4,171 research outputs found
Compaction dynamics in ductile granular media
Ductile compaction is common in many natural systems, but the temporal
evolution of such systems is rarely studied. We observe surprising oscillations
in the weight measured at the bottom of a self-compacting ensemble of ductile
grains. The oscillations develop during the first ten hours of the experiment,
and usually persist through the length of an experiment (one week). The weight
oscillations are connected to the grain--wall contacts, and are directly
correlated with the observed strain evolution and the dynamics of grain--wall
contacts during the compaction. Here, we present the experimental results and
characteristic time constants of the system, and discuss possible reasons for
the measured weight oscillations.Comment: 11 pages, 14 figure
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
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
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
Study of the semileptonic decay \Lambda_b^0 \to \Lambda_c^+ l^- \bar{\nu}_l
Within the framework of a nonrelativistic quark model we evaluate the six
form factors associated to the \Lambda_b^0 \to \Lambda_c^+ l^- \bar{\nu}_l
semileptonic decay. The baryon wave functions were evaluated using a
variational approach applied to a family of trial functions constrained by
Heavy Quark Symmetry (HQS). We use a spectator model with only one-body current
operators. For these operators we keep up to first order terms on the internal
(small) heavy quark momentum, but all orders on the transferred (large)
momentum. Our result for the partially integrated decay width is in good
agreement with lattice calculations. Comparison of our total decay width to
experiment allows us to extract the V_{cb} Cabbibo-Kobayashi-Maskawa matrix
element for which we obtain a value of |V_{cb}|=0.047\pm 0.005 in agreement
with a recent determination by the DELPHI Collaboration. Furthermore, we obtain
the universal Isgur-Wise function with a slope parameter \rho^2=0.98 in
agreement with lattice results.Comment: 4 pages, 3 figures. Presented at 6th International Conference on
Hyperons, Charm and Beauty Hadrons (BEACH 2004), Chicago, Illinois, 27 Jun -
3 Jul 200
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
Explicitly Covariant Light-Front Dynamics and Relativistic Few-Body Systems
The wave function of a composite system is defined in relativity on a
space-time surface. In the explicitly covariant light-front dynamics, reviewed
in the present article, the wave functions are defined on the plane \omega \cd
x=0, where is an arbitrary four-vector with . The
standard non-covariant approach is recovered as a particular case for . Using the light-front plane is of crucial importance, while the
explicit covariance gives strong advantages emphasized through all the review.
The properties of the relativistic few-body wave functions are discussed in
detail and are illustrated by examples in a solvable model. The
three-dimensional graph technique for the calculation of amplitudes in the
covariant light-front perturbation theory is presented.
The structure of the electromagnetic amplitudes is studied. We investigate
the ambiguities which arise in any approximate light-front calculations, and
which lead to a non-physical dependence of the electromagnetic amplitude on the
orientation of the light-front plane. The elastic and transition form factors
free from these ambiguities are found for spin 0, 1/2 and 1 systems.
The formalism is applied to the calculation of the relativistic wave
functions of two-nucleon systems (deuteron, scattering state), with particular
attention to the role of their new components in the deuteron elastic and
electrodisintegration form factors and to their connection with meson exchange
currents. Straigthforward applications to the pion and nucleon form factors and
the transition are also made.Comment: latex.tar.gz file, 162 pages, 42 figures, to be published in Physics
Reports (next issues
Rare exclusive semileptonic b -> s transitions in the Standard Model
We study long-distance effects in rare exclusive semileptonic decays B -> (K,
K*) (l+ l-, nu bar{nu}) and analyze dilepton spectra and asymmetries within the
framework of the Standard Model. The form factors, describing the meson
transition amplitudes of the effective Hamiltonian are calculated within the
lattice-constrained dispersion quark model: the form factors are given by
dispersion representations through the wave functions of the initial and final
mesons, and these wave functions are chosen such that the B -> K* transition
form factors agree with the lattice results at large q**2. We calculate
branching ratios of semileptonic B -> K, K* transition modes and study the
sensitivity of observables to the long-distance contributions. The shape of the
forward-backward asymmetry and the longitudinal lepton polarization asymmetry
are found to be independent of the long-distance effects and mainly determined
by the values of the Wilson coefficients in the Standard Model.Comment: revtex, 17 pp., 5 figures with epsfig.st
Nucleon electroweak form factors in a meson-cloud model
The meson-cloud model of the nucleon consisting of a system of three valence
quarks surrounded by a meson cloud is applied to study the electroweak
structure of the proton and neutron. The electroweak nucleon form factors are
calculated within a light-front approach, by obtaining an overall good
description of the experimental data. Charge densities as a function of the
transverse distance with respect to the direction of the three-momentum
transfer are also discussed.Comment: Prepared for Proceedings of NSTAR2007, Workshop on the physics of
excited nucleons, Bonn (Germany), 5-8 September 200
Terminate Lung Cancer (TLC) Study—A Mixed-Methods Population Approach to Increase Lung Cancer Screening Awareness and Low-Dose Computed Tomography in Eastern Kentucky
For low dose CT lung cancer screening to be effective in curbing disease mortality, efforts are needed to overcome barriers to awareness and facilitate uptake of the current evidence-based screening guidelines. A sequential mixed-methods approach was employed to design a screening campaign utilizing messages developed from community focus groups, followed by implementation of the outreach campaign intervention in two high-risk Kentucky regions. This study reports on rates of awareness and screening in intervention regions, as compared to a control region
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