1,024 research outputs found
The operator product expansion on the lattice
We investigate the Operator Product Expansion (OPE) on the lattice by
directly measuring the product (where J is the vector current) and
comparing it with the expectation values of bilinear operators. This will
determine the Wilson coefficients in the OPE from lattice data, and so give an
alternative to the conventional methods of renormalising lattice structure
function calculations. It could also give us access to higher twist quantities
such as the longitudinal structure function F_L = F_2 - 2 x F_1. We use overlap
fermions because of their improved chiral properties, which reduces the number
of possible operator mixing coefficients.Comment: 7 pages, 4 postscript figures. Contribution to Lattice 2007,
Regensbur
Quark structure from the lattice Operator Product Expansion
We have reported elsewhere in this conference on our continuing project to
determine non-perturbative Wilson coefficients on the lattice, as a step
towards a completely non-perturbative determination of the nucleon structure.
In this talk we discuss how these Wilson coefficients can be used to extract
Nachtmann moments of structure functions, using the case of off-shell
Landau-gauge quarks as a first simple example. This work is done using overlap
fermions, because their improved chiral properties reduce the difficulties due
to operator mixing.Comment: 7 pages, 3 figures. Talk given at the XXVII International Symposium
on Lattice Field Theory, July 26-31 2009, Peking University, Beijing, Chin
Gap Domain Wall Fermions
I demonstrate that the chiral properties of Domain Wall Fermions (DWF) in the
large to intermediate lattice spacing regime of QCD, 1 to 2 GeV, are
significantly improved by adding to the action two standard Wilson fermions
with supercritical mass equal to the negative DWF five dimensional mass. Using
quenched DWF simulations I show that the eigenvalue spectrum of the transfer
matrix Hamiltonian develops a substantial gap and that the residual mass
decreases appreciatively. Furthermore, I confirm that topology changing remains
active and that the hadron spectrum of the added Wilson fermions is above the
lattice cutoff and therefore is irrelevant. I argue that this result should
also hold for dynamical DWF and furthermore that it should improve the chiral
properties of related fermion methods.Comment: 12 pages of text, 14 figures, added sect.6 on topology and reference
Results from 2+1 flavours of SLiNC fermions
QCD results are presented for a 2+1 flavour fermion clover action (which we
call the SLiNC action). A method of tuning the quark masses to their physical
values is discussed. In this method the singlet quark mass is kept fixed, which
solves the problem of different renormalisations (for singlet and non-singlet
quark masses) occuring for non-chirally invariant lattice fermions. This
procedure enables a wide range of quark masses to be probed, including the case
with a heavy up-down quark mass and light strange quark mass. Preliminary
results show the correct splittings for the baryon (octet and) decuplet
spectrum.Comment: 7 pages; talk given at the XXVII International Symposium on Lattice
Field Theory, July 26-31 2009, Peking University, Beijing, Chin
Hadron Spectroscopy with Dynamical Chirally Improved Fermions
We simulate two dynamical, mass degenerate light quarks on 16^3x32 lattices
with a spatial extent of 2.4 fm using the Chirally Improved Dirac operator. The
simulation method, the implementation of the action and signals of
equilibration are discussed in detail. Based on the eigenvalues of the Dirac
operator we discuss some qualitative features of our approach. Results for
ground state masses of pseudoscalar and vector mesons as well as for the
nucleon and delta baryons are presented.Comment: 26 pages, 17 figures, 10 table
Nucleon structure in terms of OPE with non-perturbative Wilson coefficients
Lattice calculations could boost our understanding of Deep Inelastic
Scattering by evaluating moments of the Nucleon Structure Functions. To this
end we study the product of electromagnetic currents between quark states. The
Operator Product Expansion (OPE) decomposes it into matrix elements of local
operators (depending on the quark momenta) and Wilson coefficients (as
functions of the larger photon momenta). For consistency with the matrix
elements, we evaluate a set of Wilson coefficients non-perturbatively, based on
propagators for numerous momentum sources, on a 24^3 x 48 lattice. The use of
overlap quarks suppresses unwanted operator mixing and lattice artifacts.
Results for the leading Wilson coefficients are extracted by means of Singular
Value Decomposition.Comment: 7 pages, 3 figures, contribution to the XXVI International Symposium
on Lattice Field Theory, July 14-19 Williamsburg, Virginia, US
Non-perturbative improvement of stout-smeared three flavour clover fermions
We discuss a 3-flavour lattice QCD action with clover improvement in which
the fermion matrix has single level stout smearing for the hopping terms
together with unsmeared links for the clover term. With the (tree-level)
Symanzik improved gluon action this constitutes the Stout Link Non-perturbative
Clover or SLiNC action. To cancel O(a) terms the clover term coefficient has to
be tuned. We present here results of a non-perturbative determination of this
coefficient using the Schroedinger functional and as a by-product a
determination of the critical hopping parameter. Comparisons of the results are
made with lowest order perturbation theory.Comment: 30 pages, 13 figures, minor changes, published versio
Low-Dimensional Long-Range Topological Charge Structure in the QCD Vacuum
While sign-coherent 4-dimensional structures cannot dominate topological
charge fluctuations in the QCD vacuum at all scales due to reflection
positivity, it is possible that enhanced coherence exists over extended
space-time regions of lower dimension. Using the overlap Dirac operator to
calculate topological charge density, we present evidence for such structure in
pure-glue SU(3) lattice gauge theory. It is found that a typical equilibrium
configuration is dominated by two oppositely-charged sign-coherent connected
structures (``sheets'') covering about 80% of space-time. Each sheet is built
from elementary 3-d cubes connected through 2-d faces, and approximates a
low-dimensional curved manifold (or possibly a fractal structure) embedded in
the 4-d space. At the heart of the sheet is a ``skeleton'' formed by about 18%
of the most intense space-time points organized into a global long-range
structure, involving connected parts spreading over maximal possible distances.
We find that the skeleton is locally 1-dimensional and propose that its
geometrical properties might be relevant for understanding the possible role of
topological charge fluctuations in the physics of chiral symmetry breaking.Comment: 4 pages RevTeX, 4 figures; v2: 6 pages, 5 figures, more explanations
provided, figure and references added, published versio
Quantum Dynamics of a Hydrogen Molecule Confined in a Cylindrical Potential
We study the coupled rotation-vibration levels of a hydrogen molecule in a
confining potential with cylindrical symmetry. We include the coupling between
rotations and translations and show how this interaction is essential to obtain
the correct degeneracies of the energy level scheme. We applied our formalism
to study the dynamics of H molecules inside a "smooth" carbon nanotube as
a function of tube radius. The results are obtained both by numerical solution
of the ()-component radial Schrodinger equation and by developing an
effective Hamiltonian to describe the splitting of a manifold of states of
fixed angular momentum and number of phonons, . For nanotube radius
smaller than \AA, the confining potential has a parabolic shape
and the results can be understood in terms of a simple toy model. For larger
radius, the potential has the "Mexican hat" shape and therefore the H
molecule is off-centered, yielding radial and tangential translational dynamics
in addition to rotational dynamics of H molecule which we also describe
by a simple model. Finally, we make several predictions for the the neutron
scattering observation of various transitions between these levels.Comment: 36 pages, 8 figures, submitted to Phys. Rev. B on 12 December 200
Measurement of the branching ratios of the decays Xi0 --> Sigma+ e- nubar and anti-Xi0 --> anti-Sigma+ e+ nu
From 56 days of data taking in 2002, the NA48/1 experiment observed 6316 Xi0
--> Sigma+ e- nubar candidates (with the subsequent Sigma+ --> p pi0 decay) and
555 anti-Xi0 --> anti-Sigma+ e+ nu candidates with background contamination of
215+-44 and 136+-8 events, respectively. From these samples, the branching
ratios BR(Xi0 --> Sigma+ e- nubar)= (2.51+-0.03stat+-0.09syst)E(-4) and
BR(anti-Xi0 --> anti-Sigma+ e+ nu)= (2.55+-0.14stat+-0.10syst)E(-4) were
measured allowing the determination of the CKM matrix element |Vus| =
0.209+0.023-0.028. Using the Particle Data Group average for |Vus| obtained in
semileptonic kaon decays, we measured the ratio g1/f1 = 1.20+-0.05 of the
axial-vector to vector form factors.Comment: 16 pages, 11 figures Submitted to Phys.Lett.
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