92,192 research outputs found
Lattice QCD Calculations of Hadron Structure: Constituent Quarks and Chiral Symmetry
New data from parity-violating experiments on the deuteron now allow
isolation of the strange-quark contribution to the nucleon magnetic moment,
G_M^s(0), without the uncertainty surrounding the anapole moment of the
nucleon. Still, best estimates place G_M^s(0) > 0. It is illustrated how this
experimental result challenges the very cornerstone of the constituent quark
model. The chiral physics giving rise to G_M^s(0) \sim 0 is illustrated.Comment: Invited talk presented by DBL at the 16th Int. Conf. on Few Body
Problems (Taipei, March 6-10, 2000); 9 pages, 5 figure
Proton Decay Constraints on Low Scale AdS/CFT Unification
Dark matter candidates and proton decay in a class of models based on the
AdS/CFT correspondence are discussed. We show that the present bound on the
proton decay lifetime is inconsistent with SUSY, and strongly
constrains non-SUSY, low scale trinification type unification of
orbifolded AdS models.Comment: 10 page
The First Moments of Nucleon Generalized Parton Distributions
We extrapolate the first moments of the generalized parton distributions
using heavy baryon chiral perturbation theory. The calculation is based on the
one loop level with the finite range regularization. The description of the
lattice data is satisfactory and the extrapolated moments at physical pion mass
are consistent with the results obtained with dimensional regularization,
although the extrapolation in the momentum transfer to does show
sensitivity to form factor effects which lie outside the realm of chiral
perturbation theory. We discuss the significance of the results in the light of
modern experiments as well as QCD inspired models.Comment: 14 pages, 9 figure
Unified chiral analysis of the vector meson spectrum from lattice QCD
The chiral extrapolation of the vector meson mass calculated in
partially-quenched lattice simulations is investigated. The leading one-loop
corrections to the vector meson mass are derived for partially-quenched QCD. A
large sample of lattice results from the CP-PACS Collaboration is analysed,
with explicit corrections for finite lattice spacing artifacts. To incorporate
the effect of the opening decay channel as the chiral limit is approached, the
extrapolation is studied using a necessary phenomenological extension of chiral
effective field theory. This chiral analysis also provides a quantitative
estimate of the leading finite volume corrections. It is found that the
discretisation, finite-volume and partial quenching effects can all be very
well described in this framework, producing an extrapolated value of M_\rho in
excellent agreement with experiment. This procedure is also compared with
extrapolations based on polynomial forms, where the results are much less
enlightening.Comment: 30 pages, 13 fig
Physical Baryon Resonance Spectroscopy from Lattice QCD
We complement recent advances in the calculation of the masses of excited
baryons in quenched lattice QCD with finite-range regulated chiral effective
field theory enabling contact with the physical quark mass region. We examine
the P-wave contributions to the low-lying nucleon and delta resonances.Comment: Contributed paper at FB17, the 17th International Conference on
Few-Body Problems in Physics, Durham, NC, June 5-10, 2003. 3 pages, 6 figure
The No-Boundary Measure of the Universe
We consider the no-boundary proposal for homogeneous isotropic closed
universes with a cosmological constant and a scalar field with a quadratic
potential. In the semi-classical limit, it predicts classical behavior at late
times if the initial scalar field is more than a certain minimum. If the
classical late time histories are extended back, they may be singular or bounce
at a finite radius. The no-boundary proposal provides a probability measure on
the classical solutions which selects inflationary histories but is heavily
biased towards small amounts of inflation. This would not be compatible with
observations. However we argue that the probability for a homogeneous universe
should be multiplied by exp(3N) where N is the number of e-foldings of slow
roll inflation to obtain the probability for what we observe in our past light
cone. This volume weighting is similar to that in eternal inflation. In a
landscape potential, it would predict that the universe would have a large
amount of inflation and that it would start in an approximately de Sitter state
near a saddle-point of the potential. The universe would then have always been
in the semi-classical regime.Comment: 4 pages, revtex4, minor corrections to accord with published versio
A Schottky/2-DEG varactor diode for millimeter and submillimeter wave multiplier applications
A new Schottky diode is investigated for use as a multiplier element in the millimeter and submillimeter wavelength regions. The new diode is based on the Schottky contact at the edge of a 2-dimensional electron gas (2-DEG). As a negative voltage is applied to the Schottky contact, the depletion layer between the Schottky contact and the 2-DEG expands and the junction capacitance decreases, resulting in a nonlinear capacitance-voltage characteristic. In this paper, we outline the theory, design, fabrication, and evaluation of the new device. Recent results include devices having cutoff frequencies of 1 THz and above. Preliminary multiplier results are also presented
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