46 research outputs found
The role of interaction vertices in bound state calculations
In recent studies of the one and two-body Greens' function for scalar
interactions it was shown that crossed ladder and ``crossed rainbow'' (for the
one-body case) exchanges play a crucial role in nonperturbative dynamics. In
this letter we use exact analytical and numerical results to show that the
contribution of vertex dressings to the two-body bound state mass for scalar
QED are cancelled by the self-energy and wavefunction normalization. This
proves, for the first time, that the mass of a two-body bound state given by
the full theory can in a very good approximation be obtained by summing only
ladder and crossed ladder diagrams using a bare vertex and a constant dressed
mass. We also discuss the implications of the remarkable cancellation between
rainbow and crossed rainbow diagrams that is a feature of one-body
calculations.Comment: 9 pages, 5 figure
Pion photoproduction on nucleons in a covariant hadron-exchange model
We present a relativistic dynamical model of pion photoproduction on the
nucleon in the resonance region. It offers several advances over the existing
approaches. The model is obtained by extending our -scattering
description to the electromagnetic channels. The resulting photopion amplitude
is thus unitary in the , \ga N channel space, Watson's theorem is
exactly satisfied. At this stage we have included the pion, nucleon,
\De(1232)-resonance degrees of freedom. The and meson
exchanges are also included, but play a minor role in the considered energy
domain (up to GeV). In this energy range the model provides a
good description of all the important multipoles. We have allowed for only two
free parameters -- the photocouplings of the -resonance. These
couplings are adjusted to reproduce the strength of corresponding
resonant-multipoles and at the resonance position.Comment: 17 pages, 4 figs, version to appear in Phys. Rev. C 70 (2004
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
Confinement and the analytic structure of the one body propagator in Scalar QED
We investigate the behavior of the one body propagator in SQED. The self
energy is calculated using three different methods: i) the simple bubble
summation, ii) the Dyson-Schwinger equation, and iii) the Feynman-Schwinger
represantation. The Feynman-Schwinger representation allows an {\em exact}
analytical result. It is shown that, while the exact result produces a real
mass pole for all couplings, the bubble sum and the Dyson-Schwinger approach in
rainbow approximation leads to complex mass poles beyond a certain critical
coupling. The model exhibits confinement, yet the exact solution still has one
body propagators with {\it real} mass poles.Comment: 5 pages 2 figures, accepted for publication in Phys. Rev.
Scaling of Dirac Fermions and the WKB approximation
We discuss a new method for obtaining the WKB approximation to the Dirac
equation with a scalar potential and a time-like vector potential. We use the
WKB solutions to investigate the scaling behavior of a confining model for
quark-hadron duality. In this model, a light quark is bound to a heavy di-quark
by a linear scalar potential. Absorption of virtual photons promotes the quark
to bound states. The analog of the parton model for this case is for a virtual
photon to eject the bound, ground-state quark directly into free continuum
states. We compare the scaling limits of the response functions for these two
transitions
Detection and localization of early- and late-stage cancers using platelet RNA
Cancer patients benefit from early tumor detection since treatment outcomes are more favorable for less advanced cancers. Platelets are involved in cancer progression and are considered a promising biosource for cancer detection, as they alter their RNA content upon local and systemic cues. We show that tumor-educated platelet (TEP) RNA-based blood tests enable the detection of 18 cancer types. With 99% specificity in asymptomatic controls, thromboSeq correctly detected the presence of cancer in two-thirds of 1,096 blood samples from stage I–IV cancer patients and in half of 352 stage I–III tumors. Symptomatic controls, including inflammatory and cardiovascular diseases, and benign tumors had increased false-positive test results with an average specificity of 78%. Moreover, thromboSeq determined the tumor site of origin in five different tumor types correctly in over 80% of the cancer patients. These results highlight the potential properties of TEP-derived RNA panels to supplement current approaches for blood-based cancer screening