257 research outputs found
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 Electroproduction
Exclusive 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
electroproduction, where the axial charge can enter the amplitudes. Contrarily
the tensor charge enters the 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 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
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