Form factors of hadronic systems in various forms of relativistic quantum mechanics

The form factor of hadronic systems in various forms of relativistic quantum mechanics is considered. Motivated by the agreement of the nucleon ``point-form'' results with experiment, results for a toy model corresponding to the simplest Feynman diagram are first presented. These ones include the results for this diagram, which plays the role of an experiment, for the front-form and instant-form in standard kinematics (q^+=0 and Breit frame), but also in unconventional kinematics and finally a Dirac's point-form inspired approach. Results for an earlier ``point-form'' approach are reminded. Results are also presented for the pion charge form factor. Conclusions as for the efficiency of various approaches are given.Comment: 8 pages, 5 figures; invited talk at N*2004, Grenoble, March 24-27 (2004); to be published in the proceedings (World Scientific

Two-pion-exchange parity-violating potential and n⃗p→dγ\vec{n} p \to d \gamma

We calculate the parity-violating nucleon-nucleon potential in heavy-baryon chiral perturbation theory up to the next-to-next-to-leading order. The one-pion exchange comes in the leading order and the next-to-next-to-leading order consists of two-pion-exchange and the two-nucleon contact terms. In order to investigate the effect of the higher order contributions, we calculate the parity-violating asymmetry in $\vec{n} p \to d \gamma$ at the threshold. The one-pion dominates the physical observable and the two-pion contribution is about or less than 10% of the one-pion contribution.Comment: 3 pages, contribution to the workshop PAVI06 held in Milos island, Greece, May 16-20, 200

Pion charge form factor and constraints from space-time translations

The role of Poincar\'e covariant space-time translations is investigated in the case of a relativistic quantum mechanics approach to the pion charge form factor. It is shown that the related constraints are generally inconsistent with the assumption of a single-particle current, which is most often referred to. The only exception is the front-form approach with $q^+=0$. How accounting for the related constraints, as well as restoring the equivalence of different RQM approaches in estimating form factors, is discussed. Some extensions of this work and, in particular, the relationship with a dispersion-relation approach, are presented. Conclusions relative to the underlying dynamics are given.Comment: 6 pages, 2 figures, proceedings of the QNP2009 international conference (Beijing, oct. 2009), to be published in Chinese Physics

About the parity-non-conserving asymmetry in n+p--->d+gamma

The parity-non-conserving (pnc) asymmetry in \vec{n} + p \to d+\gamma at thermal energies has recently been calculated using effective field-theory methods. A comparison of this calculation with much more elaborate calculations performed in the 70's is made. This allows one to assess the validity of this new approach as presently used. It is found to overshoot the almost exact calculations by a factor close to 2 for the contribution involving the ^3S_1 component of both the initial and final states. This is much larger than anticipated by the authors. This discrepancy is analyzed and found to originate from the over-simplified description of the deuteron and capture states which underlies the new approach. The claim that earlier determinations of the sign would be in error is also examined. It is found that the sign discrepancy is most probably due, instead, to the fact that the pion-nucleon interaction referred to by the authors corresponds to a parity-non-conserving potential with a sign opposite to what is currently used. Some estimates and constraints relative to the pnc piNN coupling, h^1_{\pi}, which the above asymmetry is dependent on, are reviewed. Further details are given in an Appendix.Comment: 17 pages, 3 figures, submitted for publication in a shorter versio

Validity of the one-body current for the calculation of form factors in the point form of relativistic quantum mechanics

Form factors are calculated in the point form of relativistic quantum mechanics for the lowest energy states of a system made of two scalar particles interacting via the exchange of a massless boson. They are compared to the exact results obtained by using solutions of the Bethe-Salpeter equation which are well known in this case (Wick-Cutkosky model). Deficiencies of the point-form approach together with the single-particle current are emphasised. They point to the contribution of two-body currents which are required in any case to fulfil current conservation.Comment: 11 pages, 1 eps figur

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

Parity violation and the nature of charges

The origin of parity violation in physics is still unknown. At the present time, it is introduced in the theory by requiring that the SU(2) subgroup entering the description of interactions involves the left components. In the present contribution, one elaborates upon a suggestion made by Landau that particles and antiparticles could be like "stereo-isomeric" molecules, which would naturally provides parity violation. Particles and antiparticles could thus be combinations of the parity doublets associated with a chiral symmetry realized in the Wigner-Weyl mode. Consequences of such a description and the possible problems it could raise are examined.Comment: 3 pages, contribution to the 3rd international workshop: "From parity violation to hadronic structure and more ..." (PAVI06), to appear in the proceedings (EPJA

RQM description of PS meson form factors, constraints from space-time translations, and underlying dynamics

The role of Poincar\'e covariant space-time translations is investigated in the case of the pseudoscalar-meson charge form factors. It is shown that this role extends beyond the standard energy-momentum conservation, which is accounted for in all relativistic quantum mechanics calculations. It implies constraints that have been largely ignored until now but should be fulfilled to ensure the full Poincar\'e covariance. The violation of these constraints, which is more or less important depending on the form of relativistic quantum mechanics that is employed, points to the validity of using a single-particle current, which is generally assumed in calculations of form factors. In short, these constraints concern the relation of the momentum transferred to the constituents to the one transferred to the system. How to account for the related constraints, as well as restoring the equivalence of different relativistic quantum mechanics approaches in estimating form factors, is discussed. Some conclusions relative to the underlying dynamics are given in the pion case.Comment: 37 pages, 13 figures; figures completed for notations, revised text with better emphasis on differences with previous works; accepted for publication in EPJ

"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