26 research outputs found
Electromagnetic form factors of the meson in a light-front constituent quark model
The electromagnetic form factors of the meson are evaluated adopting a
relativistic constituent quark model based on the light-front formalism, and
using a meson wave function with the high-momentum tail generated by the
one-gluon-exchange interaction. The breakdown of the rotational covariance for
the one-body component of the current operator is investigated and the
sensitivity of the ratio of the -meson form factors to the pion (charge)
form factor to the spin-dependent component of the effective
interaction is illustrated.Comment: 8 pages, latex file, 4 figures available as a separate .uu fil
Charmed decays of the B-meson in the quark model
Exclusive and inclusive, semileptonic and non-leptonic, charmed decays of the
B-meson are investigated in the context of a phenomenological quark model.
Bound-state effects are taken care of by adopting a single (model-dependent)
non-perturbative wave function, describing the motion of the light spectator
quark in the B-meson. A nice reproduction of both exclusive and inclusive
semileptonic data is obtained. Our predictions for the electron spectrum are
presented and compared with those of the Isgur-Scora-Grinstein-Wise quark
model. Finally, our approach is applied to the calculation of inclusive
non-leptonic widths, obtaining a remarkable agreement with experimental
findings.Comment: to appear in the Proc. of the 2^{nd} Int. Conf. on Hyperons, Charm
and Beauty Hadrons, Montreal, Canada, 27-30 August 199
Frame-Independence of Exclusive Amplitudes in the Light-Front Quantization
While the particle-number-conserving convolution formalism established in the
Drell-Yan-West reference frame is frequently used to compute exclusive
amplitudes in the light-front quantization, this formalism is limited to only
those frames where the light-front helicities are not changed and the good
(plus) component of the current remains unmixed. For an explicit demonstration
of such criteria, we present the relations between the current matrix elements
in the two typical reference frames used for calculations of the exclusive
amplitudes, i.e. the Drell-Yan-West and Breit frames and investigate both
pseudoscalar and vector electromagnetic currents in detail. We find that the
light-front helicities are unchanged and the good component of the current does
not mix with the other components of the current under the transformation
between these two frames. Thus, the pseudoscalar and vector form factors
obtained by the diagonal convolution formalism in both frames must indeed be
identical. However, such coincidence between the Drell-Yan-West and Breit
frames does not hold in general. We give an explicit example in which the
light-front helicities are changed and the plus component of the current is
mixed with other components under the change of reference frame. In such a
case, the relationship between the frames should be carefully analyzed before
the established convolution formalism in the Drell-Yan-West frame is used.Comment: 14 pages, 4 figure
A connection between inclusive semileptonic decays of bound and free heavy quarks
A relativistic constituent quark model, formulated on the light-front, is
used to derive a new parton approximation for the inclusive semileptonic decay
width of the B-meson. A simple connection between the decay rate of a free
heavy-quark and the one of a heavy-quark bound in a meson or in a baryon is
established. The main features of the new approach are the treatment of the
b-quark as an on-mass-shell particle and the inclusion of the effects arising
from the b-quark transverse motion in the B-meson. In a way conceptually
similar to the deep-inelastic scattering case, the B-meson inclusive width is
expressed as the integral of the free b-quark partial width multiplied by a
bound-state factor related to the b-quark distribution function in the B-meson.
The non-perturbative meson structure is described through various quark-model
wave functions, constructed via the Hamiltonian light-front formalism using as
input both relativized and non-relativistic potential models. A link between
spectroscopic quark models and the B-meson decay physics is obtained in this
way. Our predictions for the B -> X_c l nu_l and B -> X_u l nu_l decays are
used to extract the CKM parameters |V_cb| and |V_ub| from available inclusive
data. After averaging over the various quark models adopted and including
leading-order perturbative QCD corrections, we obtain |V_cb| = (43.0 +/-
0.7_exp +/- 1.8_th) 10^-3 and |V_ub| = (3.83 +/- 0.48_exp +/- 0.14_th) 10^-3,
implying |V_ub / V_cb| = 0.089 +/- 0.011_exp +/- 0.005_th, in nice agreement
with existing predictions.Comment: revised version with pQCD corrections included, to appear in Physical
Review
Comparison among Hamiltonian light-front formalisms at q+ = 0 and q+ <> 0: space-like elastic form factors of pseudoscalar and vector mesons
The electromagnetic elastic form factors of pseudoscalar and vector mesons
are analyzed for space-like momentum transfers in terms of relativistic quark
models based on the Hamiltonian light-front formalism elaborated in different
reference frames (q+ 0 and q+ 0). As far as the one-body approximation for
the electromagnetic current operator is concerned, it is shown that the
predictions of the light-front approach at q+=0 should be preferred,
particularly in case of light hadrons, because of: i) the relevant role played
by the Z-graph at q+ 0, and ii) the appropriate elimination of spurious
effects, related to the orientation of the null hyperplane where the
light-front wave function is defined.Comment: version to appear in Phys. Rev. C. No change in the results and in
the conclusion
Radiative and transition form factors in a light-front constituent quark model
The form factors of the and radiative transitions are
evaluated within a light-front constituent quark model, using for the first
time the eigenfunctions of a light-front mass operator reproducing the meson
mass spectrum and including phenomenological Dirac and Pauli quark form factors
in the one-body electromagnetic current operator. The sensitivity of the
transition form factors both to the meson wave functions and to the constituent
quark form factors is illustrated. It is shown that the measurement of the and radiative transitions could help in discriminating among
various models of the meson structure.Comment: 9 pages, latex, 4 figures available as separate .uu file, to appear
in Phys. Lett.
Hard Constituent Quarks and Electroweak Properties of Pseudoscalar Mesons
The high momentum components generated in the wave function of pseudoscalar
mes* by the one-gluon-exchange interaction are investigated within a
relativistic constituent quark model. Adopting the light-cone formalism, the
sensitivity of the weak decay constant and the charge form factor to hard
constituent quarks is illustrated.Comment: 11 pages and 5 figs. (to be requested), LaTeX, INFN-ISS 94/3. To
appear in Physics Lett.
The Vector Meson Form Factor Analysis in Light-Front Dynamics
We study the form factors of vector mesons using a covariant fermion field
theory model in dimensions. Performing a light-front calculation in the
frame in parallel with a manifestly covariant calculation, we note the
existence of a nonvanishing zero-mode contribution to the light-front current
and find a way of avoiding the zero-mode in the form factor calculations.
Upon choosing the light-front gauge (\ep^+_{h=\pm}=0) with circular
polarization and with spin projection , only the
helicity zero to zero matrix element of the plus current receives zero-mode
contributions. Therefore, one can obtain the exact light-front solution of the
form factors using only the valence contribution if only the helicity
components, , and , are used. We also compare our
results obtained from the light-front gauge in the light-front helicity basis
(i.e. ) with those obtained from the non-LF gauge in the instant form
linear polarization basis (i.e. ) where the zero-mode contributions to
the form factors are unavoidable.Comment: 33 pages; typo in Eq.(15) is corrected; comment on Ref.[9] is
corrected; version to appear in Phys. Rev.
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
Angular Conditions,Relations between Breit and Light-Front Frames, and Subleading Power Corrections
We analyze the current matrix elements in the general collinear (Breit)
frames and find the relation between the ordinary (or canonical) helicity
amplitudes and the light-front helicity amplitudes. Using the conservation of
angular momentum, we derive a general angular condition which should be
satisfied by the light-front helicity amplitudes for any spin system. In
addition, we obtain the light-front parity and time-reversal relations for the
light-front helicity amplitudes. Applying these relations to the spin-1 form
factor analysis, we note that the general angular condition relating the five
helicity amplitudes is reduced to the usual angular condition relating the four
helicity amplitudes due to the light-front time-reversal condition. We make
some comments on the consequences of the angular condition for the analysis of
the high- deuteron electromagnetic form factors, and we further apply the
general angular condition to the electromagnetic transition between spin-1/2
and spin-3/2 systems and find a relation useful for the analysis of the
N- transition form factors. We also discuss the scaling law and the
subleading power corrections in the Breit and light-front frames.Comment: 24 pages,2 figure