96 research outputs found
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
Comparison of Different Boost Transformations for the Calculation of Form Factors in Relativistic Quantum Mechanics
The effect of different boost expressions, pertinent to the instant, front
and point forms of relativistic quantum mechanics, is considered for the
calculation of the ground-state form factor of a two-body system in simple
scalar models. Results with a Galilean boost as well as an explicitly covariant
calculation based on the Bethe-Salpeter approach are given for comparison. It
is found that the present so-called point-form calculations of form factors
strongly deviate from all the other ones. This suggests that the formalism
which underlies them requires further elaboration. A proposition in this sense
is made.Comment: Invited talk given at the 18th European Conference on Few-Body
Problems in Physics, Bled, Slovenia, 8-14 Sep 2002. Submitted to Few Body
Syst.Supp
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
Form factors in RQM approaches: constraints from space-time translations
Different relativistic quantum mechanics approaches have recently been used
to calculate properties of various systems, form factors in particular. It is
known that predictions, which most often rely on a single-particle current
approximation, can lead to predictions with a very large range. It was shown
that accounting for constraints related to space-time translations could
considerably reduce this range. It is shown here that predictions can be made
identical for a large range of cases. These ones include the following
approaches: instant form, front form, and "point-form" in arbitrary momentum
configurations and a dispersion-relation approach which can be considered as
the approach which the other ones should converge to. This important result
supposes both an implementation of the above constraints and an appropriate
single-particle-like current. The change of variables that allows one to
establish the equivalence of the approaches is given. Some points are
illustrated with numerical results for the ground state of a system consisting
of scalar particles.Comment: 37 pages, 7 figures; further comments in ps 16 and 19; further
references; modified presentation of some formulas; corrected misprint
Form factors of heavy-light systems in point-form relativistic quantum mechanics: the Isgur-Wise function
We investigate electromagnetic and weak form factors of heavy-light mesons in
the context of point-form relativistic quantum mechanics. To this aim we treat
the physical processes from which such electroweak form factors are extracted
by means of a coupled channel approach which accounts for the dynamics of the
intermediate gauge bosons. It is shown that heavy-quark symmetry is respected
by this formulation. A simple analytical expression is obtained for the
Isgur-Wise function in the heavy-quark limit. Breaking of heavy-quark symmetry
due to realistic values of the heavy-quark mass are studied numerically.Comment: Presented at the 21st European Conference on Few-Body Problems in
Physics, Salamanca, Spain, 30 August - 3 September 201
Point-form quantum field theory and meson form factors
We shortly review point-form quantum field theory, i.e. the canonical
quantization of a relativistic field theory on a Lorentz-invariant surface of
the form . As an example of how point-form quantum field
theory may enter the framework of relativistic quantum mechanics we discuss the
calculation of the electromagnetic form factor of a confined quark-antiquark
pair (e.g. the pion).Comment: 3 pages, 2 figures. Based on a talk presented by W. Schweiger at the
20th European Conference on Few-Body Problems in Physics, September 10-14
2007, Pisa, Ital
The Balian-Br\'ezin Method in Relativistic Quantum Mechanics
The method suggested by Balian and Br\'ezin for treating angular momentum
reduction in the Faddeev equations is shown to be applicable to the
relativistic three-body problem.Comment: 14 pages in LaTe
RQM description of the charge form factor of the pion and its asymptotic behavior
The pion charge and scalar form factors, and , are first
calculated in different forms of relativistic quantum mechanics. This is done
using the solution of a mass operator that contains both confinement and
one-gluon-exchange interactions. Results of calculations, based on a one-body
current, are compared to experiment for the first one. As it could be expected,
those point-form, and instant and front-form ones in a parallel momentum
configuration fail to reproduce experiment. The other results corresponding to
a perpendicular momentum configuration (instant form in the Breit frame and
front form with ) do much better. The comparison of charge and scalar
form factors shows that the spin-1/2 nature of the constituents plays an
important role. Taking into account that only the last set of results
represents a reasonable basis for improving the description of the charge form
factor, this one is then discussed with regard to the asymptotic QCD-power-law
behavior . The contribution of two-body currents in achieving the right
power law is considered while the scalar form factor, , is shown to
have the right power-law behavior in any case. The low- behavior of the
charge form factor and the pion-decay constant are also discussed.}Comment: 30 pages, 10 figure
Constraints of cluster separability and covariance on current operators
Realistic models of hadronic systems should be defined by a dynamical unitary
representation of the Poincare group that is also consistent with cluster
properties and a spectral condition. All three of these requirements constrain
the structure of the interactions. These conditions can be satisfied in
light-front quantum mechanics, maintaining the advantage of having a kinematic
subgroup of boosts and translations tangent to a light front. The most
straightforward construction of dynamical unitary representations of the
Poincare group due to Bakamjian and Thomas fails to satisfy the cluster
condition for more than two particles. Cluster properties can be restored, at
significant computational expense, using a recursive method due to Sokolov. In
this work we report on an investigation of the size of the corrections needed
to restore cluster properties in Bakamjian-Thomas models with a light-front
kinematic symmetry. Our results suggest that for models based on nucleon and
meson degrees of freedom these corrections are too small to be experimentally
observed.Comment: Contribution to Light Cone 2011, Dallas TX, 4 pages, 2 figure
- …