Do Wilson Fermions Induce an Adjoint Gauge Coupling?

Expansions of the Wilson determinant in lattice QCD with quarks produce gauge action terms which shift the coupling constant of the fundamental representation plaquette action and induce an adjoint representation plaquette action. We study the magnitude of these induced couplings with two flavors of Wilson fermions. We utilize a microcanonical demon method, which allows us to measure the induced couplings directly from gauge configurations generated by full fermionic simulations.Comment: 3 pages postscript, proceedings for LATTICE '9

Theory of incommensurate magnetic correlations across the insulator-superconductor transition in underdoped La_{2-x}Sr_xCuO_4

The main feature in the elastic neutron scattering of La_{2-x}Sr_xCuO_4 is the existence of incommensurate peaks with positions that jump from 45 to 0 degrees at 5% doping. We show that the spiral state of the t-t'-t''-J model with realistic parameters describes this data perfectly. We explain why in the insulator the peak is at 45 degrees while it switches to 0 degrees precisely at the insulator-metal transition. The calculated positions of the peaks are in agreement with the data in both phases.Comment: 5 pages, 1 figure; minor change

A numerical study of the RG equation for the deformed O(3)O(3) nonlinear sigma model

The Renormalization Group equation describing the evolution of the metric of the nonlinear sigma model poses some nice mathematical problems involving functional analysis, differential geometry and numerical analysis. In this article we briefly report some results obtained from the numerical study of the solutions in the case of a two dimensional target space (deformation of the $O(3)$ sigma model). In particular, our analysis shows that the so-called sausages define an attracting manifold in the $U(1)$-symmetric case, at one-loop level. Moreover, data from two-loop evolution are used to test the association put forward in Nucl. Phys., B406 (1993) 521 between the so-called $SSM_{\nu}$ field theory and a certain $U(1)$-symmetric, factorized scattering theory (FST).Comment: LaTex (RevTex style), 16 pages, 6 uuencoded figures. Minor technical changes

B_K from quenched overlap QCD

We present an exploratory calculation of the standard model Delta S=2 matrix element relevant for indirect CP violation in K -> pi pi decays. The computation is performed with overlap fermions in the quenched approximation at beta=6.0 on a 16^3x32 lattice. The resulting bare matrix element is renormalized non-perturbatively. Our preliminary result is B_K^{NDR}(2 GeV)=0.61(7), where the error does not yet include an estimate of systematic uncertainties.Comment: 3 pages, 2 figures, Lattice2002(matrixel). Parallel session talk by L. Lellouch at the 20th International Symposium on Lattice Field Theory (Lattice 2002), Boston, Massachusetts, 24-29 June 200

Localized eigenmodes of the overlap operator and their impact on the eigenvalue distribution

In a system where chiral symmetry is spontaneously broken, the low energy eigenmodes of the continuum Dirac operator are extended. On the lattice, due to discretization effects, the Dirac operator can have localized eigenmodes that affect physical quantities sensitive to chiral symmetry. While the infrared eigenmodes of the Wilson Dirac operator are usually extended even on coarse lattices, the chiral overlap operator has many localized eigenmodes in the physical region, especially in mixed action simulations. Depending on their density, these modes can introduce strong lattice artifacts. The effect can be controlled by changing the parameters of the overlap operator, in particular the clover improvement term and the center of the overlap projection.Comment: 16 pages, 6 figure

Conformal or Walking? Monte Carlo renormalization group studies of SU(3) gauge models with fundamental fermions

Strongly coupled gauge systems with many fermions are important in many phenomenological models. I use the 2-lattice matching Monte Carlo renormalization group method to study the fixed point structure and critical indexes of SU(3) gauge models with 8 and 12 flavors of fundamental fermions. With an improved renormalization group block transformation I am able to connect the perturbative and confining regimes of the N_f=8 flavor system, thus verifying its QCD-like nature. With N_f=12 flavors the data favor the existence of an infrared fixed point and conformal phase, though the results are also consistent with very slow walking. I measure the anomalous mass dimension in both systems at several gauge couplings and find that they are barely different from the free field value.Comment: 26 pages, 11 figure

't Hooft loops and consistent order parameters for confinement

We study ratios of partition functions in two types of sectors of SU(2), with fixed temporal center flux and with static fundamental charge. Both can be used as bona fide order parameters for the deconfinement transition.Comment: 3 Pages, LaTeX 2.09 with espcrc2.sty, 4 Figures (epsfig), for Lattice2001(confinement

Role of the σ\sigma-resonance in determining the convergence of chiral perturbation theory

The dimensionless parameter $\xi = M_\pi^2/(16 \pi^2 F_\pi^2)$, where $F_\pi$ is the pion decay constant and $M_\pi$ is the pion mass, is expected to control the convergence of chiral perturbation theory applicable to QCD. Here we demonstrate that a strongly coupled lattice gauge theory model with the same symmetries as two-flavor QCD but with a much lighter $\sigma$-resonance is different. Our model allows us to study efficiently the convergence of chiral perturbation theory as a function of $\xi$. We first confirm that the leading low energy constants appearing in the chiral Lagrangian are the same when calculated from the $p$-regime and the $\epsilon$-regime as expected. However, $\xi \lesssim 0.002$ is necessary before 1-loop chiral perturbation theory predicts the data within 1%. For $\xi > 0.0035$ the data begin to deviate dramatically from 1-loop chiral perturbation theory predictions. We argue that this qualitative change is due to the presence of a light $\sigma$-resonance in our model. Our findings may be useful for lattice QCD studies.Comment: 5 pages, 6 figures, revtex forma

Testing the self-duality of topological lumps in SU(3) lattice gauge theory

We discuss a simple formula which connects the field-strength tensor to a spectral sum over certain quadratic forms of the eigenvectors of the lattice Dirac operator. We analyze these terms for the near zero-modes and find that they give rise to contributions which are essentially either self-dual or anti self-dual. Modes with larger eigenvalues in the bulk of the spectrum are more dominated by quantum fluctuations and are less (anti) self-dual. In the high temperature phase of QCD we find considerably reduced (anti) self-duality for the modes near the edge of the spectral gap.Comment: Remarks added, to appear in Phys. Rev. Let