88 research outputs found
Relationship of field-theory based single boson exchange potentials to current ones
It is shown that field-theory based single boson exchange potentials cannot
be identified to those of the Yukawa or Coulomb type that are currently
inserted in the Schr\"odinger equation. The potential which is obtained rather
corresponds to this current single boson exchange potential corrected for the
probability that the system under consideration is in a two-body component,
therefore missing contributions due to the interaction of these two bodies
while bosons are exchanged. The role of these contributions, which involve at
least two boson exchanges, is examined. The conditions that allow one to
recover the usual single boson exchange potential are given. It is shown that
the present results have some relation: i) to the failure of the Bethe-Salpeter
equation in reproducing the Dirac or Klein-Gordon equations in the limit where
one of the constituent has a large mass, ii) to the absence of corrections of
relative order alpha log(1/alpha) to a full calculation of the binding energy
in the case of neutral massless bosons or iii) to large corrections of
wave-functions calculated perturbatively in some light-front approaches.Comment: LaTeX, 37 pages, 14 figures, submitted for publicatio
Nucleon-nucleon interaction models and non-locality
The effect of non-locality in the NN interaction models is examined. It is
shown that this feature can explain differences in predictions made from models
evidencing a difference with this respect. This is done for both static and
dynamical observables, taking into account that a non-local term can be
transformed away by performing a unitary transformation. Some results for the
deuteron form factors, the A(Q^2) structure function and the T_{20}(Q^2) tensor
polarization are given as an example. A few cases where discrepancies cannot be
explained are also considered. They point to differences in the models as for
the deuteron asymptotic normalizations, A_S and A_D, which are not affected by
the present analysis.Comment: 6 pages, 6 figures, file.tar, Invited talk (XVIIIth European
Conference on Few-Body Problems in Physics), to appear in Few-Body Systems
Supplemen
"Point-form" estimate of the pion form factor revisited
The pion form factor calculation in the ``point-form'' of relativistic
quantum mechanics is re-considered. Particular attention is given to the
relation between the momentum of the system and the momentum transfer as well
as to the quark current.Comment: 3 pages, 2 figures, contribution to the 17th International IUAP
Conference on Few-Body Problems in Physics, 5-10 June 2003, Durham (USA
Comparison of Form Factors Calculated with Different Expressions for the Boost Transformation
The effect of different boost expressions is considered for the calculation
of the ground-state form factor of a two-body system made of scalar particles
interacting via the exchange of a scalar boson. The aim is to provide an
uncertainty range on methods employed in implementing these effects as well as
an insight on their relevance when an ``exact'' calculation is possible. Using
a wave function corresponding to a mass operator that has the appropriate
properties to construct the generators of the Poincar\'{e} algebra in the
framework of relativistic quantum mechanics, form factors are calculated using
the boost transformations pertinent to the instant, front and point forms of
this approach. Moderately and strongly bound systems are considered with masses
of the exchanged boson taken as zero, 0.15 times the constituent mass , and
infinity. In the first and last cases, a comparison with ``exact'' calculations
is made (Wick-Cutkosky model and Feynman triangle diagram). Results with a
Galilean boost are also given. Momentum transfers up to are
considered. Emphasis is put on the contribution of the single-particle current,
as usually done. It is found that the present point-form calculations of form
factors strongly deviate from all the other ones, requiring large contributions
from two-body currents. Different implementations of the point-form approach,
where the role of these two-body currents would be less important, are
sketched.Comment: Version as accepted for publication, added 6 pages of explanatorial
materia
The form factor of the pion in "point-form" of relativistic dynamics revisited
The electromagnetic form factor of the pion is calculated in the "point-form"
of relativistic quantum mechanics using simple, phenomenological wave
functions. It is found that the squared charge radius of the pion is predicted
one order of magnitude larger than the experimental value and the asymptotic
behavior expected from QCD cannot be reproduced. The origin of these
discrepancies is analyzed. The present results confirm previous ones obtained
from a theoretical model and call for major improvements in the implementation
of the "point-form" approach.Comment: 8 pages, 3 eps figure
More about the comparison of local and non-local NN interaction models
The effect of non-locality in the NN interaction with an off-energy shell
character has been studied in the past in relation with the possibility that
some models could be approximately phase-shifts equivalent. This work is
extended to a non-locality implying terms that involve an anticommutator with
the operator p^2. It includes both scalar and tensor components. The most
recent ``high accuracy'' models are considered in the analysis. After studying
the deuteron wave functions, electromagnetic properties of various models are
compared with the idea that these ones differ by their non-locality but are
equivalent up to a unitary transformation. It is found that the extra non-local
tensor interaction considered in this work tends to re-enforce the role of the
term considered in previous works, allowing one to explain almost completely
the difference in the deuteron D-state probabilities evidenced by the
comparison of the Bonn-QB and Paris models for instance. Conclusions for the
effect of the non-local scalar interaction are not so clear. In many cases, it
was found that these terms could explain part of the differences that the
comparison of predictions for various models evidences but cases where they
could not were also found. Some of these last ones have been analyzed in order
to pointing out the origin of the failure.Comment: 32 pages, 24 figure
Parity-Violating Interaction Effects in the np System
We investigate parity-violating observables in the np system, including the
longitudinal asymmetry and neutron-spin rotation in np elastic scattering, the
photon asymmetry in np radiative capture, and the asymmetries in deuteron
photo-disintegration d(gamma,n)p in the threshold region and
electro-disintegration d(e,e`)np in quasi-elastic kinematics. To have an
estimate of the model dependence for the various predictions, a number of
different, latest-generation strong-interaction potentials--Argonne v18, Bonn
2000, and Nijmegen I--are used in combination with a weak-interaction potential
consisting of pi-, rho-, and omega-meson exchanges--the model known as DDH. The
complete bound and scattering problems in the presence of parity-conserving,
including electromagnetic, and parity-violating potentials is solved in both
configuration and momentum space. The issue of electromagnetic current
conservation is examined carefully. We find large cancellations between the
asymmetries induced by the parity-violating interactions and those arising from
the associated pion-exchange currents. In the np capture, the model dependence
is nevertheless quite small, because of constraints arising through the Siegert
evaluation of the relevant E1 matrix elements. In quasi-elastic electron
scattering these processes are found to be insignificant compared to the
asymmetry produced by gamma-Z interference on individual nucleons.Comment: 65 pages, 26 figures, submitted to PR
Dynamic Response of Two-Electrode Distributed Feedback Laser for Stable Signal Mode Operation
The longitudinal spatial hole burning (LSHB) effect has been known to limit the performance of distributed feedback (DFB) semiconductor lasers to achieve a better dynamic signal mode operation (DSMO). So, in order to ensure a stable (DSMO), we propose a novel device design of two electrode DFB lasers with longitudinal variation in the coupling coefficient (distributed coupling coefficient (DCC)), the structure also contains a phase shifted in middle of the cavity. By means of the finite difference time domain (FDTD) numerical method, we analyze dynamic response of our structure and we also compare the results with the conventional two electrode DFB laser (TE-DFB). The numerical simulation shows that, a better dynamic signal mode has been achieved by TE-DCC-DFB lasers in comparison with TE-DFB laser due to its better and high side mode suppression ratio (SMSR). Therefore, the TE-DCC-DFB lasers will be useful to extend the transmission distance in optical fiber communication systems
An Efficient Face Recognition Using SIFT Descriptor in RGB-D Images
Automatic face recognition has known a very important evolution in the last decade, due to its huge usage in the security systems. The most of facial recognition approaches use 2D image, but the problem is that this type of image is very sensible to the illumination and lighting changes. Another approach uses the 3D camera and stereo cameras as well, but it’s rarely used because it requires a relatively long processing duration. A new approach rise in this field, which is based on RGB-D images produced by Kinect, this type of cameras cost less and it can be used in any environment and under any circumstances. In this work we propose a new algorithm that combines the RGB image with Depth map which is less sensible to illumination changes. We got a recognition rate of 96, 63% in rank 2
Cross-ladder effects in Bethe-Salpeter and Light-Front equations
Bethe-Salpeter (BS) equation in Minkowski space for scalar particles is
solved for a kernel given by a sum of ladder and cross-ladder exchanges. The
solution of corresponding Light-Front (LF) equation, where we add the
time-ordered stretched boxes, is also obtained. Cross-ladder contributions are
found to be very large and attractive, whereas the influence of stretched boxes
is negligible. Both approaches -- BS and LF -- give very close results.Comment: 11 pages, 7 figure
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