6,488 research outputs found
On the efficiency of stochastic volume sources for the determination of light meson masses
We investigate the efficiency of single timeslice stochastic sources for the
calculation of light meson masses on the lattice as one varies the quark mass.
Simulations are carried out with Nf = 2 flavours of non-perturbatively O(a)
improved Wilson fermions for pion masses in the range of 450 - 760 MeV. Results
for pseudoscalar and vector meson two-point correlation functions computed
using stochastic as well as point sources are presented and compared. At fixed
computational cost the stochastic approach reduces the variance considerably in
the pseudoscalar channel for all simulated quark masses. The vector channel is
more affected by the intrinsic stochastic noise. In order to obtain stable
estimates of the statistical errors and a more pronounced plateau for the
effective vector meson mass, a relatively large number of stochastic sources
must be used.Comment: 18 pages, 6 figure
STM contrast inversion of the Fe(110) surface
We extend the orbital-dependent electron tunneling model implemented within
the three-dimensional (3D) Wentzel-Kramers-Brillouin (WKB) atom-superposition
approach to simulate spin-polarized scanning tunneling microscopy (SP-STM)
above magnetic surfaces. The tunneling model is based on the electronic
structure data of the magnetic tip and surface obtained from first principles.
Applying our method, we analyze the orbital contributions to the tunneling
current, and study the nature of atomic contrast reversals occurring on
constant-current SP-STM images above the Fe(110) surface. We find an interplay
of orbital-dependent tunneling and spin-polarization effects responsible for
the contrast inversion, and we discuss its dependence on the bias voltage, on
the tip-sample distance, and on the tip orbital composition.Comment: 20 pages manuscript, 5 figure
Symmetries and pre-metric electromagnetism
The equations of pre-metric electromagnetism are formulated as an exterior
differential system on the bundle of exterior differential 2-forms over the
spacetime manifold. The general form for the symmetry equations of the system
is computed and then specialized to various possible forms for an
electromagnetic constitutive law, namely, uniform linear, non-uniform linear,
and uniform nonlinear. It is shown that in the uniform linear case, one has
four possible ways of prolonging the symmetry Lie algebra, including
prolongation to a Lie algebra of infinitesimal projective transformations of a
real four-dimensional projective space. In the most general non-uniform linear
case, th effect of non-uniformity on symmetry seems inconclusive in the absence
of further specifics, and in the uniform nonlinear case, the overall difference
from the uniform linear case amounts to a deformation of the electromagnetic
constitutive tensor by the electromagnetic fields strengths, which induces a
corresponding deformation of the symmetry Lie algebra that was obtained in the
uniform linear case.Comment: 53 pages. Annalen der Physik (Leipzig) (2005), to be publishe
Power sum expansion of chromatic quasisymmetric functions
The chromatic quasisymmetric function of a graph was introduced by Shareshian
and Wachs as a refinement of Stanley's chromatic symmetric function. An
explicit combinatorial formula, conjectured by Shareshian and Wachs, expressing
the chromatic quasisymmetric function of the incomparability graph of a natural
unit interval order in terms of power sum symmetric functions, is proven. The
proof uses a formula of Roichman for the irreducible characters of the
symmetric group.Comment: Final version, 9 pages; comments by a referee incorporate
Hadron-Hadron Interactions from Lattice QCD: isospin-2 scattering length
We present results for the scattering length using
twisted mass lattice QCD for three values of the lattice spacing and a range of
pion mass values. Due to the use of Laplacian Heaviside smearing our
statistical errors are reduced compared to previous lattice studies. A detailed
investigation of systematic effects such as discretisation effects, volume
effects, and pollution of excited and thermal states is performed. After
extrapolation to the physical point using chiral perturbation theory at NLO we
obtain .Comment: Edited for typos, overhauled figures, more detailed comparison to
existing lattice result
The elastic -wave nucleon-pion scattering amplitude and the resonance from lattice QCD
We present the first direct determination of meson-baryon resonance
parameters from a scattering amplitude calculated using lattice QCD. In
particular, we calculate the elastic , -wave nucleon-pion amplitude
on a single ensemble of Wilson-clover fermions with
and . At these quark masses,
the resonance pole is found close to the threshold and a
Breit-Wigner fit to the amplitude gives in agreement with phenomenological determinations.Comment: 7 pages, 2 figures. Agrees with published version, one additional
phase shift point and clarification of different coupling convention
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