392 research outputs found

### Nucleon and Pion Form Factors in Different Forms of Relativistic Quantum Mechanics

Calculations of form factors in different forms of relativistic kinematics
are presented. They involve the instant, front and point forms. In the two
first cases, different kinematical conditions are considered while in the
latter case, both a Dirac-inspired approach and a hyperplane-based one are
incorporated in our study. Numerical results are presented for the pion form
factors with emphasis on both the low and high Q**2 range. A new argument is
presented, explaining why some approaches do considerably much better than
other ones whenonly a single-particle current is considered.Comment: To appear in the proceedings of MENU04 (Beijing, Aug. 29- Sept. 4,
2004

### 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

### 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

### 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 $\vec{n} p \to d \gamma$

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

### 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

### Form factors in relativistic quantum mechanics: constraints from space-time translations

The comparison of form factors calculated from a single-particle current in
different relativistic quantum mechanic approaches evidences tremendous
discrepancies. The role of constraints coming from space-time translations is
considered here with this respect. It is known that invariance under these
translations implies the energy-momentum conservation relation that is usually
assumed to hold globally. Transformations of the current under these
translations, which lead to this result, also imply constraints that have been
ignored so far in relativistic quantum mechanic approaches. An implementation
of these constraints is discussed in the case of a model with two scalar
constituents. It amounts to incorporate selected two-body currents to all
orders in the interaction. Discrepancies for form factors in different
approaches can thus be removed, contributing to restore the equivalence of
different approaches. Results for the standard front-form approach ($q^+=0$)
are found to fulfill the constraints and are therefore unchanged. The relation
with results from a dispersion-relation approach is also made.Comment: 8 pages, 5 figures; to be published in the proceedings of LC2008;
Light Cone 2008. Relativistic Nuclear and Particle Physics, Mulhouse : France
(2008

### 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

### 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

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