1,348 research outputs found
Crossed-boson exchange contribution and Bethe-Salpeter equation
The contribution to the binding energy of a two-body system due to the
crossed two-boson exchange contribution is calculated, using the Bethe-Salpeter
equation. This is done for distinguishable, scalar particles interacting via
the exchange of scalar massive bosons. The sensitivity of the results to the
off-shell behavior of the operator accounting for this contribution is
discussed. Large corrections to the Bethe-Salpeter results in the ladder
approximation are found. For neutral scalar bosons, the mass obtained for the
two-body system is close to what has been calculated with various forms of the
instantaneous approximation, including the standard non-relativistic approach.
The specific character of this result is demonstrated by a calculation
involving charged bosons, which evidences a quite different pattern. Our
results explain for some part those obtained by Nieuwenhuis and Tjon on a
different basis. Some discrepancy appears with increasing coupling constants,
suggesting the existence of sizeable contributions involving more than
two-boson exchanges.Comment: 13 pages, 5 .eps figures, submitted to 'Few Body Systems
influence.ME: tools for detecting influential data in mixed effects models
influence.ME provides tools for detecting influential data in mixed effects models. The application of these models has become common practice, but the development of diagnostic tools has lagged behind. influence.ME calculates standardized measures of influential data for the point estimates of generalized mixed effects models, such as DFBETAS, Cookâs distance, as well as percentile change and a test for changing levels of significance. influence.ME calculates these measures of influence while accounting for the nesting structure of the data. The package and measures of influential data\ud
are introduced, a practical example is given, and strategies for dealing with influential data are suggested
Adolescents' future in the balance of family, school, and the neighborhood:A multidimensional application of two theoretical perspectives
OBJECTIVE: Family, school, and neighborhood contexts provide cultural resources that may foster children's ambitions and bolster their academic performance. Reference group theory instead highlights how seemingly positive settings can depress educational aspirations, expectations, and performance. We test these competing claims.
METHODS: We test these claims using the British Avon Longitudinal Study of Parents and Children (N = 4968).
RESULTS: Results are broadly in line with the cultural resource perspective. However, important exceptions to this pattern point to reference group processes for children from low-educated parents, whose academic aspirations are especially low when they either attended an affluent school or lived in an affluent neighborhoodâbut not both, and for children from highly educated parents attending poor schools, whose realistic expectations of the future are higher than their peers in affluent schools.
CONCLUSION: The resource perspective strongly predicts adolescentsâ (ideas about) education, but reference group processes also play an important role in neighborhoods and schools.https://onlinelibrary.wiley.com/doi/10.1111/ssqu.13137Published versio
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
Study of relativistic bound states for scalar theories in Bethe-Salpeter and Dyson-Schwinger formalism
The Bethe-Salpeter equation for Wick-Cutkosky like models is solved in
dressed ladder approximation. The bare vertex truncation of the Dyson-Schwinger
equations for propagators is combined with the dressed ladder Bethe-Salpeter
equation for the scalar S-wave bound state amplitudes. With the help of
spectral representation the results are obtained directly in Minkowski space.
We give a new analytic formula for the resulting equation simplifying the
numerical treatment. The bare ladder approximation of Bethe-Salpeter equation
is compared with the one with dressed ladder. The elastic electromagnetic form
factors is calculated within the relativistic impulse approximation.Comment: 30 pages, 10 figures, accepted for publication in Phys. Rev.
Exact spinor-scalar bound states in a QFT with scalar interactions
We study two-particle systems in a model quantum field theory, in which
scalar particles and spinor particles interact via a mediating scalar field.
The Lagrangian of the model is reformulated by using covariant Green's
functions to solve for the mediating field in terms of the particle fields.
This results in a Hamiltonian in which the mediating-field propagator appears
directly in the interaction term. It is shown that exact two-particle
eigenstates of the Hamiltonian can be determined. The resulting relativistic
fermion-boson equation is shown to have Dirac and Klein-Gordon one-particle
limits. Analytic solutions for the bound state energy spectrum are obtained for
the case of massless mediating fields.Comment: 12 pages, RevTeX, 1 figur
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