, 8 and 12. We consider light, strange and charm quarks. We use p4 action for valence quarks and gauge configurations generated with p4 action with physical value of the strange quark mass and light quark mass mq=0.1ms generated by the RBC-Bielefeld collaboration. We observe that for all quark masses the quark number fluctuations rapidly get close to the corresponding ideal gas limits. We compare our results to predictions of a quasi-particle model and resummed high temperature perturbative calculations. We also investigate correlations among different flavor channels.

hep-lat↗

arXiv2009

S-wave pi-K scattering length from lattice QCD

The $S$-wave $πK$ scattering lengths are calculated for both the isospin 1/2 and 3/2 channels in the lattice QCD by using the finite size formula. We perform the calculation with $N_f=2+1$ gauge configurations generated on $32^3 \times 64$ lattice using the Iwasaki gauge action and nonperturbatively $O(a)$-improved Wilson action at $1/a = 2.17$ GeV. The quark masses correspond to $m_π= 0.30 - 0.70$ GeV. For $I=1/2$, to separate the contamination from excited states, we construct a $2 \times 2$ matrix of the time correlation function and diagonalize it. Here, we adopt the two kinds of operators, $\bar{s}u$ and $πK$. It is found that the signs of the scattering lengths are in agreement with experiment, namely attraction in $I=1/2$ and repulsion in $I=3/2$. We investigate the quark-mass dependence of the scattering lengths and also discuss the limitation of chiral perturbation theory.

hep-lat↗

arXiv2009

Vortex Content of SU(2) Calorons and Multi-Calorons

We use Laplacian Center Gauge to reveal the vortex content of single SU(2) calorons and multi-caloron systems at different holonomies. The vortex surfaces in a single SU(2) caloron consist of two parts that are induced by the constituent dyon charges and by the twist between the dyons, respectively. The latter part percolates in a caloron ensemble at maximal nontrivial holonomy. This finding fits perfectly in the confinement scenario of vortices and shows that calorons are suitable to facilitate the vortex confinement mechanism.

hep-lat↗

arXiv2009

Quarkonium mass splittings with Fermilab heavy quarks and 2+1 flavors of improved staggered sea quarks

We present results from an ongoing lattice study of the lowest lying charmonium and bottomonium level splittings using the Fermilab heavy quark formalism. Our objective is to test the performance of this action on MILC-collaboration ensembles of (2+1) flavors of light improved staggered (asqtad) quarks. Measurements are done on 16 ensembles with degenerate up and down quarks of various masses, thus permitting a chiral extrapolation, and over lattice spacings ranging from 0.09 fm to 0.18 fm, thus permitting study of lattice-spacing dependence. We examine combinations of the mass splittings that are sensitive to components of the effective quarkonium potential.

hep-lat↗

arXiv2009

SU(2) chiral fits to light pseudoscalar masses and decay constants

We present the results of fits to recent asqtad data in the light pseudoscalar sector using SU(2) partially-quenched staggered chiral perturbation theory. Superfine (a~0.06 fm) and ultrafine (a~0.045 fm) ensembles are used, where light sea quark masses and taste splittings are small compared to the strange quark mass. Our fits include continuum NNLO chiral logarithms and analytic terms. We give preliminary results for the pion decay constant, SU(2) low-energy constants and the chiral condensate in the two-flavor chiral limit.

hep-lat↗

arXiv2009

Monte Carlo approach to turbulence

The behavior of the one-dimensional random-force-driven Burgers equation is investigated in the path integral formalism on a discrete space-time lattice. We show that by means of Monte Carlo methods one may evaluate observables, such as structure functions, as ensemble averages over different field realizations. The regularization of shock solutions to the zero-viscosity limit (Hopf-equation) eventually leads to constraints on lattice parameters required for the stability of the simulations. Insight into the formation of localized structures (shocks) and their dynamics is obtained.

hep-lat↗

arXiv2009

Simulations of supersymmetric Yang-Mills theory

Results of a numerical simulation concerning the low-lying spectrum of four-dimensional N=1 SU(2) Supersymmetric Yang-Mills (SYM) theory on the lattice with light dynamical gluinos are reported. We use the tree-level Symanzik improved gauge action and Wilson fermions with stout smearing of the gauge links in the Wilson-Dirac operator. The configurations are produced with the Two-Step Polynomial Hybrid Monte Carlo (TS-PHMC) algorithm. We performed simulations on lattices up to a size of 24^3x48 at β=1.6. Using QCD units with the Sommer scale being set to r_0=0.5 fm, the lattice spacing is about a~0.09 fm, and the spatial extent of the lattice corresponds to 2.1 fm to control finite size effects. At the lightest simulated gluino mass our results indicate a mass for the lightest gluino-glue bound state, which is considerably heavier than the values obtained for its possible superpartners. Whether supermultiplets are formed remains to be studied in upcoming simulations.

hep-lat↗

arXiv2009

Scaling properties of many-fermion systems from MCRG studies

Monte Carlo renormalization group methods were designed to study the phase structure and critical behavior of statistical systems. They are well suited to determine the running coupling and to investigate the properties of fixed points of gauge-fermion models, including the existence of conformal infrared fixed points in many-fermion systems. I discuss the implementation of the 2-lattice matching method and present results for SU(3) gauge theories with N_f=0,12 and 16 fundamental fermion flavors.

hep-lat↗

arXiv2009

Gauge-independent derivation of "Abelian" dominance and magnetic monopole dominance in the string tension

Recently, we have developed a reformulation of the lattice Yang-Mills theory based on the change of variables a la Cho-Faddeev-Niemi combined with a non-Abelian Stokes theorem. In this talk, we give a new procedure (called reduction) for obtaining the color field which plays the central role in this reformulation. In the 4D SU(2) lattice Yang-Mills theory, we confirm the gauge-independent "abelian" dominance and gauge-independent magnetic-monopole dominance in the string tension extracted from the Wilson loop in the fundamental representation.

hep-lat↗

arXiv2009

The charmed-strange meson spectrum from overlap fermions on domain wall dynamical fermion configurations

The charmed-strange meson spectrum is calculated with the overlap valence fermions on 2+1 flavor domain wall dynamical configurations for $32^3\times 64$ lattices with a spatial size of 2.7 fm. Both charm and strange quark propagators are calculated with the overlap fermion action. The calculated scalar meson at 2304(22) MeV and axial-vector meson at 2546(27) MeV are in good agreement with the experimental masses of $D{s0}^*$(2317) and $D_{s1}$(2536).

hep-ph↗