Light hadron properties with improved staggered quarks

Preliminary results from simulations with 2+1 dynamical quark flavors at a lattice spacing of 0.09 fm are combined with earlier results at a=0.13 fm. We examine the approach to the continuum limit and investigate the dependence of the pseudoscalar masses and decay constants as the sea and valence quark masses are separately varied.Comment: Lattice2002(spectrum

Analytic estimates for penguin operators in quenched QCD

Strong penguin operators are singlets under the right-handed flavor symmetry group SU(3)_R. However, they do not remain singlets when the operator is embedded in (partially) quenched QCD, but instead they become linear combinations of two operators with different transformation properties under the (partially) quenched symmetry group. This is an artifact of the quenched approximation. Each of these two operators is represented by a different set of low-energy constants in the chiral effective theory. In this paper, we give analytic estimates for the leading low-energy constants, in quenched and partially quenched QCD. We conclude that the effects of quenching on Q_6 are large.Comment: 6 pages. Typo fixed and an explanatory footnote adde

Threshold Photo/Electro Pion Production - Working Group Summary

We summarize the pertinent experimental and theoretical developments in the field of pion photo- and electroproduction in the threshold region. We discuss which experiments and which calculations should be done/performed in the future.Comment: plain TeX (macro included), 6pp, summary talk presented at the workshop on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199

A Classification of random Dirac fermions

We present a detailed classification of random Dirac hamiltonians in two spatial dimensions based on the implementation of discrete symmetries. Our classification is slightly finer than that of random matrices, and contains thirteen classes. We also extend this classification to non-hermitian hamiltonians with and without Dirac structure.Comment: 15 pages, version2: typos in the table of classes are correcte

Unitarized pion-nucleon scattering within Heavy Baryon Chiral Perturbation Theory

By means of the Inverse Amplitude Method we unitarize the elastic pion-nucleon scattering amplitudes obtained from Heavy Baryon Chiral Perturbation Theory to O(q^3). Within this approach we can enlarge their applicability range and generate the Delta(1232) resonance. We can find a reasonable description of the pion nucleon phase shifts with (q^2) parameters in agreement with the resonance saturation hypothesis. However, the uncertainties in the analysis of the low energy data as well as the large number of chiral parameters, which can have strong correlations, allow us to obtain very good fits with rather different sets of chiral constants.Comment: Shortened version to appear in Phys. Rev. D. Brief Report

A Cosmic Watershed: the WVF Void Detection Technique

On megaparsec scales the Universe is permeated by an intricate filigree of clusters, filaments, sheets and voids, the Cosmic Web. For the understanding of its dynamical and hierarchical history it is crucial to identify objectively its complex morphological components. One of the most characteristic aspects is that of the dominant underdense Voids, the product of a hierarchical process driven by the collapse of minor voids in addition to the merging of large ones. In this study we present an objective void finder technique which involves a minimum of assumptions about the scale, structure and shape of voids. Our void finding method, the Watershed Void Finder (WVF), is based upon the Watershed Transform, a well-known technique for the segmentation of images. Importantly, the technique has the potential to trace the existing manifestations of a void hierarchy. The basic watershed transform is augmented by a variety of correction procedures to remove spurious structure resulting from sampling noise. This study contains a detailed description of the WVF. We demonstrate how it is able to trace and identify, relatively parameter free, voids and their surrounding (filamentary and planar) boundaries. We test the technique on a set of Kinematic Voronoi models, heuristic spatial models for a cellular distribution of matter. Comparison of the WVF segmentations of low noise and high noise Voronoi models with the quantitatively known spatial characteristics of the intrinsic Voronoi tessellation shows that the size and shape of the voids are succesfully retrieved. WVF manages to even reproduce the full void size distribution function.Comment: 24 pages, 15 figures, MNRAS accepted, for full resolution, see http://www.astro.rug.nl/~weygaert/tim1publication/watershed.pd

Is Heavy Baryon Approach Necessary?

It is demonstrated that using an appropriately chosen renormalization condition one can respect power counting within the relativistic baryon chiral perturbation theory without appealing to the technique of the heavy baryon approach. Explicit calculations are performed for diagrams including two-loops. It is argued that the introduction of the heavy baryon chiral perturbation theory was useful but not necessary.Comment: 9 pages, 2 figures, minor changes, references adde

Heavy-Light Semileptonic Decays in Staggered Chiral Perturbation Theory

We calculate the form factors for the semileptonic decays of heavy-light pseudoscalar mesons in partially quenched staggered chiral perturbation theory (\schpt), working to leading order in $1/m_Q$, where $m_Q$ is the heavy quark mass. We take the light meson in the final state to be a pseudoscalar corresponding to the exact chiral symmetry of staggered quarks. The treatment assumes the validity of the standard prescription for representing the staggered ``fourth root trick'' within \schpt by insertions of factors of 1/4 for each sea quark loop. Our calculation is based on an existing partially quenched continuum chiral perturbation theory calculation with degenerate sea quarks by Becirevic, Prelovsek and Zupan, which we generalize to the staggered (and non-degenerate) case. As a by-product, we obtain the continuum partially quenched results with non-degenerate sea quarks. We analyze the effects of non-leading chiral terms, and find a relation among the coefficients governing the analytic valence mass dependence at this order. Our results are useful in analyzing lattice computations of form factors $B\to\pi$ and $D\to K$ when the light quarks are simulated with the staggered action.Comment: 53 pages, 8 figures, v2: Minor correction to the section on finite volume effects, and typos fixed. Version to be published in Phys. Rev.

Comparison of finite-size-scaling functions for 3d O(N) spin models to QCD

We calculate numerically universal finite-size-scaling functions of the magnetization for the three-dimensional O(4) and O(2) spin models. The approach of these functions to the infinite-volume scaling functions is studied in detail on the critical and pseudocritical lines. For this purpose we determine the pseudocritical line in two different ways. We find that the asymptotic form of the finite-size-scaling functions is already reached at small values of the scaling variable. A comparison with QCD lattice data for two flavours of staggered fermions shows a similar finite-size behaviour which is compatible with that of the spin models.Comment: Lattice2001(hightemp), 3 pages, 5 figures, acknowledgements completed, minor typographical errors correcte

The pion charge radius from charged pion electroproduction

We analyze a low-energy theorem of threshold pion electroproduction which allows one to determine the charge radius of the pion. We show that at the same order where the radius appears, pion loops induce a correction to the momentum dependence of the longitudinal dipole amplitude $L_{0+}^{(-)}$. This model-independent correction amounts to an increase of the pion charge radius squared from the electroproduction data by about 0.26~fm$^2$. It sheds light on the apparent discrepancy between the recent determination of the pion radius from electroproduction data and the one based on pion-electron scattering.Comment: 3 pp, REVTeX, uses eps