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At least 667 records · Page 37Linked to original sources
arXiv2009

$Λ_{QCD}$ from gluon and ghost propagators

Fundamental quantities of QCD, such as the strong coupling and $Λ_{QCD}$, are studied in the framework of lattice QCD with $N_f=2$ twisted mass fermions. In particular, the contact between lattice and continuous calculations is made by comparing the renormalized ghost-gluon vertex in MOM scheme with 4-loop perturbative results. A power correction is needed in order to have agreement between the two descriptions. This suggests the presence of a dimension-two $\VEV{A^2}$ gluon condensate whose value is found to be higher than in the quanched case.

hep-lat↗
arXiv2009

Topological configurations of Yang-Mills field responsible for magnetic-monopole loops as quark confiner

We have given a new description of the lattice Yang-Mills theory a la Cho-Faddeev-Niemi-Shabanov, which has enabled us to confirm in a gauge-independent manner "Abelian"-dominance and magnetic-monopole dominance in the Wilson loop average, yielding a gauge-independent dual superconductor picture for quark confinement. In particular, we have given a new procedure (called reduction) for obtaining a gauge-independent magnetic monopole from a given Yang-Mills field. In this talk, we demonstrate how some of known topological configurations in the SU(2) Yang-Mills theory such as merons and instantons generate closed loops of magnetic-monopole current as the quark confiner, both of which are characterized by the gauge-invariant topological index, topological charge (density) and magnetic charge (density), respectively. We also try to detect which type of topological configurations exist in the lattice data involving magnetic-monopole loops generated by Monte Carlo simulation. Here we apply a new geometrical algorithm based on "computational homology" to discriminating each closed loop from clusters of magnetic-monopole current, since the magnetic-monopole current on a lattice is integer valued.

hep-lat↗
arXiv2009

Pure gauge glueballs at finite temperature

Pure gauge glueballs at finite temperature are investigated in a large temperature range from $0.3T_c$ to $1.9T_c$ on anisotropic lattices. Optimized glueball operators are used to obtain better signals. It is found in all 20 symmetry channels that the pole masses $M_G$ of glueballs remain almost constants when the temperature approaches the critical temperature $T_c$ from below, and start to reduce gradually with the temperature going above $T_c$. The glueball correlators in $0^{++}$, $0^{-+}$, and $2^{++}$ channels, are also analyzed based on the Breit-Wigner ansatz by assuming a thermal width $Γ$ to the pole mass $ω_0$. While $ω_0$'s are insensitive to $T$ in the whole temperature range, $Γ$'s exhibit distinct behavior below and above $T_c$: They are only few percents of $ω_0$ when $T T_c$ and reach values of roughly $Γ\sim ω_0/2$ at $T\approx 1.9T_c$.

hep-lat↗
arXiv2009

Pion form factors from lattice QCD with exact chiral symmetry

We calculate pion vector and scalar form factors in two-flavor lattice QCD and study the chiral behavior of the vector and scalar radii _{V,S}. For a direct comparison with chiral perturbation theory (ChPT), chiral symmetry is exactly preserved by employing the overlap quark action. We utilize the all-to-all quark propagator in order to calculate the scalar form factor including the contributions of disconnected diagrams. A detailed comparison with ChPT reveals that two-loop contributions are important to describe the chiral behavior of the radii in our region of the pion mass M_π\gtrsim 290 MeV. From chiral extrapolation based on two-loop ChPT, we obtain _V = 0.409(23)(37) fm^2 and _S = 0.617(79)(66) fm^2, which are consistent with phenomenological analyses.

hep-ph↗
arXiv2009

B-Bbar Mixing and Matching with Fermilab Heavy Quarks

We discuss the matching procedure for heavy-light 4-quark operators using the Fermilab method for heavy quarks and staggered fermions for light quarks. These ingredients enable us to construct the continuum-limit operator needed to determine the oscillation frequency of neutral B mesons. The matching is then carried out at the one-loop level. We also present an updated preliminary result for the SU(3)-breaking ratio ξ, based on calculations using the MILC Collaboration's ensembles of lattice gauge fields.

hep-lat↗
arXiv2009

The QCD equation of state and transition at finite temperature

We present the latest results for the equation of state and the crossover transition in 2+1 flavor QCD from the HotQCD Collaboration. Bulk thermodynamic quantities - energy density, pressure, entropy density, and the speed of sound - are calculated on lattices with temporal extent $N_t = 8$ in the temperature range 140 < T < 540 MeV. We utilize two improved staggered fermion actions, asqtad and p4, with the mass for the two degenerate light quarks chosen to be $m_{ud} = 0.1 m_s$, corresponding to $m_π\approx 220$ MeV for the lightest pion. We also calculate observables that are sensitive to the chiral and deconfing transitions - the light and strange quark number susceptibilities, the chiral condensate, and the renormalized Polyakov loop - finding that deconfinement and chiral symmetry restoration occur in the same narrow temperature interval.

hep-lat↗
arXiv2009

N=1 super Yang-Mills on the lattice

We present results from a numerical study of N=1 supersymmetric Yang-Mills theory using domain wall fermions. A set of dynamical simulations were performed for the gauge group SU(2) using the Wilson gauge action on 8^3x8 and 16^3x32 lattices. We considered a range of gluino masses (i.e., fifth dimension extents L_s=16-28 and input gluino mass values m_f=0.01-0.04) in order to perform chiral limit extrapolations of physical quantities. In these proceedings, we summarize our findings from a study of the Dirac spectrum and present new results for the topological charge on beta=2.3, 2.353 and 2.4 ensembles.

hep-lat↗
arXiv2009

Progress on charm semileptonic form factors from 2+1 flavor lattice QCD

Lattice calculations of the form factors for the charm semileptonic decays D to K l nu and D to pi l nu provide inputs to direct determinations of the CKM matrix elements |V(cs)| and |V(cd)| and can be designed to validate calculations of the form factors for the bottom semileptonic decays B to pi l nu and B to K l l-bar. We are using Fermilab charm (bottom) quarks and asqtad staggered light quarks on the 2+1 flavor asqtad MILC ensembles to calculate the charm (bottom) form factors. We outline improvements to the previous calculation of the charm form factors and detail our progress. We expect our current round of data production to allow us to reduce the theoretical uncertainties in |V(cs)| and |V(cd)| from 10.5% and 11%, respectively, to about 7%.

hep-lat↗
arXiv2009

$π^0$ to two-photon decay in lattice QCD

We calculate the neutral pion ($π^0$) to off-shell two photon ($γ^*γ^*$) transition form factor in lattice QCD. The transition form factor can be extracted from the three-point function of the form (axial-vector)-(vector)-(vector) as a function of off-shell two-photon momentum. Since the axial-anomaly plays an important role in the $π^0\toγγ$ decay process, we employ the overlap fermion, which preserves the exact chiral symmetery on the lattice. After extrapolating to the chiral and the vanishing photon momentum limit with a fit function based on vector meson dominance (VMD) model, we find that the Adler-Bell-Jackiw anomaly is correctly reproduced.

hep-lat↗
arXiv2010

Non-lattice simulation of supersymmetric gauge theories as a probe to quantum black holes and strings

In the past decade we have witnessed remarkable developments in the gauge-gravity duality, which suggested a new approach to superstring theory and quantum space-time. In this context it is important to study supersymmetric large-N gauge theories in the strongly coupled regime. I will summarize the results and insights obtained so far by non-lattice simulations. A simple example of the gauge-gravity duality is the one between 1d U(N) gauge theory with 16 supercharges and the so-called black 0-brane solution in type IIA supergravity. In order for this duality to be valid, one has to take the 't Hooft large-N limit and to take the strong coupling limit on the gauge theory side. The gauge theory can be regularized by fixing the gauge completely thanks to one dimension, and by introducing a Fourier mode cutoff. One can then use the standard RHMC algorithm to simulate the system. The energy calculated as a function of the temperature was compared with the results obtained from the gravity side based on the black hole thermodynamics. This confirmed the gauge-gravity duality with high accuracy and provided the microscopic origin of the black hole thermodynamics. From the calculation of the Wilson loop, one obtains the Schwarzschild radius of the dual geometry. One can actually use the present 1d model with supersymmetric mass deformation to study \mathcal{N}=4 super Yang-Mills theory on R \times S^3 based on a novel large-N reduction, which generalizes the original idea of Eguchi and Kawai. It is remarkable that we can now simulate the 4d superconformal field theory, which appears in the most typical case of the gauge-gravity duality known as the AdS/CFT correspondence. In particular, no fine-tuning is required unlike previous proposals based on the lattice regularization.

hep-lat↗
arXiv2009

Integrable structures in LGTs near the deconfinement transition

In this contribution we review some recent results about the emergence of 2D integrable systems in 3D Lattice Gauge Theories near the deconfinement transition. We focus on some concrete examples involving the flux tube thickness, the ratio of k-string tensions and Polyakov loops correlators in various models.

hep-lat↗
arXiv2009

Lattice QCD Analysis for Gluons

Nonperturbative properties of gluons are studied in SU(3) lattice QCD at the quenched level. The first subject is a functional-form analysis of the gluon propagator $D_{μν}^{ab}(x)$ in the Landau gauge. We find that the gluon propagator $D_{μμ}^{aa}(r)$ obtained in lattice QCD is well described by the four-dimensional (4D) Yukawa-type function $e^{-mr}/r$ with $m \simeq 600$MeV for the Euclidean 4D distance $r = 0.1 \sim 1.0$ fm. In momentum space, the gluon propagator $\tilde D_{μμ}^{aa}(p^2)$ ($p= 0.5 \sim 3$ GeV) is found to be well approximated with a new-type propagator of $(p^2+m^2)^{-3/2}$, which corresponds to 4D Fourier image of the Yukawa-type function. Associated with the Yukawa-type gluon propagator, we derive analytical expressions for the zero-spatial-momentum propagator $D_0(t)$, the effective mass $M_{\rm eff}(t)$, and the spectral function $ρ(ω)$ of the gluon field. The mass parameter $m$ turns out to be the infrared effective mass of gluons. The obtained gluon spectral function $ρ(ω)$ is almost negative-definite for $ω>m$, except for a positive $δ$-functional peak at $ω=m$. The second subject is a lattice-QCD determination of the relevant gluonic momentum-component for color confinement. As a remarkable fact, the string tension is found to be almost unchanged even after cutting off the high-momentum gluon component above 1.5 GeV in the Landau gauge. In fact, the relevant gluonic scale for color confinement is concluded to be below 1.5 GeV.

hep-lat↗
arXiv2009

The strange quark content of the nucleon in 2+1 flavor lattice QCD

The strangeness of the nucleon, - <0|ss|0>, is a quantity of interest for interpreting the results of dark matter detection experiments as well as for exploring the structure of the nucleon itself. We present a calculation of this quantity in 2+1 flavor lattice QCD using a range of lattice spacings and quark masses. The method is based on calculating quark-line disconnected contributions on the MILC lattice configurations, which include the effects of dynamical strange quarks. After continuum and chiral extrapolations, the value is - <0|ss|0> = 0.69 +/- 0.07(stat) +/- 0.09(sys) in the msbar(2GeV) regularization.

hep-lat↗
arXiv2009

Schwinger-Dyson equations and the quark-antiquark static potential

In lattice QCD, a confining potential for a static quark-antiquark pair can be computed with the Wilson loop technique. This potential, dominated by a linear potential at moderate distances, is consistent with the confinement with a flux tube, an extended and scalar system also directly observable in lattice QCD. Quantized flux tubes have also been observed in another class of confinement, the magnetic confinement in type II superconductors. On the other hand the solution of Schwinger Dyson Equations, say with the Landau gauge fixing and the truncation of the series of Feynman diagrams, already at the rainbow level for the self energy and at the ladder level for the Bethe Salpeter equation, provides a signal of a possible inverse quartic potential in momentum space derived from one gluon and one ghost exchange, consistent with confinement. Here we address the successes, difficulties and open problems of the matching of these two different perspectives of confinement, the Schwinger-Dyson perspective versus the flux tube perspective.

hep-lat↗
arXiv2009

Delta I=3/2, K to Pi Pi Decays with Light, Non-Zero Momentum Pions

Delta I=3/2, K to Pi Pi matrix elements are calculated on 68 configurations of quenched 24^3 x 64 lattices using the DBW2 action, and domain wall fermions with L_s=16. The lattice spacing is a^(-1)=1.3 GeV, corresponding to a physical volume of (3.6 fm)^3, which allows us to simulate a pion mass of m_Pi=227.6(6) MeV and a kaon mass of m_K=564(2) MeV. Twisted boundary conditions are used to give the two pions momentum. One twist corresponds to a pion momentum of p=Pi/L=170 MeV, which represents a decay that is nearly on-shell. Results for time separations of 20, 24, 28, and 32 between the kaon and the two pions are computed and an error weighted average is performed to reduced the error. The matrix elements are then found to have errors of order 3-4% for momentum 0 and Pi/L, 7% for momentum sqrt(2)*Pi/L, and 15% for momentum sqrt(3)*Pi/L.

hep-lat↗
arXiv2009

The role of multigrid algorithms for LQCD

We report on the first successful QCD multigrid algorithm which demonstrates constant convergence rates independent of quark mass and lattice volume for the Wilson Dirac operator. The new ingredient is the adaptive method for constructing the near null space on which the coarse grid multigrid Dirac operator acts. In addition we speculate on future prospects for extending this algorithm to the Domain Wall and Staggered discretizations, its exceptional suitability for high performance GPU code and its potential impact on simulations at the physical pion mass.

hep-lat↗
arXiv2009

The Landau gauge gluon propagator in 4D SU(2) lattice gauge theory revisited: Gribov copies and scaling properties

Lattice results for the gluon propagator in SU(2) pure gauge theory obtained on large lattices are presented. Simulated annealing is used throughout to fix the Landau gauge. We concentrate on checks for Gribov copy effects and for scaling properties. Our findings are similar to the ones in the SU(3) case, supporting the decoupling-type infrared behaviour of the gluon propagator.

hep-lat↗