55 research outputs found
A cluster-separable Born approximation for the 3D reduction of the three-fermion Bethe-Salpeter equation
We perform a 3D reduction of the two-fermion Bethe-Salpeter equation based on
Sazdjian's explicitly covariant propagator, combined with a covariant
substitute of the projector on the positive-energy free states. We use this
combination in the two fermions in an external potential and in the
three-fermion problems. The covariance of the two-fermion propagators insures
the covariance of the two-body equations obtained by switching off the external
potential, or by switching off all interactions between any pair of two
fermions and the third one, even if the series giving the 3D potential is
limited to the Born term or more generally truncated. The covariant substitute
of the positive energy projector preserves the equations against continuum
dissolution without breaking the covariance.Comment: 21 pages, 1 figure This article has been deeply modified after
refereeing. The presentation has been improved and examples have been added.
Three subsections have been added (transition matrix elements, two-body
limits, covariant Salpeter's equation). submitted to Journal of Physics
3D reduction of the N-body Bethe-Salpeter equation
We perform a 3D reduction of the two-fermion Bethe-Salpeter equation, by
series expansion around a positive-energy instantaneous approximation of the
Bethe-Salpeter kernel, followed by another series expansion at the 3D level in
order to get a manifestly hermitian 3D potential. It turns out that this
potential does not depend on the choice of the starting approximation of the
kernel anymore, and can be written in a very compact form. This result can also
be obtained directly by starting with an approximation of the free propagator,
based on integrals in the relative energies instead of the more usual
delta-constraint. Furthermore, the method can be generalized to a system of N
particles, consisting in any combination of bosons and fermions. As an example,
we write the 3D equation for systems of two or three fermions exchanging
photons, in Feynman or Coulomb's gauge.Comment: 22 pages, 3 figures in one single ps file. In the first revision, the
self-energy corrections to the propagator have been taken into account. The
three figures were gathered in a single ps file instead of three eps. In this
second revision (after submission to Nuclear Physics A and refereeing) some
explanations have been added, plus a new subsection about the scattering of a
particle by a bound stat
Supersymmetry and superalgebra for the two-body system with a Dirac oscillator interaction
Some years ago, one of the authors~(MM) revived a concept to which he gave
the name of single-particle Dirac oscillator, while another~(CQ) showed that it
corresponds to a realization of supersymmetric quantum mechanics. The Dirac
oscillator in its one- and many-body versions has had a great number of
applications. Recently, it included the analytic expression for the eigenstates
and eigenvalues of a two-particle system with a new type of Dirac oscillator
interaction of frequency~. By considering the latter together with its
partner corresponding to the replacement of~ by~, we are able
to get a supersymmetric formulation of the problem and find the superalgebra
that explains its degeneracy.Comment: 21 pages, LaTeX, 1 figure (can be obtained from the authors), to
appear in J. Phys.
Closed expression of the interaction kernel in the Bethe-Salpeter equation for quark-antiquark bound states
The interaction kernel in the Bethe-Salpeter equation for quark-antiquark
bound states is derived from the Bethe-Salpeter equations satisfied by the
quark-antiquark four-point Green's function. The latter equations are
established based on the equations of motion obeyed by the quark and antiquark
propagators, the four-point Green's function and some other kinds of Green's
functions which follow directly from the QCD generating functional. The B-S
kernel derived is given an exact and explicit expression which contains only a
few types of Green's functions. This expression is not only convenient for
perturbative calculations, but also suitable for nonperturbative
investigations.Comment: 27 pages,no figure
Weak Pion Production off the Nucleon
We develop a model for the weak pion production off the nucleon, which
besides the Delta pole mechanism (weak excitation of the
resonance and its subsequent decay into ), includes also some background
terms required by chiral symmetry. We re-fit the form factor to
the flux averaged ANL differential cross
section data, finding a substantially smaller contribution of the Delta pole
mechanism than traditionally assumed in the literature. Within this scheme, we
calculate several differential and integrated cross sections, including pion
angular distributions, induced by neutrinos and antineutrinos and driven both
by charged and neutral currents. In all cases we find that the background terms
produce quite significant effects and that they lead to an overall improved
description of the data, as compared to the case where only the Delta pole
mechanism is considered. We also show that the interference between the Delta
pole and the background terms produces parity-violating contributions to the
pion angular differential cross section, which are intimately linked to odd
correlations in the contraction between the leptonic and hadronic tensors.
However, these latter correlations do not imply a genuine violation of time
reversal invariance because of the existence of strong final state interaction
effects.Comment: Typos corrected; comments adde
Nucleon-to-Delta axial transition form factors in relativistic baryon chiral perturbation theory
We report a theoretical study of the axial Nucleon to Delta(1232)
() transition form factors up to one-loop order in relativistic
baryon chiral perturbation theory. We adopt a formalism in which the
couplings obey the spin-3/2 gauge symmetry and, therefore, decouple the
unphysical spin-1/2 fields. We compare the results with phenomenological form
factors obtained from neutrino bubble chamber data and in quark models.Comment: A few clarifying remarks added; version to appear in Physical Review
N-Delta(1232) axial form factors from weak pion production
The N-Delta axial form factors are determined from neutrino induced pion
production ANL & BNL data by using a state of the art theoretical model, which
accounts both for background mechanisms and deuteron effects. We find
violations of the off diagonal Goldberger-Treiman relation at the level of 2
sigma which might have an impact in background calculations for T2K and
MiniBooNE low energy neutrino oscillation precision experiments.Comment: 4 pages, 1 figur
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