354 research outputs found
Quark-binding effects in inclusive decays of heavy mesons
We present a new approach to the analysis of quark-binding effects in
inclusive decays of heavy mesons within the relativistic dispersion quark
model. Various differential distributions, such as electron energy spectrum,
- and -distributions, are calculated in terms of the meson soft
wave function which also determines long-distance effects in exclusive
transition form factors. Using the quark-model parameters and the meson
wave function previously determined from the description of the exclusive transitions within the same dispersion approach, we provide numerical
results on various distributions in the inclusive decays.Comment: revtex, 18 pages, preprint HD-THEP-99-50 (Heidelberg) and
RM3-TH/99-13 (Roma
Heavy-to-light form factors: sum rules on the light cone and beyond
We report the first systematic analysis of the off-light-cone effects in sum
rules for heavy-to-light form factors. These effects are investigated in a
model based on scalar constituents, which allows a technically rather simple
analysis but has the essential features of the analogous QCD calculation. The
correlator relevant for the extraction of the heavy-to-light form factor is
calculated in two different ways: first, by adopting the full Bethe-Salpeter
amplitude of the light meson and, second, by performing the expansion of this
amplitude near the light cone . We demonstrate that the contributions to
the correlator from the light-cone term and the off-light-cone terms
have the same order in the expansion. The light-cone
correlator, corresponding to , is shown to systematically overestimate
the full correlator, the difference being , with
the continuum subtraction parameter of order 1 GeV. Numerically, this
difference is found to be 10-20%.Comment: revtex 14 pages, version to be published in Phys. Rev. D (discussion
in Sect. 3 extended, example in Sect. 4 added
Electromagnetic form factors in the light-front formalism and the Feynman triangle diagram: spin-0 and spin-1 two-fermion systems
The connection between the Feynman triangle diagram and the light-front
formalism for spin-0 and spin-1 two-fermion systems is analyzed. It is shown
that in the limit q+ = 0 the form factors for both spin-0 and spin-1 systems
can be uniquely determined using only the good amplitudes, which are not
affected by spurious effects related to the loss of rotational covariance
present in the light-front formalism. At the same time, the unique feature of
the suppression of the pair creation process is maintained. Therefore, a
physically meaningful one-body approximation, in which all the constituents are
on their mass-shells, can be consistently formulated in the limit q+ = 0.
Moreover, it is shown that the effects of the contact term arising from the
instantaneous propagation of the active constituent can be canceled out from
the triangle diagram by means of an appropriate choice of the off-shell
behavior of the bound state vertexes; this implies that in case of good
amplitudes the Feynman triangle diagram and the one-body light-front result
match exactly. The application of our covariant light-front approach to the
evaluation of the rho-meson elastic form factors is presented.Comment: corrected typos in the reference
Width of the J^P=1/2^+ pentaquark in the quark-diquark model
We analyse the width of the pentaquark assuming that it
is a bound state of two extended spin-zero -diquarks and the
antiquark (the Jaffe-Wilczek scenario). The width obtained when the size
parameters of the pentaquark wave function are taken to be close to the
parameters of the nucleon is found to be MeV, i.e. it has a normal
value for a -wave hadron decay with the corresponding energy
release.However, we found a strong dynamical suppression of the decay width if
the pentaquark has an asymmetric "peanut" structure with the strange antiquark
in the center and the two diquarks rotating around. In this case a decay width
of 1 MeV is a natural possibility.Comment: 3 new references added, version accepted to Physics Letters
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