539 research outputs found
Relation between Light Cone Distribution Amplitudes and Shape Function in B mesons
The Bakamjian-Thomas relativistic quark model provides a Poincar\'e
representation of bound states with a fixed number of constituents and, in the
heavy quark limit, form factors of currents satisfy covariance and Isgur-Wise
scaling. We compute the Light Cone Distribution Amplitudes of mesons
as well as the Shape Function , that enters
in the decay , that are also covariant in this class of
models. The LCDA and the SF are related through the quark model wave function.
The former satisfy, in the limit of vanishing constituent light quark mass, the
integral relation given by QCD in the valence sector of Fock space. Using a
gaussian wave function, the obtained is identical to the so-called
Roman Shape Function. From the parameters for the latter that fit the spectrum we predict the behaviour of . We
discuss the important role played by the constituent light quark mass. In
particular, although for vanishing light quark mass, a
non-vanishing mass implies the unfamiliar result . Moreover,
we incorporate the short distance behaviour of QCD to ,
which has sizeable effects at large . We obtain the values for the
parameters GeV and
GeV. We compare with other theoretical approaches and illustrate the
great variety of models found in the literature for the functions ; hence the necessity of imposing further constraints as in the
present paper. We briefly review also the different phenomena that are
sensitive to the LCDA.Comment: 6 figure
Quenching of the Deuteron in Flight
We investigate the Lorentz contraction of a deuteron in flight. Our starting
point is the Blankenbecler-Sugar projection of the Bethe-Salpeter equation to a
3-dimensional quasi potential equation, wqhich we apply for the deuteron bound
in an harmonic oscillator potential (for an analytical result) and by the Bonn
NN potential for a more realistic estimate. We find substantial quenching with
increasing external momenta and a significant modification of the high momentum
spectrum of the deuteron.Comment: 11 pages, 4 figure
Spatial distributions in static heavy-light mesons: a comparison of quark models with lattice QCD
Lattice measurements of spatial distributions of the light quark bilinear
densities in static mesons allow to test directly and in detail the wave
functions of quark models. These distributions are gauge invariant quantities
directly related to the spatial distribution of wave functions. We make a
detailed comparison of the recent lattice QCD results with our own quark
models, formulated previously for quite different purposes. We find a striking
agreement not only between our two quark models, but also with the lattice QCD
data for the ground state in an important range of distances up to about 4/GeV.
Moreover the agreement extends to the L=1 states [j^P=(1/2)^+]. An explanation
of several particular features completely at odds with the non-relativistic
approximation is provided. A rather direct, somewhat unexpected and of course
approximate relation between wave functions of certain quark models and QCD has
been established.Comment: 40 pages, 5 figures (version published in PRD
Space-like and time-like pion electromagnetic form factor and Fock state components within the Light-Front dynamics
The simultaneous investigation of the pion electromagnetic form factor in the
space- and time-like regions within a light-front model allows one to address
the issue of non-valence components of the pion and photon wave functions. Our
relativistic approach is based on a microscopic vector meson dominance (VMD)
model for the dressed vertex where a photon decays in a quark-antiquark pair,
and on a simple parametrization for the emission or absorption of a pion by a
quark. The results show an excellent agreement in the space like region up to
-10 , while in time-like region the model produces reasonable
results up to 10 .Comment: 74 pages, 11 figures, use revtex
The Rotation Average in Lightcone Time-Ordered Perturbation Theory
We present a rotation average of the two-body scattering amplitude in the
lightcone time()-ordered perturbation theory. Using a rotation average
procedure, we show that the contribution of individual time-ordered diagram can
be quantified in a Lorentz invariant way. The number of time-ordered diagrams
can also be reduced by half if the masses of two bodies are same. In the
numerical example of theory, we find that the higher Fock-state
contribution is quite small in the lightcone quantization.Comment: 25 pages, REVTeX, epsf.sty, 69 eps file
Rotational covariance and light-front current matrix elements
Light-front current matrix elements for elastic scattering from hadrons with
spin~1 or greater must satisfy a nontrivial constraint associated with the
requirement of rotational covariance for the current operator. Using a model
meson as a prototype for hadronic quark models, this constraint and its
implications are studied at both low and high momentum transfers. In the
kinematic region appropriate for asymptotic QCD, helicity rules, together with
the rotational covariance condition, yield an additional relation between the
light-front current matrix elements.Comment: 16 pages, [no number
Basis States for Relativistic, Dynamically-Entangled Particles
In several recent papers on entanglement in relativistic quantum systems and
relativistic Bell's inequalities, relativistic Bell-type two-particle states
have been constructed in analogy to non-relativistic states. These
constructions do not have the form suggested by relativistic invariance of the
dynamics. Two relativistic formulations of Bell-type states are shown for
massive particles, one using the standard Wigner spin basis and one using the
helicity basis. The construction hinges on the use of Clebsch-Gordan
coefficients of the Poincar\'e group to reduce the direct product of two
unitary irreducible representations (UIRs) into a direct sum of UIRs.Comment: 19 pages, three tables, revte
On algebraic models of relativistic scattering
In this paper we develop an algebraic technique for building relativistic
models in the framework of direct-interaction theories. The interacting mass
operator M in the Bakamjian-Thomas construction is related to a quadratic
Casimir operator C of a non-compact group G. As a consequence, the S matrix can
be gained from an intertwining relation between Weyl-equivalent representations
of G. The method is illustrated by explicit application to a model with SO(3,1)
dynamical symmetry.Comment: 10 pages, to appear in J. Phys. A : Math. Theo
Baryon Current Matrix Elements in a Light-Front Framework
Current matrix elements and observables for electro- and photo-excitation of
baryons from the nucleon are studied in a light-front framework. Relativistic
effects are estimated by comparison to a nonrelativistic model, where we use
simple basis states to represent the baryon wavefunctions. Sizeable
relativistic effects are found for certain transitions, for example, to radial
excitations such as that conventionally used to describe to the Roper
resonance. A systematic study shows that the violation of rotational covariance
of the baryon transition matrix elements stemming from the use of one-body
currents is generally small.Comment: 32 pages, LaTeX, 10 postscript figures, uses epsf.sty; figures
uuencoded with uufiles (or available by request in .ps or hardcopy form
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