4,870 research outputs found
Analysis of Ultrasonic Scattering from Simply Shaped Objects
The simple shapes referred to in my title are actually restricted to spheres and spheroids , that is ellipsoids of revolution. Actually , I will not even say too much about the spheroids except to t he extent that I will concentrate on the sphere as the limiting case and as the jumping-off point for the complete analysis of the spheroid
New Relations for Excited Baryons in Large N_c QCD
We show that excited baryons in large N_c QCD form multiplets, within which
masses are first split at O(1/N_c). The dominant couplings of resonances to
various mesons are highly constrained: The N(1535) decays at leading 1/N_c
order exclusively to eta-N rather than pi-N, and vice versa for the N(1650).
This multiplet structure is reproduced by a simple large N_c quark model, well
studied in the literature, that describes resonances as single-quark
excitations.Comment: 4 pages, no figures, ReVTeX 4. Includes new discussion of previous
work on excited baryon tower
SU(3) Clebsch-Gordan Coefficients for Baryon-Meson Coupling at Arbitrary N_c
We present explicit formulae for the SU(3) Clebsch-Gordan coefficients that
are relevant for the couplings of large N_c baryons to mesons. In particular,
we compute the Clebsch-Gordan series for the coupling of the octet (associated
with mesons, and remains the correct representation at large N_c) to the large
N_c analogs of the baryon octet and decuplet representations.Comment: 8 pages, no figures, ReVTe
Our Knowledge of the Atomic Constants F, N, m, and h in 1947, and of Other Constants Derivable Therefrom
The present paper aims to deal only with a
limited portion of the subject, the evaluation of
the so-called atomic constants: e the electronic
charge, m the electronic mass, and h Planck's
constant of action together with certain auxiliary
constants intimately associated with them. A
number of useful physical constants which can
be computed from the above data will also be
evaluated
On the Existence of Heavy Pentaquarks: The large Nc and Heavy Quark Limits and Beyond
We present a very general argument that the analogue of a heavy pentaquark (a
state with the quantum numbers of a baryon combined with an additional light
quark and a heavy antiquark) must exist as a particle stable under strong
interactions in the combined heavy quark and large Nc limits of QCD. Moreover,
in the combined limit these heavy pentaquark states fill multiplets of
SU(4)xO(8)xSU(2). We explore the question of whether corrections in the
combined 1/Nc and 1/mQ expansions are sufficiently small to maintain this
qualitative result. Since no model-independent way is known to answer this
question, we use a class of ``realistic'' hadronic models in which a pentaquark
can be formed via nucleon-heavy meson binding through a pion-exchange
potential. These models have the virtue that they necessarily yield the correct
behavior in the combined limit, and the long-distance parts of the interactions
are model independent. If the long-distance attraction in these models were to
predict bound states in a robust way (i.e., largely insensitive to the details
of the short-range interaction), then one could safely conclude that heavy
pentaquarks do exist. However, in practice the binding does depend very
strongly on the details of the short-distance physics, suggesting that the real
world is not sufficiently near the combined large Nc, mQ limit to use it as a
reliable guide. Whether stable heavy pentaquarks exist remains an open
question.Comment: 11 pages; references adde
Calcitonin inhibits phospholipase A2 and collagenase activity of human osteoarthritic chondrocytes
SummaryCalcitonin (CT) is a known potent inhibitor of bone resorption but its effect on cartilage enzymatic degradation has been incompletely studied. Salmon CT, at a concentration of 0, 0.1, 0.25, 0.5, 2.5 and 50 ng/ml, was added at 24 or 72 h to the culture medium of chondrocytes from human osteoarthritic hips and knees. The spontaneous collagenolytic activity, measured using a radiolabeled type II collagen, was inhibited by CT in a dose-dependent manner. However, CT had no effect on the total collagenolytic activity assayed after APMA activation. Stromelysin and plasmin activity, measured by degradation of casein and a synthetic substrate, were also unaffected by CT. Chondrocyte phospholipase A2 activity, assayed using a labeled specific substrate, was decreased by CT. Chondrocyte pre-incubation with CT significantly decreased the cell binding of labeled TNFα, but did not affect IL-1β cell binding. Attachment of chondrocytes on fibronectin was markedly stimulated by CT, while attachment to type II collagen was not. Significant effects were obtained using at least 2 or 5 ng/ml of CT. CT appears to decrease collagenolytic activity by decreasing its activation and/or increasing its inhibition by tissue inhibitors of metalloproteinases (TIMP). CT might act on osteoarthritic chondrocyte activation via mechanisms such as phospholipase A2 activity, human necrosis factor-α or fibronectin receptor expression
Physical characterization and origin of binary near-Earth asteroid (175706) 1996 FG3
The near-Earth asteroid (NEA) (175706) 1996 FG3 is a particularly interesting
spacecraft target: a binary asteroid with a low-DeltaV heliocentric orbit. The
orbit of its satellite has provided valuable information about its mass density
while its albedo and colors suggest it is primitive or part of the C-complex
taxonomic grouping. We extend the physical characterization of this object with
new observations of its emission at mid-Infrared (IR) wavelengths and with
near-IR reflection spectroscopy. We derive an area-equivalent system diameter
of 1.90 \pm 0.28 km (corresponding to approximate component diameters of 1.83
km and 0.51 km, respectively) and a geometric albedo of 0.039 \pm 0.012.
1996 FG3 was previously classified as a C-type asteroid, though the combined
0.4--2.5 micron spectrum with thermal correction indicates classification as
B-type; both are consistent with the low measured albedo. Dynamical studies
show that 1996 FG3 has most probably originated in the inner main asteroid
belt. Recent work has suggested the inner Main Belt (142) Polana family as the
possible origin of another low-DeltaV B-type NEA, (101955) 1999 RQ36. A similar
origin for 1996 FG3 would require delivery by the overlapping Jupiter 7:2 and
Mars 5:9 mean motion resonances rather than the nu-6 resonance, and we find
this to be a low probability, but possible, origin.Comment: Published in Ap
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