916 research outputs found
Nonperturbative dynamics of scalar field theories through the Feynman-Schwinger representation
In this paper we present a summary of results obtained for scalar field
theories using the Feynman-Schwinger (FSR) approach. Specifically, scalar QED
and chi^2phi theories are considered. The motivation behind the applications
discussed in this paper is to use the FSR method as a rigorous tool for testing
the quality of commonly used approximations in field theory. Exact calculations
in a quenched theory are presented for one-, two-, and three-body bound states.
Results obtained indicate that some of the commonly used approximations, such
as Bethe-Salpeter ladder summation for bound states and the rainbow summation
for one body problems, produce significantly different results from those
obtained from the FSR approach. We find that more accurate results can be
obtained using other, simpler, approximation schemes.Comment: 25 pags, 19 figures, prepared for the volume celebrating the 70th
birthday of Yuri Simono
Solving the Bethe-Salpeter equation for bound states of scalar theories in Minkowski space
We apply the perturbation theory integral representation (PTIR) to solve for
the bound state Bethe-Salpeter (BS) vertex for an arbitrary scattering kernel,
without the need for any Wick rotation. The results derived are applicable to
any scalar field theory (without derivative coupling). It is shown that solving
directly for the BS vertex, rather than the BS amplitude, has several major
advantages, notably its relative simplicity and superior numerical accuracy. In
order to illustrate the generality of the approach we obtain numerical
solutions using this formalism for a number of scattering kernels, including
cases where the Wick rotation is not possible.Comment: 28 pages of LaTeX, uses psfig.sty with 5 figures. Also available via
WWW at
http://www.physics.adelaide.edu.au/theory/papers/ADP-97-10.T248-abs.html or
via anonymous ftp at
ftp://bragg.physics.adelaide.edu.au/pub/theory/ADP-97-10.T248.ps A number of
(crucial) typographical errors in Appendix C corrected. To be published in
Phys. Rev. D, October 199
Nonperturbative study of generalized ladder graphs in a \phi^2\chi theory
The Feynman-Schwinger representation is used to construct scalar-scalar bound
states for the set of all ladder and crossed-ladder graphs in a \phi^2\chi
theory in (3+1) dimensions. The results are compared to those of the usual
Bethe-Salpeter equation in the ladder approximation and of several
quasi-potential equations. Particularly for large couplings, the ladder
predictions are seen to underestimate the binding energy significantly as
compared to the generalized ladder case, whereas the solutions of the
quasi-potential equations provide a better correspondence. Results for the
calculated bound state wave functions are also presented.Comment: 5 pages revtex, 3 Postscripts figures, uses epsf.sty, accepted for
publication in Physical Review Letter
Feynman-Schwinger representation approach to nonperturbative physics
The Feynman-Schwinger representation provides a convenient framework for the
cal culation of nonperturbative propagators. In this paper we first investigate
an analytically solvable case, namely the scalar QED in 0+1 dimension. With
this toy model we illustrate how the formalism works. The analytic result for
the self energy is compared with the perturbative result. Next, using a
interaction, we discuss the regularization of various divergences
encountered in this formalism. The ultraviolet divergence, which is common in
standard perturbative field theory applications, is removed by using a
Pauli-Villars regularization. We show that the divergence associated with large
values of Feynman-Schwinger parameter is spurious and it can be avoided by
using an imaginary Feynman parameter .Comment: 26 pages, 9 figures, minor correctio
Light-Front Bethe-Salpeter Equation
A three-dimensional reduction of the two-particle Bethe-Salpeter equation is
proposed. The proposed reduction is in the framework of light-front dynamics.
It yields auxiliary quantities for the transition matrix and the bound state.
The arising effective interaction can be perturbatively expanded according to
the number of particles exchanged at a given light-front time. An example
suggests that the convergence of the expansion is rapid. This result is
particular for light-front dynamics. The covariant results of the
Bethe-Salpeter equation can be recovered from the corresponding auxiliary
three-dimensional ones. The technical procedure is developed for a two-boson
case; the idea for an extension to fermions is given. The technical procedure
appears quite practicable, possibly allowing one to go beyond the ladder
approximation for the solution of the Bethe-Salpeter equation. The relation
between the three-dimensional light-front reduction of the field-theoretic
Bethe-Salpeter equation and a corresponding quantum-mechanical description is
discussed.Comment: 42 pages, 5 figure
Implementation and evaluation of a fall risk screening strategy among frail older adults for the primary care setting:A study protocol
Background: Falls are an increasing problem among older people. There are several evidence-based interventions available to prevent falls. However, these are not always well implemented in the primary care setting. General practitioners (GPs) are often the first point of contact for health issues, making them the designated professionals for providing falls prevention. Because GPs are often unaware which patients have a high fall risk and patients themselves do not always know they have a high fall risk, this study aims to evaluate the implementation of a targeted fall risk screening strategy among independently living, frail older people in the primary care setting. Materials and methods: The targeted fall risk screening strategy used in this study consists of tools for screening high fall risk and for identifying the underlying cause(s) of the high fall risk, an accredited training course in falls prevention for professionals, and service provision by certified physio- and exercise therapists who are able to offer evidence-based falls prevention interventions. This targeted fall risk screening strategy will be implemented in the primary care setting and evaluated at the level of the GP practice and at the level of the patient by using the RE-AIM model of Glasgow et al. In a pre-posttest design, data will be collected of the total number of frail older people who are screened, referred and enrolled for fall-preventive care. Furthermore, barriers and facilitators of the implementation of the fall risk screening strategy will be identified by conducting focus groups and interviews with the care providers and frail older patients. Additionally, the influence of the falls prevention interventions on frail older patients will be evaluated by using a pre-posttest design with a 12-month follow-up period during which data are collected regarding patients' stability, mobility, strength, balance, self-efficacy, health status, and daily activities. Study Registration: This study is approved by the Medical Ethics Committee Brabant, the Netherlands (NL61582.028.17/ P1732) and registered at the Netherlands Trial Register, NL7917
Restoration of rotational invariance of bound states on the light front
We study bound states in a model with scalar nucleons interacting via an
exchanged scalar meson using the Hamiltonian formalism on the light front. In
this approach manifest rotational invariance is broken when the Fock space is
truncated. By considering an effective Hamiltonian that takes into account two
meson exchanges, we find that this breaking of rotational invariance is
decreased from that which occurs when only one meson exchange is included. The
best improvement occurs when the states are weakly bound.Comment: 20 pages, 6 figures, uses feynMF; changed typos, clarified use of
angular momentu
The Generalized Gell-Mann--Low Theorem for Relativistic Bound States
The recently established generalized Gell-Mann--Low theorem is applied in
lowest perturbative order to bound-state calculations in a simple scalar field
theory with cubic couplings. The approach via the generalized Gell-Mann--Low
Theorem retains, while being fully relativistic, many of the desirable features
of the quantum mechanical approaches to bound states. In particular, no
abnormal or unphysical solutions are found in the model under consideration.
Both the non-relativistic and one-body limits are straightforward and
consistent. The results for the spectrum are compared to those of the
Bethe-Salpeter equation (in the ladder approximation) and related equations.Comment: 24 pages, 6 pspicture diagrams, 4 postscript figure
Variational Worldline Approximation for the Relativistic Two-Body Bound State in a Scalar Model
We use the worldline representation of field theory together with a
variational approximation to determine the lowest bound state in the scalar
Wick-Cutkosky model where two equal-mass constituents interact via the exchange
of mesons. Self-energy and vertex corrections are included approximately in a
consistent way as well as crossed diagrams. Only vacuum-polarization effects of
the heavy particles are neglected. In a path integral description of an
appropriate current-current correlator an effective, retarded action is
obtained by integrating out the meson field. As in the polaron problem we
employ a quadratic trial action with variational functions to describe
retardation and binding effects through multiple meson exchange.The variational
equations for these functions are derived, discussed qualitatively and solved
numerically. We compare our results with the ones from traditional approaches
based on the Bethe-Salpeter equation and find an enhanced binding contrary to
some claims in the literature. For weak coupling this is worked out
analytically and compared with results from effective field theories. However,
the well-known instability of the model, which usually is ignored, now appears
at smaller coupling constants than in the one-body case and even when
self-energy and vertex corrections are turned off. This induced instability is
investigated analytically and the width of the bound state above the critical
coupling is estimated.Comment: 62 pages, 7 figures, FBS style, published versio
Mutation-specific reporter for optimization and enrichment of prime editing
Prime editing is a versatile genome-editing technique that shows great promise for the generation and repair of patient mutations. However, some genomic sites are difficult to edit and optimal design of prime-editing tools remains elusive. Here we present a fluorescent prime editing and enrichment reporter (fluoPEER), which can be tailored to any genomic target site. This system rapidly and faithfully ranks the efficiency of prime edit guide RNAs (pegRNAs) combined with any prime editor variant. We apply fluoPEER to instruct correction of pathogenic variants in patient cells and find that plasmid editing enriches for genomic editing up to 3-fold compared to conventional enrichment strategies. DNA repair and cell cycle-related genes are enriched in the transcriptome of edited cells. Stalling cells in the G1/S boundary increases prime editing efficiency up to 30%. Together, our results show that fluoPEER can be employed for rapid and efficient correction of patient cells, selection of gene-edited cells, and elucidation of cellular mechanisms needed for successful prime editing
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