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

"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

Dirac's inspired point form and hadron form factors

Noticing that the point-form approach referred to in many recent works implies physics described on hyperplanes, an approach inspired from Dirac's one, which involves a hyperboloid surface, is presented. A few features pertinent to this new approach are emphasized. Consequences as for the calculation of form factors are discussed.Comment: 4 pages, 2 figures, to be published in the proceedings of BARYON0

Relativistic quantum mechanics: A Dirac's point-form inspired approach

This paper describes a tentative relativistic quantum mechanics approach inspired by Dirac's point-form, which is based on the physics description on a hyperboloid surface. It is mainly characterized by a non-standard relation of the constituent momenta of some system to its total momentum. Contrary to instant- and front-form approaches, where it takes the form of a 3-dimensional delta function, the relation is given here by a Lorentz-scalar constraint. Thus, in the c.m. frame, the sum of the constituent momenta, which differs from zero off-energy shell, has no fixed direction, in accordance with the absence of preferred direction on a hyperboloid surface. To some extent, this gives rise to an extra degree of freedom entering the description of the system of interest. The development of a consistent formalism within this picture is described. Comparison with other approaches is made.Comment: 26 pages, 3 figures, to be submitte

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 $m$, 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 $Q^2=100 m^2$ 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

Cement remnants removal in implant restorations: A preliminary in vitro study

Treball Final de Grau d'Odontologia, Facultat de Medicina i Ciències de la Salut, Universitat de Barcelona, Curs: 2016-2017, Director: Rui Pedro Barbosa De FigueiredoObjectives: To evaluate the efficacy of stainless steel explorers, waxed dental floss and teflon scalers in removing cement remnants from fixed implant restorations; To evaluate if these instruments produce surface alterations in the abutment surface. Materials and Methods: A preliminary in vitro study, involving 9 identical single-unit implant-supported crowns (upper central incisor), manufactured with CAD/CAM technology, was perfomed. After cementation with a zinc oxide eugenol cement, each crown was randomly allocated into 3 study groups, according to the instrument used to remove the cement remnants (teflon scaler, stainless steel explorer and waxed dental floss). An experienced professional removed the cement remnants. Afterwards, a blinded researcher evaluated the presence of cement using direct visual observation, standardized digital photographs and Scanning Electron Microscopy (SEM) images. In the latter analysis (SEM 75x), surface alterations in the abutment surface caused by the employed instruments were also registered. A descriptive analysis of the data was performed. Results: After cementation, cement remnants were observed in all samples. The instrument that eliminated a higher percentage of cement remnants was the stainless steel explorer (59.4%) and the worst, the waxed dental floss (27.6%). The stainless steel explorer produced scratches in 100% of the samples (5 to 10 scratches in each sample). Conclusions: Even though cement debris was observed in all samples, stainless steel explorers and teflon scalers seem to be the most effective instruments in the removal of cement remnants in implant-supported restorations. Since stainless steel explorers scratched the abutment surface, teflon scalers seem to be the best option for this particular indication. Dental floss is not a suitable alternative since it removed less cement than the other 2 instruments, and also left some material debris (small threads) over the abutment. A larger sample is required to confirm the present findings

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