Browsing by Author "Amghar, A."

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The form factor of the pion in "point-form" of relativistic dynamics revisited
(Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 2003) Amghar, A.; Desplanques, B.; Theußl, L.
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. The role of essential ingredients for the description of the pion form factor is reminded. © 2003 Published by Elsevier B.V
From the Bethe-Salpeter equation to nonrelativistic approaches with effective two-body interactions
(Nuclear Physics A, 2001) Amghar, A.; Desplanques, B.; Theubi, L.
It is known that binding energies calculated from the Bethe-Salpeter equation in ladder approximation can be reasonably well accounted for by an energy-dependent interaction, at least for the lowest states. It is also known that none of these approaches gives results close to what is obtained by using the same interaction in the so-called instantaneous approximation, which is often employed in nonrelativistic calculations. However, a recently proposed effective interaction was shown to account for the main features of both the Bethe-Salpeter equation and the energy-dependent approach. In the present work, a detailed comparison of these different methods for calculating binding energies of a two-particle system is made. Some improvement, previously incorporated for the zero-mass boson case in the derivation of the effective interaction, is also employed for massive bosons. The constituent particles are taken to be distinguishable and spinless. Different masses of the exchanged boson (including a zero mass) as well as states with different angular momenta are considered and the contribution of the crossed two-boson-exchange diagram is discussed. With this respect, the role played by the charge of the exchanged boson is emphasized. It is shown that the main difference between the Bethe-Salpeter results and the instantaneous approximation ones are not due to relativity as often conjectured. © 2001 Elsevier Science B.V
Relationship of field-theory based single-boson-exchange potentials to static ones
(Few-Body Systems, 2000) Amghar, A.; Desplanques, B.
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 Schrödinger 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 constituents has a large mass, (ii) to the absence of corrections of relative order α log 1/α 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