35,411 research outputs found
Heavy Baryons in a Quark Model
A quark model is applied to the spectrum of baryons containing heavy quarks.
The model gives masses for the known heavy baryons that are in agreement with
experiment, but for the doubly-charmed baryon Cascade_{cc}, the model
prediction is too heavy. Mixing between the Cascade_Q and Cascade_Q^\prime
states is examined and is found to be small for the lowest lying states. In
contrast with this, mixing between the Cascade_{bc} and Cascade_{bc}^\prime
states is found to be large, and the implication of this mixing for properties
of these states is briefly discussed. We also examine heavy-quark spin-symmetry
multiplets, and find that many states in the model can be placed in such
multiplets. We compare our predictions with those of a number of other authors.Comment: Version published in International Journal of Modern Physics
Comparison of T1 mapping techniques for ECV quantification. histological validation and reproducibility of ShMOLLI versus multibreath-hold T1 quantification equilibrium contrast CMR
BACKGROUND: Myocardial extracellular volume (ECV) is elevated in fibrosis or infiltration and can be quantified by measuring the haematocrit with pre and post contrast T1 at sufficient contrast equilibrium. Equilibrium CMR (EQ-CMR), using a bolus-infusion protocol, has been shown to provide robust measurements of ECV using a multibreath-hold T1 pulse sequence. Newer, faster sequences for T1 mapping promise whole heart coverage and improved clinical utility, but have not been validated.
METHODS: Multibreathhold T1 quantification with heart rate correction and single breath-hold T1 mapping using Shortened Modified Look-Locker Inversion recovery (ShMOLLI) were used in equilibrium contrast CMR to generate ECV values and compared in 3 ways.Firstly, both techniques were compared in a spectrum of disease with variable ECV expansion (n=100, 50 healthy volunteers, 12 patients with hypertrophic cardiomyopathy, 18 with severe aortic stenosis, 20 with amyloid). Secondly, both techniques were correlated to human histological collagen volume fraction (CVF%, n=18, severe aortic stenosis biopsies). Thirdly, an assessment of test:retest reproducibility of the 2 CMR techniques was performed 1 week apart in individuals with widely different ECVs (n=10 healthy volunteers, n=7 amyloid patients).
RESULTS: More patients were able to perform ShMOLLI than the multibreath-hold technique (6% unable to breath-hold). ECV calculated by multibreath-hold T1 and ShMOLLI showed strong correlation (r(2)=0.892), little bias (bias -2.2%, 95%CI -8.9% to 4.6%) and good agreement (ICC 0.922, range 0.802 to 0.961, p<0.0001). ECV correlated with histological CVF% by multibreath-hold ECV (r(2)= 0.589) but better by ShMOLLI ECV (r(2)= 0.685). Inter-study reproducibility demonstrated that ShMOLLI ECV trended towards greater reproducibility than the multibreath-hold ECV, although this did not reach statistical significance (95%CI -4.9% to 5.4% versus 95%CI -6.4% to 7.3% respectively, p=0.21).
CONCLUSIONS: ECV quantification by single breath-hold ShMOLLI T1 mapping can measure ECV by EQ-CMR across the spectrum of interstitial expansion. It is procedurally better tolerated, slightly more reproducible and better correlates with histology compared to the older multibreath-hold FLASH techniques
Polarization Observables for Two-Pion Production off the Nucleon
We develop polarization observables for the processes
and , using both a helicity and hybrid helicity-transversity
basis. Such observables are crucial if processes that produce final states
consisting of a spin-1/2 baryon and two pseudoscalar mesons are to be fully
exploited for baryon spectroscopy. We derive relationships among the
observables, as well as inequalities that they must satisfy. We also discuss
the observables that must be measured in `complete' experiments, and briefly
examine the prospects for measurement of some of these observables in the near
future.Comment: 20 pages, using revtex
Light Assisted Collisional Loss in a Rb Ultracold Optical Trap
We have studied hetero- and homonuclear excited state/ground state collisions
by loading both Rb and Rb into a far off resonant trap (FORT).
Because of the relatively weak confinement of the FORT, we expect the hyperfine
structure of the different isotopes to play a crucial role in the collision
rates. This dependence on hyperfine structure allows us to measure collisions
associated with long range interatomic potentials of different structure: such
as long and short ranged; or such as purely attractive, purely repulsive, or
mixed attractive and repulsive. We observe significantly different loss rates
for different excited state potentials. Additionally, we observe that some
collisional channels' loss rates are saturated at our operating intensities
(~15 mW/cm). These losses are important limitations in loading dual
isotope optical traps.Comment: about 8 pages, 5 figure
CLTs and asymptotic variance of time-sampled Markov chains
For a Markov transition kernel P and a probability distribution
μ on nonnegative integers, a time-sampled Markov chain evolves according
to the transition kernel Pμ = Σkμ(k)Pk. In this note we obtain CLT
conditions for time-sampled Markov chains and derive a spectral formula
for the asymptotic variance. Using these results we compare efficiency of
Barker's and Metropolis algorithms in terms of asymptotic variance
Interaction of intermediate energy protons with light nuclei
Interaction of intermediate energy protons with light nucle
Quasi-Two-Body Decays of Nonstrange Baryons
We examine the decays of nonstrange baryons to the final states ,
, , , , , and
, in a relativized pair-creation() model which has
been developed in a previous study of the decays of the same baryon
states. As it is our goal to provide a guide for the possible discovery of new
baryon states at CEBAF and elsewhere, we examine the decays of resonances which
have already been seen in the partial-wave analyses, along with those of states
which are predicted by the quark model but which remain undiscovered. The level
of agreement between our calculation and the available widths from the
partial-wave analyses is encouraging.Comment: 41 pages, CEBAF-TH-93-1
Peculiar Features of the Velocity Field of OB Associations and the Spiral Structure of the Galaxy
Some of the peculiar features of the periodic velocity-field structure for OB
associations can be explained by using the model of Roberts and Hausman (1984),
in which the behavior of a system of dense clouds is considered in a perturbed
potential. The absence of statistically significant variations in the azimuthal
velocity across the Carina arm, probably, results from its sharp increase
behind the shock front, which is easily blurred by distance errors. The
existence of a shock wave in the spiral arms and, at the same time, the
virtually free motion of OB associations in epicycles can be reconciled in the
model of particle clouds with a mean free path of 0.2-2 kpc. The velocity field
of OB associations exhibits two appreciable nonrandom deviations from an ideal
spiral pattern: a 0.5-kpc displacement of the Cygnus- and Carina-arm fragments
from one another and a weakening of the Perseus arm in quadrant III. However,
the identified fragments of the Carina, Cygnus, and Perseus arms do not belong
to any of the known types of spurs.Comment: 14 pages, 3 postscript figures, to be published in Astronomy Letter
Engineering evaluations and studies. Volume 3: Exhibit C
High rate multiplexes asymmetry and jitter, data-dependent amplitude variations, and transition density are discussed
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