454 research outputs found
Does adjunct beta-blockade enhance the efficacy of membrane in patients with inducible ventricular tachycardia?
Electromagnetic form factors of pion and rho in the three forms of relativistic kinematics
The electromagnetic form factors of the and the are obtained
using the three forms of relativistic kinematics, instant form, point form and
(light) front form. Simple representations of the mass operator together with
single quark currents are employed with all the forms. The Poincar\'e covariant
current operators are generated by the dynamics from single-quark currents that
are covariant under the kinematic subgroup. Front and instant forms allow to
reproduce the available data for the pion form factor. On the other hand point
form is not able to reproduce qualitatively the experimental data with
reasonable values for the wave function parameters. For the
electromagnetic form factors, instant and front forms provide a consistent
picture. The obtained results do not depend appreciably on the wave function
used.Comment: 10 pages, color figures. Revised references and discussion. Accepted
in Phys. Letts.
Vector Meson Production in the Golec-Biernat Wuesthoff Model
We apply the Golec-Biernat Wuesthoff model in the calculation of vector meson
photo- and electroproduction. Starting from very simple non-relativistic wave
functions we show that the model provides a good description of cross
sections in wide and ranges. For the light mesons one obtains the
approximately correct dependence and ratio of longitudinal to transverse
cross sections, although in this case the normalization, affected mainly by the
wave function employed, is not in good agreement with data
Are megaquakes clustered?
We study statistical properties of the number of large earthquakes over the
past century. We analyze the cumulative distribution of the number of
earthquakes with magnitude larger than threshold M in time interval T, and
quantify the statistical significance of these results by simulating a large
number of synthetic random catalogs. We find that in general, the earthquake
record cannot be distinguished from a process that is random in time. This
conclusion holds whether aftershocks are removed or not, except at magnitudes
below M = 7.3. At long time intervals (T = 2-5 years), we find that
statistically significant clustering is present in the catalog for lower
magnitude thresholds (M = 7-7.2). However, this clustering is due to a large
number of earthquakes on record in the early part of the 20th century, when
magnitudes are less certain.Comment: 5 pages, 5 figure
Boost-Invariant Running Couplings in Effective Hamiltonians
We apply a boost-invariant similarity renormalization group procedure to a
light-front Hamiltonian of a scalar field phi of bare mass mu and interaction
term g phi^3 in 6 dimensions using 3rd order perturbative expansion in powers
of the coupling constant g. The initial Hamiltonian is regulated using momentum
dependent factors that approach 1 when a cutoff parameter Delta tends to
infinity. The similarity flow of corresponding effective Hamiltonians is
integrated analytically and two counterterms depending on Delta are obtained in
the initial Hamiltonian: a change in mu and a change of g. In addition, the
interaction vertex requires a Delta-independent counterterm that contains a
boost invariant function of momenta of particles participating in the
interaction. The resulting effective Hamiltonians contain a running coupling
constant that exhibits asymptotic freedom. The evolution of the coupling with
changing width of effective Hamiltonians agrees with results obtained using
Feynman diagrams and dimensional regularization when one identifies the
renormalization scale with the width. The effective light-front Schroedinger
equation is equally valid in a whole class of moving frames of reference
including the infinite momentum frame. Therefore, the calculation described
here provides an interesting pattern one can attempt to follow in the case of
Hamiltonians applicable in particle physics.Comment: 24 pages, LaTeX, included discussion of finite x-dependent
counterterm
The Relativistic Bound State Problem in QCD: Transverse Lattice Methods
The formalism for describing hadrons using a light-cone Hamiltonian of SU(N)
gauge theory on a coarse transverse lattice is reviewed. Physical gauge degrees
of freedom are represented by disordered flux fields on the links of the
lattice. A renormalised light-cone Hamiltonian is obtained by making a
colour-dielectric expansion for the link-field interactions. Parameters in the
Hamiltonian are renormalised non-perturbatively by seeking regions in parameter
space with enhanced Lorentz symmetry. In the case of pure gauge theories to
lowest non-trivial order of the colour-dielectric expansion, this is sufficient
to determine accurately all parameters in the large-N limit. We summarize
results from applications to glueballs. After quarks are added, the Hamiltonian
and Hilbert space are expanded in both dynamical fermion and link fields.
Lorentz and chiral symmetry are not sufficient to accurately determine all
parameters to lowest non-trivial order of these expansions. However, Lorentz
symmetry and one phenomenological input, a chiral symmetry breaking scale, are
enough to fix all parameters unambiguously. Applications to light-light and
heavy-light mesons are described.Comment: 55 pp, revised version, to appear in `Progress in Particle and
Nuclear Physics
Mesons in (2+1) Dimensional Light Front QCD. II. Similarity Renormalization Approach
Recently we have studied the Bloch effective Hamiltonian approach to bound
states in 2+1 dimensional gauge theories. Numerical calculations were carried
out to investigate the vanishing energy denominator problem. In this work we
study similarity renormalization approach to the same problem. By performing
analytical calculations with a step function form for the similarity factor, we
show that in addition to curing the vanishing energy denominator problem,
similarity approach generates linear confining interaction for large transverse
separations. However, for large longitudinal separations, the generated
interaction grows only as the square root of the longitudinal separation and
hence produces violations of rotational symmetry in the spectrum. We carry out
numerical studies in the G{\l}azek-Wilson and Wegner formalisms and present low
lying eigenvalues and wavefunctions. We investigate the sensitivity of the
spectra to various parameterizations of the similarity factor and other
parameters of the effective Hamiltonian, especially the scale . Our
results illustrate the need for higher order calculations of the effective
Hamiltonian in the similarity renormalization scheme.Comment: 31 pages, 4 figures, to be published in Physical Review
Vector mesons in a relativistic point-form approach
We apply the point form of relativistic quantum mechanics to develop a
Poincare invariant coupled-channel formalism for two-particle systems
interacting via one-particle exchange. This approach takes the exchange
particle explicitly into account and leads to a generalized eigenvalue equation
for the Bakamjian-Thomas type mass operator of the system. The coupling of the
exchange particle is derived from quantum field theory. As an illustrative
example we consider vector mesons within the chiral constituent quark model in
which the hyperfine interaction between the confined quark-antiquark pair is
generated by Goldstone-boson exchange. We study the effect of retardation in
the Goldstone-boson exchange by comparing with the commonly used instantaneous
approximation. As a nice physical feature we find that the problem of a too
large - splitting can nearly be avoided by taking the dynamics of
the exchange meson explicitly into account.Comment: 14 pages, 1 figur
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Glueballs in a Hamiltonian Light-Front Approach to Pure-Glue QCD
We calculate a renormalized Hamiltonian for pure-glue QCD and diagonalize it.
The renormalization procedure is designed to produce a Hamiltonian that will
yield physical states that rapidly converge in an expansion in free-particle
Fock-space sectors. To make this possible, we use light-front field theory to
isolate vacuum effects, and we place a smooth cutoff on the Hamiltonian to
force its free-state matrix elements to quickly decrease as the difference of
the free masses of the states increases. The cutoff violates a number of
physical principles of light-front pure-glue QCD, including Lorentz covariance
and gauge covariance. This means that the operators in the Hamiltonian are not
required to respect these physical principles. However, by requiring the
Hamiltonian to produce cutoff-independent physical quantities and by requiring
it to respect the unviolated physical principles of pure-glue QCD, we are able
to derive recursion relations that define the Hamiltonian to all orders in
perturbation theory in terms of the running coupling. We approximate all
physical states as two-gluon states, and use our recursion relations to
calculate to second order the part of the Hamiltonian that is required to
compute the spectrum. We diagonalize the Hamiltonian using basis-function
expansions for the gluons' color, spin, and momentum degrees of freedom. We
examine the sensitivity of our results to the cutoff and use them to analyze
the nonperturbative scale dependence of the coupling. We investigate the effect
of the dynamical rotational symmetry of light-front field theory on the
rotational degeneracies of the spectrum and compare the spectrum to recent
lattice results. Finally, we examine our wave functions and analyze the various
sources of error in our calculation.Comment: 75 pages, 17 figures, 1 tabl
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