1,202 research outputs found
Self-Energy of Decuplet Baryons in Nuclear Matter
We calculate, in chiral perturbation theory, the change in the self-energy of
decuplet baryons in nuclear matter. These self-energy shifts are relevant in
studies of meson-nucleus scattering and of neutron stars. Our results are
leading order in an expansion in powers of the ratio of characteristic momenta
to the chiral symmetry-breaking scale (or the nucleon mass). Included are
contact diagrams generated by 4-baryon operators, which were neglected in
earlier studies for the isomultiplet but contribute to the self-energy
shifts at this order in chiral perturbation theory.Comment: 11 pages, 2 eps figures, REVTe
Existence of a Semiclassical Approximation in Loop Quantum Gravity
We consider a spherical symmetric black hole in the Schwarzschild metric and
apply Bohr-Sommerfeld quantization to determine the energy levels. The
canonical partition function is then computed and we show that the entropy
coincides with the Bekenstein-Hawking formula when the maximal number of states
for the black hole is the same as computed in loop quantum gravity, proving in
this case the existence of a semiclassical limit and obtaining an independent
derivation of the Barbero-Immirzi parameter.Comment: 6 pages, no figures. Final version accepted for publication in
General Relativity and Gravitatio
Family Symmetry, Gravity, and the Strong CP Problem
We show how in a class of models Peccei--Quinn symmetry can be realized as an
automatic consequence of a gauged family symmetry. These models provide
a solution to the strong CP problem either via a massless --quark or via the
DFSZ invisible axion. The local family symmetry protects against potentially
large corrections to induced by quantum gravitational
effects. In a supersymmetric extension, the `--problem' is shown to have a
natural solution in the context of gravitationally induced operators. We also
present a plausible mechanism which can explain the inter--generational mass
hierarchy in such a context.Comment: BA-92-79, 14 pages, in LaTeX, no figure
Curvature induced toroidal bound states
Curvature induced bound state (E < 0) eigenvalues and eigenfunctions for a
particle constrained to move on the surface of a torus are calculated. A limit
on the number of bound states a torus with minor radius a and major radius R
can support is obtained. A condition for mapping constrained particle wave
functions on the torus into free particle wave functions is established.Comment: 6 pages, no figures, Late
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
(S)fermion Masses in Fat Brane Scenario
We discuss the fermion mass hierarchy and the flavor mixings in the fat brane
scenario of a five dimensional SUSY theory. Assuming that the matter fields
lives in the bulk, their zero mode wave functions are Gaussians, and Higgs
fields are localized on the brane, we find simple various types of the matter
configurations generating the mass matrices consistent with experimental data.
Sfermion mass spectrum is also discussed using the matter configurations found
above. Which type of squark mass spectra (the degeneracy, the decoupling and
the alignment) is realized depends on the relative locations of SUSY breaking
brane and the brane where Higgs fields are localized.Comment: 18 pages, LaTe
Strange form factors of the proton: a new analysis of the neutrino (antineutrino) data of the BNL-734 experiment
We consider ratios of elastic neutrino(antineutrino)-proton cross sections
measured by the Brookhaven BNL-734 experiment and use them to obtain the
neutral current (NC) over charged current (CC) neutrino-antineutrino asymmetry.
We discuss the sensitivity of these ratios and of the asymmetry to the
electric, magnetic and axial strange form factors of the nucleon and to the
axial cutoff mass M_A. We show that the effects of the nuclear structure and
interactions on the asymmetry and, in general, on ratios of cross sections are
negligible. We find some restrictions on the possible values of the parameters
characterizing the strange form factors. We show that a precise measurement of
the neutrino-antineutrino asymmetry would allow the extraction of the axial and
vector magnetic strange form factors in a model independent way. The
neutrino-antineutrino asymmetry turns out to be almost independent on the
electric strange form factor and on the axial cutoff mass.Comment: 12 page
Composite Inelastic Dark Matter
Peaking consistently in June for nearly eleven years, the annual modulation
signal reported by DAMA/NaI and DAMA/LIBRA offers strong evidence for the
identity of dark matter. DAMA's signal strongly suggest that dark matter
inelastically scatters into an excited state split by O(100 keV). We propose
that DAMA is observing hyperfine transitions of a composite dark matter
particle. As an example, we consider a meson of a QCD-like sector, built out of
constituent fermions whose spin-spin interactions break the degeneracy of the
ground state. An axially coupled U(1) gauge boson that mixes kinetically with
hypercharge induces inelastic hyperfine transitions of the meson dark matter
that can explain the DAMA signal.Comment: 5 pages (two-column), 1 figure, revised version, references adde
Diffraction Symmetry in Crystalline, Close-Packed C60
We have grown crystals of the carbon structure C60 by sublimation. In contrast to solution-grown crystals, the sublimed crystals have long range order with no evidence of solvent inclusions. Sublimed C60 forms three dimensional, faceted crystals with a close-packed, face-centered cubic unit cell. We have refined a crystal structure using the "soccer ball" model of the C60 molecule. The results indicate that the C60 molecule has the expected spherical shape, however the data are not sufficiently accurate to unambiguously determine atomic positions
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