Lattice Gauge Field Interpolation for Chiral Gauge Theories

The importance of lattice gauge field interpolation for our recent non-perturbative formulation of chiral gauge theory is emphasized. We illustrate how the requisite properties are satisfied by our recent four-dimensional non-abelian interpolation scheme, by going through the simpler case of U(1) gauge fields in two dimensions.Comment: 3 pages. Based on talk presented at LATTICE96(Chiral Gauge

Nucleon Tensor Charge from Exclusive πo\pi^o Electroproduction

Exclusive $\pi^o$ electroproduction from nucleons is suggested for extracting the tensor charge and other quantities related to transversity from experimental data. This process isolates C-parity odd and chiral odd combinations of t-channel exchange quantum numbers. In a hadronic picture it connects the meson production amplitudes to C-odd Regge exchanges with final state interactions. In a description based on partonic degrees of freedom, the helicity structure for this C-odd process relates to the quark helicity flip, or chiral odd generalized parton distributions. This differs markedly from deeply virtual Compton scattering, and both vector meson and charged $\pi$ electroproduction, where the axial charge can enter the amplitudes. Contrarily the tensor charge enters the $\pi^o$ process. The connection through the helicity description of the process to both the partonic and hadronic perspectives is studied and exploited in model calculations to indicate how the tensor charge and other transversity parameters can be related to cross section and spin asymmetry measurements over a broad range of kinematics.Comment: 40 pages, 14 figures Revised text clarifying main points, fixing typos, adding reference

QED: Chiral transition and the issue of triviality

I give a review and progress report on studies of lattice QED. I emphasize analytical results and methods that are applied in data analysis. Also, I derive some bounds for the critical exponents and establish their connection with scaling violations. Triviality, as realized in $\phi^4$ theory, is ruled out on theoretical grounds. I show that the present data, if analyzed correctly, all lead to the same conclusions. They are compatible with power law scaling with nongaussian exponents.Comment: 6 pages, 7 figures (not included), ILL-(TH)-93-2

Gluon contributions to the pion mass and light cone momentum fraction

We calculate the matrix elements of the gluonic contributions to the energy-momentum tensor for a pion of mass 600 < Mpi < 1100 MeV in quenched lattice QCD. We find that gluons contribute (37 +/- 8 +/- 12)% of the pion's light cone momentum. The bare matrix elements corresponding to the trace anomaly contribution to the pion mass are also obtained. The discretizations of the energy-momentum tensor we use have other promising applications, ranging from calculating the origin of hadron spin to QCD thermodynamics.Comment: 4 pages, 2 figure

On the Triviality of Textbook Quantum Electrodynamics

By adding a small, irrelevant four fermi interaction to the action of lattice Quantum Electrodynamics (QED), the theory can be simulated with massless quarks in a vacuum free of lattice monopoles. This allows an ab initio high precision, controlled study of the existence of "textbook" Quantum Electrodynamics with several species of fermions. The lattice theory possesses a second order chiral phase transition which we show is logarithmically trivial. The logarithms of triviality, which modify mean field scaling laws, are pinpointed in several observables. The result supports Landau's contention that perturbative QED suffers from complete screening and would have a vanishing fine structure constant in the absence of a cutoff.Comment: reference to Phys. Rev. Lett.80, 4119(1998) adde

Using NSPT for the Removal of Hypercubic Lattice Artifacts

The treatment of hypercubic lattice artifacts is essential for the calculation of non-perturbative renormalization constants of RI-MOM schemes. It has been shown that for the RI'-MOM scheme a large part of these artifacts can be calculated and subtracted with the help of diagrammatic Lattice Perturbation Theory (LPT). Such calculations are typically restricted to 1-loop order, but one may overcome this limitation and calculate hypercubic corrections for any operator and action beyond the 1-loop order using Numerical Stochastic Perturbation Theory (NSPT). In this study, we explore the practicability of such an approach and consider, as a first test, the case of Wilson fermion bilinear operators in a quenched theory. Our results allow us to compare boosted and unboosted perturbative corrections up to the 3-loop order.Comment: 7 pages, 6 figures, talk presented at the 32nd International Symposium on Lattice Field Theory (Lattice 2014), 23-28 June 2014, New York, USA; PoS(LATTICE2014)29

Leading and higher twists in proton, neutron and deuteron unpolarized structure functions

We summarize the results of a recent global analysis of proton and deuteron F2 structure function world data performed over a large range of kinematics, including recent measurements done at JLab with the CLAS detector. From these data the lowest moments (n <= 10) of the unpolarized structure functions are determined with good statistics and systematics. The Q**2 evolution of the extracted moments is analyzed in terms of an OPE based twist expansion, taking into account soft-gluon effects at large x. A clean separation among the Leading and Higher-Twist terms is achieved. By combining proton and deuteron measurements the lowest moments of the neutron F2 structure function are determined and its leading twist term is extracted. Particular attention is paid to nuclear effects in the deuteron, which become increasingly important for the higher moments. Our results for the non-singlet, isovector (p - n) combination of the leading twist moments are used to test recent lattice simulations. We also determine the lowest few moments of the higher twist contributions, and find these to be approximately isospin independent, suggesting the possible dominance of ud correlations over uu and dd in the nucleon.Comment: 8 pages, 4 figures; to appear in the Proceedings of the IVth International Conference on Quark and Nuclear Physics (QNP06), Madrid (Spain), June 5-10, 200

Spontaneous Symmetry Breaking in Fermion-Gauge Systems- A Non Standard Approach

We propose a new method for the study of the chiral properties of the ground state in QFT's based on the computation of the probability distribution function of the chiral condensate. It can be applied directly in the chiral limit and therefore no mass extrapolations are needed. Furthermore this approach allows to write up equations relating the chiral condensate with quantities computable by standard numerical methods, the functional form of these relations depending on the broken symmetry group. As a check, we report some results for the compact Schwinger model.Comment: Latex file, 11 pages plus two figure

Excitations of the nucleon with dynamical fermions

We measure the spectrum of low-lying nucleon resonances using Bayesian fitting methods. We compare the masses obtained in the quenched approximation to those obtained with two flavours of dynamical fermions at a matched lattice spacing. At the pion masses employed in our simulations, we find that the mass of the first positive-parity nucleon excitation is always greater than that of the parity partner of the nucleon.Comment: Lattice2002(spectrum) 3 pages, 4 figure

Spectroscopy using the Anisotropic Clover Action

The calculation of the light-hadron spectrum in the quenched approximation to QCD using an anisotropic clover fermion action is presented. The tuning of the parameters of the action is discussed, using the pion and rho dispersion relation. The adoption of an anisotropic lattice provides clear advantages in the determination of the baryonic resonances, and in particular that of the so-called Roper resonance, the lightest radial excitation of the nucleon.Comment: Lattice2002(spectrum), 3 pages, 3 figures, to appear in Proceedings of Lattice 200