509 research outputs found
Contribution of spin 1/2 and 3/2 resonances to two-photon exchange effects in elastic electron-proton scattering
We calculate contributions of hadron resonances to two-photon exchange
effects in electron-proton scattering. In addition to the nucleon and P33
resonance, the following heavier resonances are included as intermediate states
in the two-photon exchange diagrams: D13, D33, P11, S11 and S31. We show that
the corrections due to the heavier resonances are smaller that the dominant
nucleon and P33 contributions. We also find that there is a partial
cancellation between the contributions from the spin 1/2 and spin 3/2
resonances, which results in a further suppression of their aggregate
two-photon exchange effect.Comment: 6 pages, 1 figure; additional comparison with data, results
unchanged; to be published in Phys. Rev.
resonance contribution to two-photon exchange in electron-proton scattering
We calculate the effects on the elastic electron-proton scattering cross
section of the two-photon exchange contribution with an intermediate
resonance. The two-photon exchange contribution is found to be smaller
in magnitude than the previously evaluated nucleon contribution, with an
opposite sign at backward scattering angles. The sum of the nucleon and
two-photon exchange corrections has an angular dependence compatible
with both the polarisation transfer and the Rosenbluth methods of measuring the
nucleon electromagnetic form factors.Comment: 9 pages, 3 figures, RevTeX4; more complete discussion of results,
conclusions unchanged; to be published in Physical Review Letter
Light Front Nuclear Physics: Toy Models, Static Sources and Tilted Light Front Coordinates
The principles behind the detailed results of a light-front mean field theory
of finite nuclei are elucidated by deriving the nucleon mode equation using a
simple general argument, based on the idea that a static source in equal time
coordinates corresponds to a moving source in light front coordinates. This
idea also allows us to solve several simple toy model examples: scalar field in
a box, 1+1 dimensional bag model, three-dimensional harmonic oscillator and the
Hulth\'en potential. The latter provide simplified versions of momentum
distributions and form factors of relevance to experiments. In particular, the
relativistic correction to the mean square radius of a nucleus is shown to be
very small. Solving these simple examples suggests another more general
approach-- the use of tilted light front coordinates. The simple examples are
made even simpler.Comment: 19 pages, references adde
The Rarita-Schwinger spin-3/2 equation in a nonuniform, central potential
The equations of motion for a massive spin-3/2 Rarita-Schwinger field in a
finite-range, central, Lorentz scalar potential are developed. It is shown that
the resulting density may not be everywhere positive definite.Comment: 9 pages, RevTe
Computational Model for Electron-Nucleon Scattering and Weak Charge of the Nucleon
We show how computational symbolic packages such as FeynArts and FormCalc can
be adopted for the evaluation of one-loop hadronic electroweak radiative
corrections for electron-nucleon scattering and applied to calculations of the
nucleon weak charge. Several numerical results are listed, and found to be in
good agreement with the current experimental data.Comment: 13 pages, 8 figures, results unchanged, minor corrections in the
appendi
Mean-field calculations of quasi-elastic responses in 4He
We present calculations of the quasi-elastic responses functions in 4He based
upon a mean-field model used to perform analogous calculations in heavier
nuclei. The meson exchange current contribution is small if compared with the
results of calculations where short-range correlations are explicitly
considered. It is argued that the presence of these correlations in the
description of the nuclear wave functions is crucial to make meson exchange
current effects appreciable.Comment: uuencoded file containing 7 LaTex peges plus 3 ps figures. To be
published in Physical Review
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
Light-Front Nuclear Physics: Mean Field Theory for Finite Nuclei
A light-front treatment for finite nuclei is developed from a relativistic
effective Lagrangian (QHD1) involving nucleons, scalar mesons and vector
mesons. We show that the necessary variational principle is a constrained one
which fixes the expectation value of the total momentum operator to be
the same as that for . This is the same as minimizing the sum of the total
momentum operators: . We obtain a new light-front version of the
equation that defines the single nucleon modes. The solutions of this equation
are approximately a non-trivial phase factor times certain solutions of the
usual equal-time Dirac equation. The ground state wave function is treated as a
meson-nucleon Fock state, and the meson fields are treated as expectation
values of field operators in that ground state. The resulting equations for
these expectation values are shown to be closely related to the usual meson
field equations. A new numerical technique to solve the self-consistent field
equations is introduced and applied to O and Ca. The computed
binding energies are essentially the same as for the usual equal-time theory.
The nucleon plus momentum distribution (probability for a nucleon to have a
given value of ) is obtained, and peaks for values of about seventy
percent of the nucleon mass. The mesonic component of the ground state wave
function is used to determine the scalar and vector meson momentum distribution
functions, with a result that the vector mesons carry about thirty percent of
the nuclear plus-momentum. The vector meson momentum distribution becomes more
concentrated at as increases.Comment: 36 pages, 2 figure
Connection between the high energy-scale evolution of the P- and T-odd coupling constant and the strong interaction
The large energy-scale behaviour of the parity and time-reversal violating
(PTV) pion-nucleon coupling constant is analyzed in a model combining
renormalization-group techniques and the dressing of the PTV vertex with a pion
loop. With the strong vertex as a mixture of the pseudovector and
pseudoscalar couplings, we show that depending on the admixture parameter, two
qualitatively distinct types of behaviour are obtained for the PTV coupling
constant at high energy scales: an asymptotic freedom or a fixed-point. We find
a critical value of the admixture parameter which delineates these two
scenarios. Several examples of the high-energy scale behaviour of the PTV constant are considered, corresponding to realistic hadronic models of the
strong pion-nucleon interaction.Comment: 5 pages, 1 figur
Effects of Short-Range Correlations in (e,e'p) reactions and nuclear overlap functions
A study of the effects of short-range correlations over the (e,e'p) reaction
for low missing energy in closed shell nuclei is presented. We use correlated,
quasi-hole overlap functions extracted from the asymptotic behavior of the
one-body density matrix, containing central correlations of Jastrow type, up to
first-order in a cluster expansion, and computed in the very high asymptotic
region, up to 100 fm. The method to extract the overlap functions is checked in
a simple shell model, where the exact results are known. We find that the
single-particle wave functions of the valence shells are shifted to the right
due to the short-range repulsion by the nuclear core. The corresponding
spectroscopic factors are reduced only a few percent with respect to the shell
model. However, the (e,e'p) response functions and cross sections are enhanced
in the region of the maximum of the missing momentum distribution due to
short-range correlations.Comment: 45 pages, 15 figure
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