10 research outputs found
Form Factors of Few-Body Systems: Point Form Versus Front Form
We present a relativistic point-form approach for the calculation of
electroweak form factors of few-body bound states that leads to results which
resemble those obtained within the covariant light-front formalism of Carbonell
et al. Our starting points are the physical processes in which such form
factors are measured, i.e. electron scattering off the bound state, or the
semileptonic weak decay of the bound state. These processes are treated by
means of a coupled-channel framework for a Bakamjian-Thomas type mass operator.
A current with the correct covariance properties is then derived from the
pertinent leading-order electroweak scattering or decay amplitude. As it turns
out, the electromagnetic current is affected by unphysical contributions which
can be traced back to wrong cluster properties inherent in the Bakamjian-Thomas
construction. These spurious contributions, however, can be separated uniquely,
as in the covariant light-front approach. In this way we end up with form
factors which agree with those obtained from the covariant light-front
approach. As an example we will present results for electroweak form factors of
heavy-light systems and discuss the heavy-quark limit which leads to the famous
Isgur-Wise function.Comment: Presented at LIGHTCONE 2011, Dallas, USA, 23 - 27 May, 201
Point-form quantum field theory
We examine canonical quantization of relativistic field theories on the
forward hyperboloid, a Lorentz-invariant surface of the form . This choice of quantization surface implies that all components of the
4-momentum operator are affected by interactions (if present), whereas rotation
and boost generators remain interaction free -- a feature characteristic of
Dirac's `` point-form\rq\rq of relativistic dynamics. Unlike previous attempts
to quantize fields on space-time hyperboloids, we keep the usual plane-wave
expansion of the field operators and consider evolution of the system generated
by the 4-momentum operator. We verify that the Fock-space representations of
the Poincar\'e generators for free scalar and spin-1/2 fields look the same as
for equal-time quantization. Scattering is formulated for interacting fields in
a covariant interaction picture and it is shown that the familiar perturbative
expansion of the S-operator is recovered by our approach. An appendix analyzes
special distributions, integrals over the forward hyperboloid, that are used
repeatedly in the paper.Comment: 30 page
Boost operators in Coulomb-gauge QCD: the pion form factor and Fock expansions in phi radiative decays
In this article we rederive the Boost operators in Coulomb-Gauge Yang-Mills
theory employing the path-integral formalism and write down the complete
operators for QCD. We immediately apply them to note that what are usually
called the pion square, quartic... charge radii, defined from derivatives of
the pion form factor at zero squared momentum transfer, are completely blurred
out by relativistic and interaction corrections, so that it is not clear at all
how to interpret these quantities in terms of the pion charge distribution. The
form factor therefore measures matrix elements of powers of the QCD boost and
Moeller operators, weighted by the charge density in the target's rest frame.
In addition we remark that the decomposition of the eta' wavefunction in
quarkonium, gluonium, ... components attempted by the KLOE collaboration
combining data from phi radiative decays, requires corrections due to the
velocity of the final state meson recoiling against a photon. This will be
especially important if such decompositions are to be attempted with data from
J/psi decays.Comment: 14 pages, 4 figure