3,030 research outputs found
Stability of bound states in the light-front Yukawa model
We show that in the system of two fermions interacting by scalar exchange,
the solutions for J= bound states are stable without any cutoff
regularization for coupling constant below some critical value.Comment: 5 pages, 2 figures, submitted to publicatio
Relativistic wave functions and energies for nonzero angular momentum states in light-front dynamics
Light-front dynamics (LFD) is a powerful approach to the theory of
relativistic composite systems (hadrons in the quark models and relativistic
nucleons in nuclei). Its explicitly covariant version has been recently applied
with success to describe the new CEBAF/TJNAF data on the deuteron
electromagnetic form factors. The solutions used in were however not obtained
from solving exactly the LFD equations but by means of a perturbative
calculation with respect to the non relativistic wave function. Since, a
consequent effort has been made to obtain exact solutions of LFD equations. The
first results concerning J=0 states in a scalar model have been published in
nucl-th/9912050. The construction of states in LFD is complicated by
the two following facts. First, the generators of the spatial rotations contain
interaction and are thus difficult to handle. Second, one is always forced to
work in a truncated Fock space, and consequently, the Poincar\'e group
commutation relations between the generators -- ensuring the correct properties
of the state vector under rotation -- are in practice destroyed. In the
standard approach, with the light-front plane defined as , this
violation of rotational invariance manifests by the fact that the energy
depends on the angular momentum projection on -axis.
We present here a method to construct states in the explicitly
covariant formulation of LFD and show how it leads to a restoration of
rotational invariance.Comment: To appear in Nucl.Phys.B, 3 pages, 2 figures, .tar.gz fil
Two-Body Bound States in Light-Front Dynamics
We present the main features of the explicitly covariant Light-Front Dynamics
formalism and a summary of our recent works on this topic. They concern the
bound states of two scalar particles in the Wick-Cutkosky model and of two
fermions interacting via the usual OBEP ladder kernels.Comment: Invited contribution to the XIVth International School on Nuclear
Physics, Varna, Bulgaria, Sept 25-30. 20 pages, 23 figure
Electromagnetic form factor via Minkowski and Euclidean Bethe-Salpeter amplitudes
The electromagnetic form factors calculated through Euclidean Bethe-Salpeter
amplitude and through the light-front wave function are compared with the one
found using the Bethe-Salpeter amplitude in Minkowski space. The form factor
expressed through the Euclidean Bethe-Salpeter amplitude (both within and
without static approximation) considerably differs from the Minkowski one,
whereas form factor found in the light-front approach is almost
indistinguishable from it.Comment: 3 pages, 2 figures. Contribution to the proceedings of the 20th
International Conference on Few-Body Problems in Physics (FB20), Pisa, Italy,
September 10-14, 2007. To be published in "Few-Body Systems
On the relation between the Deuteron Form Factor at High Momentum Transfer and the High Energy Neutron-Proton Scattering Amplitude
A non-relativistic potential-model version of the factorization assumption,
used in perturbative QCD calculations of hadronic form factors, is used, along
with the Born approximation valid at high energies, to derive a remarkably
simple relationship between the impulse approximation contribution to the
deuteron form factor at high momentum transfer and the high energy
neutron-proton scattering amplitude. The relation states that the form factor
at a given value of is proportional to the scattering amplitude at a
specific energy and scattering angle. This suggests that an accurate
computation of the form factors at large requires a simultaneous
description of the phase-shifts at a related energy, a statement that seems
reasonable regardless of any derivation. Our form factor-scattering amplitude
relation is shown to be accurate for some examples. However, if the potential
consists of a strong short distance repulsive term and a strong longer ranged
attractive term, as typically occurs in many realistic potentials, the relation
is found to be accurate only for ridiculously large values of . More general
arguments, using only the Schroedinger equation, suggest a strong, but
complicated, relationship between the form factor and scattering amplitude.
Furthermore, the use of recently obtained soft potentials, along with an
appropriate current operator, may allow calculations of form factors that are
consistent with the necessary phase shifts.Comment: 14 pages, 4 figures, The discussion has been extended by including
numerical examples and general argument
Antiproton-deuteron annihilation at low energies
Recent experimental studies of the antiproton-deuteron system at low energies
have shown that the imaginary part of the antiproton-deuteron scattering length
is smaller than the antiproton-proton one. Two- and three-body systems with
strong annihilation are investigated and a mechanism explaining this unexpected
relation between the imaginary parts of the scattering lengths is proposed.Comment: 6 pages, 3 figures, to be published in The European Physical Journal
Electromagnetic form factor via Bethe-Salpeter amplitude in Minkowski space
For a relativistic system of two scalar particles, we find the Bethe-Salpeter
amplitude in Minkowski space and use it to compute the electromagnetic form
factor. The comparison with Euclidean space calculation shows that the Wick
rotation in the form factor integral induces errors which increase with the
momentum transfer Q^2. At JLab domain (Q^2=10 GeV^2/c^2), they are about 30%.
Static approximation results in an additional and more significant error. On
the contrary, the form factor calculated in light-front dynamics is almost
indistinguishable from the Minkowski space one.Comment: 8 pages, 7 figures, to be published in Eur. Phys. J. A; Reference
[15] is adde
Relativistic bound states in Yukawa model
The bound state solutions of two fermions interacting by a scalar exchange
are obtained in the framework of the explicitly covariant light-front dynamics.
The stability with respect to cutoff of the J= and J=
states is studied. The solutions for J= are found to be stable for
coupling constants below the critical value
and unstable above it. The asymptotic behavior of the
wave functions is found to follow a law. The coefficient
and the critical coupling constant are calculated from an
eigenvalue equation. The binding energies for the J= solutions
diverge logarithmically with the cutoff for any value of the coupling constant.
For a wide range of cutoff, the states with different angular momentum
projections are weakly split.Comment: 22 pages, 13 figures, .tar.gz fil
Bethe-Salpeter equation for doubly heavy baryons in the covariant instantaneous approximation
In the heavy quark limit, a doubly heavy baryon is regarded as composed of a
heavy diquark and a light quark. We establish the Bethe-Salpeter (BS) equations
for the heavy diquarks and the doubly heavy baryons, respectively, to leading
order in a expansion. The BS equations are solved numerically under
the covariant instantaneous approximation with the kernels containing scalar
confinement and one-gluon-exchange terms. The masses for the heavy diquarks and
the doubly heavy baryons are obtained and the non-leptonic decay widths for the
doubly heavy baryons emitting a pseudo-scalar meson are calculated within the
model.Comment: Corrections to the text, two references added, version accepted for
publication in Physical Review
Nucleon form factors and moments of parton distributions in twisted mass lattice QCD
We present results on the electroweak form factors and on the lower moments
of parton distributions of the nucleon, within lattice QCD using two dynamical
flavors of degenerate twisted mass fermions. Results are obtained on lattices
with three different values of the lattice spacings, namely a=0.089 fm, a=0.070
fm and a=0.056 fm, allowing the investigation of cut-off effects. The volume
dependence is examined by comparing results on two lattices of spatial length
L=2.1 fm and L=2.8 fm. The simulations span pion masses in the range of 260-470
MeV. Our results are renormalized non-perturbatively and the values are given
in the MS-scheme at a scale mu=2 GeV.Comment: Talk presented in the XXIst International Europhysics Conference on
High Energy Physics, 21-27 July 2011, Grenoble, Rhones Alpes Franc
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