362 research outputs found
Octet-Baryon Form Factors in the Diquark Model
We present an alternative parameterization of the quark-diquark model of
baryons which particularly takes care of the most recent proton electric
form-factor data from the E136 experiment at SLAC. In addition to
electromagnetic form factors of the nucleon, for which good agreement with data
is achieved, we discuss the weak axial vector form factor of the nucleon as
well as electromagnetic form factors of and hyperons.
Technical advance in calculating the pertinent analytic expressions within
perturbative quantum chromodynamics is gained by formulating the wave function
of the quark-diquark system in a covariant way. Finally, we also comment on the
influence of Sudakov corrections within the scope of the diquark model.Comment: 16 pages, WU-B 93-07, latex, uuencoded postscript files of 7 figures
appended at the end of the latex fil
Baryon Distribution Amplitudes in QCD
We develop a new theoretical framework for the description of leading twist
light-cone baryon distribution amplitudes which is based on integrability of
the helicity evolution equation to leading logarithmic accuracy.
A physical interpretation is that one can identify a new `hidden' quantum
number which distinguishes components in the distribution
amplitudes with different scale dependence. The solution of the corresponding
evolution equation is reduced to a simple three-term recurrence relation. The
exact analytic solution is found for the component with the lowest anomalous
dimension for all moments , and the WKB-type expansion is constructed for
other levels, which becomes asymptotically exact at large . Evolution
equations for the distribution amplitudes (e.g. for the nucleon)
are studied as well. We find that the two lowest anomalous dimensions for the
operators (one for each parity) are separated from the rest of
the spectrum by a finite `mass gap'. These special states can be interpreted as
scalar diquarks.Comment: 75 pages, LaTeX style, 18 figures embedded with epsf.st
RQM description of the charge form factor of the pion and its asymptotic behavior
The pion charge and scalar form factors, and , are first
calculated in different forms of relativistic quantum mechanics. This is done
using the solution of a mass operator that contains both confinement and
one-gluon-exchange interactions. Results of calculations, based on a one-body
current, are compared to experiment for the first one. As it could be expected,
those point-form, and instant and front-form ones in a parallel momentum
configuration fail to reproduce experiment. The other results corresponding to
a perpendicular momentum configuration (instant form in the Breit frame and
front form with ) do much better. The comparison of charge and scalar
form factors shows that the spin-1/2 nature of the constituents plays an
important role. Taking into account that only the last set of results
represents a reasonable basis for improving the description of the charge form
factor, this one is then discussed with regard to the asymptotic QCD-power-law
behavior . The contribution of two-body currents in achieving the right
power law is considered while the scalar form factor, , is shown to
have the right power-law behavior in any case. The low- behavior of the
charge form factor and the pion-decay constant are also discussed.}Comment: 30 pages, 10 figure
Radiative Scalar Meson Decays in the Light-Front Quark Model
We construct a relativistic wavefunction for scalar mesons within the
framework of light-front quark model(LFQM). This scalar wavefunction is used to
perform relativistic calculations of absolute widths for the radiative decay
processes, and
which incorporate the effects of glueball-
mixing. The mixed physical states are assumed to be ,and
for which the flavor-glue content is taken from the mixing
calculations of other works. Since experimental data for these processes are
poor, our results are compared with those of a recent non-relativistic model
calculation. We find that while the relativistic corrections introduced by the
LFQM reduce the magnitudes of the decay widths by 50-70%, the relative
strengths between different decay processes are fairly well preserved. We also
calculate decay widths for the processes and
(0^{++})\to\gamma\gamm involving the light scalars and
to test the simple model of these mesons. Our results of
model for these processes are not quite consistent with well-established data,
further supporting the idea that and are not conventional
states.Comment: 10 pages, 4 figure
Analysis of the Lambda_b -> Lambda_c + l nu_l decay within a light-front constituent quark model
We present an investigation of the Isgur-Wise form factor relevant for the
semileptonic decay Lambda_b -> \Lambda_c + l nu_l performed within a
light-front constituent quark model. Adopting different baryon wave functions
it is found that the Isgur-Wise form factor depends sensitively on the baryon
structure. It is shown however that the shape of the Isgur-Wise function in the
full recoil range relevant for the Lambda_b -> \Lambda_c + l nu_l decay can be
effectively constrained using recent lattice QCD results at low recoil. Then,
the Lambda_b -> \Lambda_c + l nu_l decay is investigated including both
radiative effects and first-order power corrections in the inverse heavy-quark
mass. Our final predictions for the exclusive semileptonic branching ratio, the
longitudinal and transverse asymmetries, and the longitudinal to transverse
decay ratio are: Br(Lambda_b -> \Lambda_c + l nu_l) = (6.3 +/- 1.6) % |V_bc /
0.040|**2 ~ tau(Lambda_b) / (1.24 ps), a_L = -0.945 +/- 0.014, a_T = -0.62 +/-
0.09 and R_L/T = 1.57 +/- 0.15, respectively. Moreover, both the longitudinal
asymmetry and the (partially integrated) longitudinal to transverse decay ratio
are found to be only marginally affected by the model dependence of the
Isgur-Wise form factor as well as by first-order power corrections; therefore,
their experimental determination might be a very interesting tool for testing
the SM and for investigating possible New Physics.Comment: revised version with inclusion of PQCD corrections and improved
discussion of power corrections, to appear in Physical Review
Internal W-emmission and W-exchange Contributions to {\bf B}\to {{\bf D}^{(*)} Decays
We evaluate external -emission, internal -emission and -exchange
contributions to nonleptonic decays based on the perturbative
QCD formalism including Sudakov effects, whose ratio is found to be
at the amplitude level. We observe that the internal
-emission contribution is additive to the external -emission
contribution, and that the -exchange contribution is negligible and mainly
imaginary, its real part being at least one order of magnitude smaller than the
imaginary part. Our predictions are consistent with the CLEO data and with
those obtained by the Bauer-Stech-Wirbel method.Comment: 13 pages, Latex, 1 Postscript fil
Predictions of total and total reaction cross sections for nucleon-nucleus scattering up to 300 MeV
Total reaction cross sections are predicted for nucleons scattering from various nuclei. Projectile energies to 300 MeV are considered. So also are mass variations of those cross sections at selected energies. All predictions have been obtained from coordinate space optical potentials formed by full folding effective two-nucleon (NN) interactions with one body density matrix elements (OBDME) of the nuclear ground states. Good comparisons with data result when effective NN interactions defined by medium modification of free NN t matrices are used. Coupled with analyses of differential cross sections, these results are sensitive to details of the model ground states used to describe nuclei
Haematological and biochemical parameters in Churra-da-Terra-Quente ewes from the northeast of Portugal
Hematological and biochemical parameters, including plasma electrolytes and thyroid hormones, were determined in 73 clinically healthy Churra-da-Terra-Quente ewes, a typical breed from the northeast of Portugal. The hemogram values were: erythrocytes 9.8±1.51012/L; haemoglobin 118.1±19.1g/L; haematocrit 40.8±5.9%; leukocytes 5.7±1.8109/L; and platelets 544.3±177.2109/L. The thrombin time was 17.3±1.7 seconds. The values of biochemical parameters were: total protein 76.4±6.1g/L; glucose 2.87±0.60mmol/L; total cholesterol 1.65±0.33mmol/L; aspartate aminotransferase 155.9±49.2U/L; alanine aminotransferase 23.2±9.6U/L; γ-glutamyl transferase 48.0±18.7U/L; total alkaline phosphatase 121.6±76.1U/L; glutamate dehydrogenase 6.4±3.7U/L; urea 7.32±2.22mmol/L; creatinine 123.0±54.1μmol/L; total calcium 2.53±0.25mmol/L; phosphorus 2.10±0.46mmol/L; magnesium 1.01±0.09mmol/L; sodium 152.04±3.65mmol/L; potassium 4.7±0.4mmol/L; ionized calcium 1.32±0.07mmol/L; total thyroxine 111.75±42.29nmol/L; total triiodothyronine 1.01±0.28nmol/L; free T4 11.93±1.78pmol/L; free T3 4.22±1.33pmol/L; and thyroid-stimulating hormone 0.18±0.19μIU/mL. Although differences among the Churra-da-Terra-Quente breed and other breeds may occur, the hematological and biochemical parameters, plasma electrolytes, and thyroid hormones, for this indigenous breed, were generally situated within the reference intervals previously reported for sheep
A Critical Analysis of the Proton Form Factor with Sudakov Suppression and Intrinsic Transverse Momentum
The behavior of the proton magnetic form factor is studied within the
modified hard scattering picture, which takes into account gluonic radiative
corrections in terms of transverse separations. We parallel the analysis given
previously by Li and make apparent a number of serious objections. The
appropriate cut-off needed to render the form-factor calculation finite is both
detailed and analyzed by considering different cut-off prescriptions. The use
of the maximum interquark separation as a common infrared cut-off in the
Sudakov suppression factor is proposed, since it avoids difficulties with the
-singularities and yields a proton form factor insensitive to the
inclusion of the soft region which therefore can be confidently attributed to
perturbative QCD. Results are presented for a variety of proton wave functions
including also their intrinsic transverse momentum. It turns out that the
perturbative contribution, although theoretically self-consistent for
larger than about ~GeV to ~GeV, is too small compared to
the data.Comment: 31 pages (RevTex) + 6 figures in PS-file; preprint BUGH Wuppertal
WU-B-94-06, preprint Ruhr-Universit\"at Bochum RUB-TPII-01/9
l=0 to l=1 Transition Form Factors
A method is proposed to extend the hard scattering picture of Brodsky and
Lepage to transitions between hadrons with orbital angular momentum l=0 and
l=1. The use of covariant spin wave functions turns out to be very helpful in
formulating that method. As a first application we construct a light-cone wave
function of the nucleon resonance in the quark-diquark picture.
Using this wave function and the extended hard scattering picture, the
-- transition form factors are calculated at large momentum transfer
and the results compared to experimental data. As a further application of our
method we briefly discuss the -- form factors in an appendix.Comment: 27 pages, 6 PS-figures in uuencoded compressed file, Latex, WU-B
93-29, MZ-TH/93-2
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