635 research outputs found
Connecting physical resonant amplitudes and lattice QCD
We present a determination of the isovector, -wave scattering
phase shift obtained by extrapolating recent lattice QCD results from the
Hadron Spectrum Collaboration using MeV. The finite volume spectra
are described using extensions of L\"uscher's method to determine the infinite
volume Unitarized Chiral Perturbation Theory scattering amplitude. We exploit
the pion mass dependence of this effective theory to obtain the scattering
amplitude at MeV. The scattering phase shift is found to be in
good agreement with experiment up to center of mass energies of 1.2 GeV. The
analytic continuation of the scattering amplitude to the complex plane yields a
-resonance pole at .
The techniques presented illustrate a possible pathway towards connecting
lattice QCD observables of few-body, strongly interacting systems to
experimentally accessible quantities.Comment: 8 pages, 6 figures, equivalent to published version, added two
appendices and a figur
Scattering processes and resonances from lattice QCD
The vast majority of hadrons observed in nature are not stable under the
strong interaction, rather they are resonances whose existence is deduced from
enhancements in the energy dependence of scattering amplitudes. The study of
hadron resonances offers a window into the workings of quantum chromodynamics
(QCD) in the low-energy non-perturbative region, and in addition, many probes
of the limits of the electroweak sector of the Standard Model consider
processes which feature hadron resonances. From a theoretical standpoint, this
is a challenging field: the same dynamics that binds quarks and gluons into
hadron resonances also controls their decay into lighter hadrons, so a complete
approach to QCD is required. Presently, lattice QCD is the only available tool
that provides the required non-perturbative evaluation of hadron observables.
In this article, we review progress in the study of few-hadron reactions in
which resonances and bound-states appear using lattice QCD techniques. We
describe the leading approach which takes advantage of the periodic finite
spatial volume used in lattice QCD calculations to extract scattering
amplitudes from the discrete spectrum of QCD eigenstates in a box. We explain
how from explicit lattice QCD calculations, one can rigorously garner
information about a variety of resonance properties, including their masses,
widths, decay couplings, and form factors. The challenges which currently limit
the field are discussed along with the steps being taken to resolve them
Two-Nucleon Systems in a Finite Volume: (II) 3S1-3D1 Coupled Channels and the Deuteron
The energy spectra of two nucleons in a cubic volume provide access to the
two phase shifts and one mixing angle that define the S-matrix in the 3S1-3D1
coupled channels containing the deuteron. With the aid of recently derived
energy quantization conditions for such systems, and the known scattering
parameters, these spectra are predicted for a range of volumes. It is found
that extractions of the infinite-volume deuteron binding energy and leading
scattering parameters, including the S-D mixing angle at the deuteron pole, are
possible from Lattice QCD calculations of two-nucleon systems with boosts of
|P| <= 2pi sqrt{3}/L in volumes with 10 fm <~ L <~ 14 fm. The viability of
extracting the asymptotic D/S ratio of the deuteron wavefunction from Lattice
QCD calculations is discussed.Comment: 31 pages, 17 figure
Two-Baryon Systems with Twisted Boundary Conditions
We explore the use of twisted boundary conditions in extracting the nucleon
mass and the binding energy of two-baryon systems, such as the deuteron, from
Lattice QCD calculations. Averaging the results of calculations performed with
periodic and anti-periodic boundary conditions imposed upon the light-quark
fields, or other pair-wise averages, improves the volume dependence of the
deuteron binding energy from ~exp(-kappa*L)/L to ~exp(-sqrt(2)kappa*L)/L.
However, a twist angle of pi/2 in each of the spatial directions improves the
volume dependence from ~exp(-kappa*L)/L to ~exp(-2kappa*L)/L. Twist averaging
the binding energy with a random sampling of twist angles improves the volume
dependence from ~exp^(-kappa*L)/L to ~exp(-2kappa*L)/L, but with a standard
deviation of ~exp(-kappa*L)/L, introducing a signal-to-noise issue in modest
lattice volumes. Using the experimentally determined phase shifts and mixing
angles, we determine the expected energies of the deuteron states over a range
of cubic lattice volumes for a selection of twisted boundary conditions.Comment: 20 pages, 3 figure
An Evaluation of Distinct Volumetric and Functional MRI Contributions Toward Understanding Age and Task Performance: A Study in the Basal Ganglia
Prior work by our group and others has implicated the basal ganglia as important in age-related differences in tasks involving motor response control. The present study used structural and functional MRI approaches to analyze this region of interest (ROI) toward better understanding the contributions of structural and functional MRI measures to understanding age-related and task performance-related cognitive differences. Eleven healthy elders were compared with 11 healthy younger adults while they completed the “go” portion of a complex Go/No-go task. Separate ROI\u27s in the bilateral caudate (C) and putamen/globus pallidus (PGp) were studied based upon previous findings of age-related functional MRI differences in basal ganglia for this portion of the task. Structural volumes and functional activation (in percent area under the curve during correct responses) were independently extracted for these ROI\u27s. Results showed that age correlated with ROI volume in bilateral PGp and C, while multiple task performance measures correlated with functional activation in the left PGp. The Go/No-go task measures were also significantly correlated with traditional attention and executive functioning measures. Importantly, fMRI activation and volumes from each ROI were not significantly inter-correlated. These findings suggest that structural and functional MRI make unique contributions to the study of performance changes in aging
Isoscalar scattering and the mesons from QCD
We present the first lattice QCD study of coupled isoscalar
- and -wave scattering extracted from
discrete finite-volume spectra computed on lattices which have a value of the
quark mass corresponding to MeV. In the sector we find
analogues of the experimental and states, where the
appears as a stable bound-state below threshold, and, similar
to what is seen in experiment, the manifests itself as a dip in the
cross section in the vicinity of the threshold. For
we find two states resembling the and ,
observed as narrow peaks, with the lighter state dominantly decaying to
and the heavier state to . The presence of all these
states is determined rigorously by finding the pole singularity content of
scattering amplitudes, and their couplings to decay channels are established
using the residues of the poles
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