503 research outputs found
Magnetic Moments of the Baryon Decuplet in a Relativistic Quark Model
The magnetic moments of the baryon decuplet are calculated in a relativistic
constituent quark model using the light-front formalism. Of particular interest
are the magnetic moments of the and for which new
recent experimental measurements are available. Our calculation for the
magnetic moment ratio is in excellent agreement with
the experimental ratio, while our ratio is
slightly higher than the experimental ratio.Comment: 10 pages ReVTeX, SLAC-PUB-621
Cometary Nuclei and Tidal Disruption: The Geologic Record of Crater Chains on Callisto and Ganymede
Prominent crater chains on Ganymede and Callisto are most likely the impact scars of comets tidally disrupted by Jupiter and are not secondary crater chains. We have examined the morphology of these chains in detail in order to place constraints on the properties of the comets that formed them and the disruption process. In these chains, intercrater spacing varies by no more than a factor of 2 and the craters within a given chain show almost no deviation from linearity (although the chains themselves are on gently curved small circles). All of these crater chains occur on or very near the Jupiter-facing hemisphere. For a given chain, the estimated masses of the fragments that formed each crater vary by no more than an order of magnitude. The mean fragment masses for all the chains vary by over four orders of magnitude (W. B. McKinnon and P. M. Schenk 1995, Geophys. Res. Lett. 13, 1829-1832), however. The mass of the parent comet for each crater chain is not correlated with the number of fragments produced during disruption but is correlated with the mean mass of the fragments produced in a given disruption event. Also, the larger fragments are located near the center of each chain. All of these characteristics are consistent with those predicted by disruption simulations based on the rubble pile cometary nucleus model (in which nuclei are composed on numerous small fragments weakly bound by self-gravity), and with those observed in Comet D/Shoemaker-Levy 9. Similar crater chains have not been found on the other icy satellites, but the impact record of disrupted comets on Callisto and Ganymede indicates that disruption events occur within the Jupiter system roughly once every 200 to 400 years
A Self-Consistent Model of the Circumstellar Debris Created by a Giant Hypervelocity Impact in the HD172555 System
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of
HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the
system. Using physical arguments and constraints from observations, we rule out
the possibility of the infrared excess being created by a magma ocean planet or
a circumplanetary disk or torus. We show that the infrared excess is consistent
with a circumstellar debris disk or torus, located at approximately 6 AU, that
was created by a planetary scale hypervelocity impact. We find that radiation
pressure should remove submicron dust from the debris disk in less than one
year. However, the system's mid-infrared photometric flux, dominated by
submicron grains, has been stable within 4 percent over the last 27 years, from
IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of
the radiation pressure on fine dust in HD 172555 provide a self-consistent
explanation for this apparent contradiction. We also explore the unconfirmed
claim that 10^47 molecules of SiO vapor are needed to explain an emission
feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless
there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the
system, SiO vapor should be destroyed by photo-dissociation in less than 0.2
years. We argue that a second plausible explanation for the 8 um feature can be
emission from solid SiO, which naturally occurs in submicron silicate "smokes"
created by quickly condensing vaporized silicate.Comment: Accepted to the Astrophysical Journa
Axial vector form factor of nucleons in a light-cone diquark model
The nucleon axial vector form factor is investigated in a light-cone quark
spectator diquark model, in which Melosh rotations are applied to both the
quark and vector diquark. It is found that this model gives a very good
description of available experimental data and the results have very little
dependence on the parameters of the model. The relation between the nucleon
axial constant and the anomalous magnetic moment of nucleons is also discussed.Comment: 8 pages, Revtex4, 1 figure, version to be published in Phys. Rev.
Variation in Magnetic Fabrics at Low Shock Pressure Due to Experimental Impact Cratering
Magnetic fabrics provide important clues for understanding impact cratering processes. However, only a few magnetic fabric studies for experimentally shocked material have been reported so far. In the framework of MEMIN (Multidisciplinary Experimental and Modeling Impact Research Network), we conducted two impact experiments on blocks of Maggia gneiss with the foliation oriented perpendicular (A38) and parallel (A37) to the target surface. Maggia gneiss has plenty of biotite bands forming a strong rock foliation. The bulk magnetic susceptibility varies from 0.376 Ă 10 to 1.298 Ă 10 SI in unshocked and from 0.443 Ă 10 to 3.940 Ă 10 SI in shocked gneiss. The thermomagnetic curves reveal a Verwey transition at â147 °C and a Curie temperature between 576 and 579 °C in unshocked and shocked samples, indicating nearly pure magnetite, which carries the magnetic fabrics. In A37 and A38 kinking is prominent from the point source down to a depth of 2 and 4.2 d (projectile diameter) or 1 and 2.1 cm, respectively. Kinking, folding, and fracturing changed the position of magnetite grains with respect to each other to reorient the magnetic fabrics. Reorientation of magnetic fabrics is conspicuous down to 20 d (10 cm) in A38, where no other impactârelated deformation is visible. The reorientation of magnetic fabrics may, therefore, aid in identifying impact processes at very low pressures, starting at 0.1 GPa, when other common indicators are absent
Pion-photon and photon-pion transition form factors in light-cone formalism
We derive the minimal Fock-state expansions of the pion and the photon wave
functions in light-cone formalism, then we calculate the pion-photon and the
photon-pion transition form factors of and
processes by employing these
quark-antiquark wave functions of the pion and the photon. We find that our
calculation for the transition form factor
agrees with the experimental data at low and moderately high energy scale.
Moreover, the physical differences and inherent connections between the
transition form factors of and have been illustrated, which indicate that these
two physical processes are intrinsically related. In addition, we also discuss
the form factor and the decay width at .Comment: 20 pages, 2 figure
Electroproduction of Charmonia off Nuclei
In a recent publication we have calculated elastic charmonium production in
ep collisions employing realistic charmonia wave functions and dipole cross
sections and have found good agreement with the data in a wide range of s and
Q^2. Using the ingredients from those calculations we calculate exclusive
electroproduction of charmonia off nuclei. Here new effects become important,
(i) color filtering of the c-cbar pair on its trajectory through nuclear
matter, (ii) dependence on the finite lifetime of the c-cbar fluctuation
(coherence length) and (iii) gluon shadowing in a nucleus compared to the one
in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as
functions of s and Q^2 are presented together with some differential cross
sections. The results can be tested with future electron-nucleus colliders or
in peripheral collisions of ultrarelativistic heavy ions.Comment: 21 pages of Latex including 14 figures; few misprints are fixe
The Proton Spin and the Wigner Rotation
It is shown that in both the gluonic and strange sea explanations of the
Ellis-Jaffe sum rule violation discovered by the European Muon Collaboration
(EMC), the spin of the proton, when viewed in in its rest reference frame,
could by fully provided by quarks and antiquarks within a simple quark model
picture, taken into account the relativistic effect from the Wigner rotation.Comment: 13 latex page
Heavy quarkonium 2S states in light-front quark model
We study the charmonium 2S states and , and the bottomonium
2S states and , using the light-front quark model and the
2S state wave function of harmonic oscillator as the approximation of the 2S
quarkonium wave function. The decay constants, transition form factors and
masses of these mesons are calculated and compared with experimental data.
Predictions of quantities such as Br are made. The
2S wave function may help us learn more about the structure of these heavy
quarkonia.Comment: 5 latex pages, final version for journal publicatio
Deep Impact Mission to Tempel 1 Favours New Explosive Cosmogony of Comets
The assumption that short-period (SP) comets are fragments of massive icy
envelopes of Ganymede-like bodies saturated by products of ice electrolysis
that underwent global explosions provides a plausible explanation of all known
manifestations of comets, including the jet character of outflows, the presence
of ions in the vicinity of the nucleus, the bursts and splitting of cometary
nuclei, etc., with solar radiation initiating burning of the products of
electrolysis in the nucleus. As shown persuasively by numerical simulation
carried out in hydrodynamic approximation, the shock wave initiated by the Deep
Impact (DI) impactor in the cometary ice saturated originally by the
electrolysis products 2H2 + O2 is capable of activating under certain
conditions exothermal reactions (of the type O2 + H2 + organics = H2O + CO +
HCN + other products of incomplete burning of organics including its light and
heavy pyrolyzed compounds, soot, etc.), which will slow down shock wave damping
(forced detonation) and increase many times the energy release. As a result,
the measured energetics of ejections and outflows from the crater have to
exceed the DI energetics. Analysis of different clusters of the DI experiment
data confirms these conclusions and expectations and thus it favours the
planetary origin of comets.Comment: 21 pages incluging 3 figure
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