7,592 research outputs found
Covariant nucleon wave function with S, D, and P-state components
Expressions for the nucleon wave functions in the covariant spectator theory
(CST) are derived. The nucleon is described as a system with a off-mass-shell
constituent quark, free to interact with an external probe, and two spectator
constituent quarks on their mass shell. Integrating over the internal momentum
of the on-mass-shell quark pair allows us to derive an effective nucleon wave
function that can be written only in terms of the quark and diquark
(quark-pair) variables. The derived nucleon wave function includes
contributions from S, P and D-waves.Comment: 13 pages and 1 figur
A pure S-wave covariant model for the nucleon
Using the manifestly covariant spectator theory, and modeling the nucleon as
a system of three constituent quarks with their own electromagnetic structure,
we show that all four nucleon electromagnetic form factors can be very well
described by a manifestly covariant nucleon wave function with zero orbital
angular momentum. Since the concept of wave function depends on the formalism,
the conclusions of light-cone theory requiring nonzero angular momentum
components are not inconsistent with our results. We also show that our model
gives a qualitatively correct description of deep inelastic scattering,
unifying the phenomenology at high and low momentum transfer. Finally we review
two different definitions of nuclear shape and show that the nucleon is
spherical in this model, regardless of how shape is defined.Comment: 20 pages and 10 figures; greatly expanded version with new fits and
discussion of DIS; similar to published versio
On the Effect of Cavitation on the Radial Forces and Hydrodynamic Stiffness of a Centrifugal Pump
The asymmetric flow within a volute exerts a radial force on a centrifugal impeller. The present paper presents experimental measurements of the radial forces on the impeller in the presence of cavitation
Two-pion exchange and strong form-factors in covariant field theories
In this work improvements to the application of the Gross equation to nuclear
systems are tested. In particular we evaluate the two pion exchange diagrams,
including the crossed-box diagram, using models developed within the
spectator-on-mass-shell covariant formalism. We found that the form factors
used in these models induce spurious contributions that violate the unitary cut
requirement. We tested then some alternative form-factors in order to preserve
the unitarity condition. With this new choice, the difference between the exact
and the spectator-on-mass-shell amplitudes is of the order of the one boson
scalar exchange, supporting the idea that this difference may be parameterized
by this type of terms.Comment: RevTeX, 21 pages, 19 figures (PostScript
Rotordynamic Forces on Centrifugal Pump Impellers
The asymmetric flow around an impeller in a volute exerts a force upon the impeller. To study the rotordynamic force on an impeller which is vibrating around its machine axis of rotation, the impeller, mounted on a dynamometer, is made to whirl in a circular orbit within the volute. The measured force is expressed as the sum of a steady radial force and an unsteady force due to the eccentric motion of the impeller. These forces were measured in separate tests on a centrifugal pump with radically increased shroud clearance, a two-dimensional impeller, and an impeller with an inducer, the impeller of the HPOTP (High Pressure Oxygen Turbopump) of the SSME (Space Shuttle Main Enginer). In each case, a destabilizing force was observed over a region of positive whirl
Phase diagram of glassy systems in an external field
We study the mean-field phase diagram of glassy systems in a field pointing
in the direction of a metastable state. We find competition among a
``magnetized'' and a ``disordered'' phase, that are separated by a coexistence
line as in ordinary first order phase transitions. The coexistence line
terminates in a critical point, which in principle can be observed in numerical
simulations of glassy models.Comment: 4 pages, 5 figure
Testing replica predictions in experiments
We review the predictions of the replica approach both for the statics and
for the off-equilibrium dynamics. We stress the importance of the
Cugliandolo-Kurchan off-equilibrium fluctuation-dissipation relation in
providing a bridge between the statics and the dynamics. We present numerical
evidence for the correctness of these relations. This approach allows an
experimental determination of the basic parameters of the replica theory.Comment: To appear in Chiarotti's Festschrift Volume (8 Pages, 3 figures
Signatures of the impact of flare ejected plasma on the photosphere of a sunspot light-bridge
We investigate the properties of a sunspot light-bridge, focusing on the
changes produced by the impact of a plasma blob ejected from a C-class flare.
We observed a sunspot in active region NOAA 12544 using spectropolarimetric
raster maps of the four Fe I lines around 15655 \AA\ with the GREGOR Infrared
Spectrograph (GRIS), narrow-band intensity images sampling the Fe I 6173 \AA\
line with the GREGOR Fabry-P\'erot Interferometer (GFPI), and intensity broad
band images in G-band and Ca II H band with the High-resolution Fast Imager
(HiFI). All these instruments are located at the GREGOR telescope at the
Observatorio del Teide, Tenerife, Spain. The data cover the time before,
during, and after the flare event. The analysis is complemented with
Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI)
data from the Solar Dynamics Observatory (SDO). The physical parameters of the
atmosphere at differents heights were inferred using spectral-line inversion
techniques. We identify photospheric and chromospheric brightenings, heating
events, and changes in the Stokes profiles associated to the flare eruption and
the subsequent arrival of the plasma blob to the light bridge, after traveling
along an active region loop. The measurements suggest that these phenomena are
the result of reconnection events driven by the interaction of the plasma blob
with the magnetic field topology of the light bridge.Comment: Accepted for publication in A&
Fragile-glass behavior of a short range -spin model
In this paper we propose a short range generalization of the -spin
interaction spin-glass model. The model is well suited to test the idea that an
entropy collapse is at the bottom-line of the dynamical singularity encountered
in structural glasses. The model is studied in three dimensions through Monte
Carlo simulations, which put in evidence fragile glass behavior with stretched
exponential relaxation and super-Arrhenius behavior of the relaxation time. Our
data are in favor of a Vogel-Fulcher behavior of the relaxation time, related
to an entropy collapse at the Kauzmann temperature. We however encounter
difficulties analogous to those found in experimental systems when
extrapolating thermodynamical data at low temperatures. We study the spin glass
susceptibility investigating the behavior of the correlation length in the
system. We find that the the increase of the relaxation time is not accompanied
by any growth of the correlation length. We discuss the scaling properties of
off-equilibrium dynamics in the glassy regime, finding qualitative agreement
with the mean-field theory.Comment: 8 pages, LaTeX, 8 postscript figure
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