Journal of Physics G: Nuclear and Particle Physics, 2000
The violation of the scaling law for the electric and magnetic form factors of the proton is exam... more The violation of the scaling law for the electric and magnetic form factors of the proton is examined within the cloudy bag model. We find that the suppression of the ratio of the electric and magnetic form factors is natural in the bag model. The pion cloud plays a moderate role in understanding the recent data from TJNAF.
A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the... more A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N , ∆, ρ, and σ fields, is extended to energies up to 2 GeV by including the ηN channel and all the four stars πN resonances up to the F−waves. The effects of other 2π channels are taken into account phenomenologically. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F−waves. However, a few of the extracted resonance parameters differ considerably from the PDG values.
We show that recent experiment data for the ratios E 1+/M 1+ and S 1+/M 1+ can be explained in a ... more We show that recent experiment data for the ratios E 1+/M 1+ and S 1+/M 1+ can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare γ*NΔ form factors. Within our model we find that the bare Δ is almost spherical and the electric E2 and Coulomb C2 quardrupole excitations of the physical Δ are nearly saturated by pion cloud contribution in Q2 ≤ 4.0 GeV 2.
A meson-exchange model for pion-nucleon scattering was previously constructed using a three-dimen... more A meson-exchange model for pion-nucleon scattering was previously constructed using a three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N , ∆, ρ, and σ fields. We thereby extend our previous work by including the ηN channel and all the πN resonances with masses ∼ 2 GeV, up to the F waves. The effects of the ππN channels are taken into account by introducing an effective width in the resonance propagators. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F waves and for energies below 2 GeV. We present a new scheme to extract the properties of overlapping resonances. The predicted values for the resonance masses and widths as well as resonance pole positions and residues are compared to the listing of the Particle Data Group.
A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the... more A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N, ∆, ρ, and σ fields, is extended to energies up to 2 GeV by including the ηN channel and all the four stars πN resonances up to the F −waves. The effects of other 2π channels are taken into account phenomenologically. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F −waves. However, a few of the extracted resonance parameters differ considerably from the PDG values.
Journal of Physics G: Nuclear and Particle Physics, 1998
... Mark I Gorenstein§, Walter Greiner and Shin Nan Yang Institute for Theoretical Physics... more ... Mark I Gorenstein§, Walter Greiner and Shin Nan Yang Institute for Theoretical Physics, Goethe University, Frankfurt, Germany Department of Physics, National Taiwan University, Taipei, Taiwan 10617, Republic of China § School of Physics and ... m2T 2π2 K2 (m T ...
Pion photo- and electroproduction has been studied at threshold and in the resonance region below... more Pion photo- and electroproduction has been studied at threshold and in the resonance region below W=2GeV. At threshold pi^0 production can be very well explained within a dynamical model derived from an effective chiral Lagrangian. The final state interaction is nearly saturated by single charge exchange rescattering. In the resonance region new electroproduction data at Q^2=1GeV^2 has been analyzed with MAID and longitudinal and transverse photon helicity amplitudes have been determined for different resonances. A detailed study of the E/M and S/M ratios of the N->Delta transition shows a zero crossing of REM near Q^2=4GeV^2, whereas the RSM becomes increasingly negative at large Q^2.
The Dubna-Mainz-Taipei dynamical model for pion electromagnetic production, which can describe we... more The Dubna-Mainz-Taipei dynamical model for pion electromagnetic production, which can describe well the existing data from threshold up to 1 GeV photon lab energy, is presented and used to analyze the recent precision data in the ∆ region. We find that, within our model, the bare ∆ is almost spherical while the physical ∆ is oblate. The deformation is almost saturated by the pion cloud effects. We further find that up to Q 2 = 4.0(GeV/c) 2, the extracted helicity amplitude A 3/2 and A 1/2 remain comparable with each other, implying that hadronic helicity is not conserved at this range of Q 2. The ratio E1+/M 1 + obtained show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and to become positive with increasing Q 2. This is a possible indication of a very slow approach toward the pQCD region. Finally, we find that the bare helicity amplitude A 1/2 and S 1/2, but not A 3/2, starts exhibiting the scaling behavior at about Q 2 ≥ 2.5(G...
The $S$- and $D$-state components of the deuteron-triton overlap integral and of the form factor ... more The $S$- and $D$-state components of the deuteron-triton overlap integral and of the form factor required for a distorted-wave Born-approximation analysis of ($d$,$t$) reactions have been calculated for a number of variational triton wave functions corresponding to realistic nucleon-nucleon potentials. It is found that the calculated overlap functions decay too rapidly for large $n\ensuremath{-}d$ separations, and this has important consequences for the corresponding distorted-wave Born-approximation form factors. In particular, the parameter ${D}_{2}$, which measures the relative importance of the $D$-state form factor in the low-momentum limit, is found to be more than a factor of 2 smaller than the best experimental value. A more direct calculation of the form factors leads to a much larger value of ${D}_{2}$.
Current status of the two-boson exchange contributions to elastic electron-proton scattering, bot... more Current status of the two-boson exchange contributions to elastic electron-proton scattering, both for parity conserving and parity-violating, is briefly reviewed. How the discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments can be understood, in large part, by the two-photon exchange corrections is discussed. We also illustrate how the measurement of the ratio between positron-proton and electron-proton scattering can be used to differentiate different models of two-photon exchange. For the parity-violating electron-proton scattering, the interest is on how the two-boson exchange (TBE), γZ-exchange in particular, could affect the extraction of the long-sought strangeness form factors. Various calculations all indicate that the magnitudes of effect of TBE on the extraction of strangeness form factors is small, though can be large percentage-wise in certain kinematics.
We review the description of the lowest-energy nucleon excitation-the ∆(1232)-resonance. Much of ... more We review the description of the lowest-energy nucleon excitation-the ∆(1232)-resonance. Much of the recent experimental effort has been focused on the precision measurements of the nucleon-to-∆ transition by means of electromagnetic probes. We confront the results of these measurements with the state-of-the-art calculations based on chiral effective-field theories (EFT), lattice QCD, large-N c relations, perturbative QCD, and QCD-inspired models. We also discuss the link of the nucleon-to-∆ form factors to generalized parton distributions (GPDs). Some of the theoretical approaches are reviewed in detail, in particular, recent dynamical and unitary-isobar models of pion electroproduction, which are extensively used in the interpretation of experiments. Also, the novel extension of chiral EFTs to the energy domain of the ∆-resonance is described in detail. The two-photon exchange effects in the electroexcitation of the ∆-resonance are addressed here as well.
We evaluate the corrections of the two-photon exchange (TPE) process on the γ * N ∆ transition fo... more We evaluate the corrections of the two-photon exchange (TPE) process on the γ * N ∆ transition form factors. The contributions of the TPE process to the eN → e∆(1232) → eN π are calculated in a hadronic model with the inclusion of only the elastic nucleon intermediate states, to estimate its effects on the multipoles M (3/2) 1 + , E (3/2) 1 + , S (3/2) 1 + at the ∆ peak. We find that TPE effects on G * M is very small. G * E , and G * C are also little affected at small Q 2. For G * E , the TPE effects reach about 3 − 8% near Q 2 ∼ 4 GeV 2 , depending on the model, MAID or SAID, used to emulate the data. For G * C , the TPE effects decrease rapidly with increasing ǫ while growing with increasing Q 2 to reach ∼ 6 − 15% with Q 2 ∼ 4 GeV 2 at ǫ = 0.2. Sizeable TPE corrections to G * E and G * C found here points to the need of including TPE effects in the multipole analysis in the region of high Q 2 and small ǫ. The TPE corrections to REM and RSM obtained in our hadronic calculation are compared with those obtained in a partonic calculation for moderate momentum transfer of 2 < Q 2 < 4 GeV 2 .
We revisit the question of the contributions of two-photon exchange with ∆(1232) excitation to th... more We revisit the question of the contributions of two-photon exchange with ∆(1232) excitation to the electron-proton scattering in a hadronic model. Three improvements over the previous calculations are made, namely, correct vertex function for γN → ∆, realistic γN∆ form factors, and coupling constants. The discrepancy between the values of R ≡ µ p G E /G M extracted from Rosenbluth technique and polarization transfer method can be reasonably accounted for if the data of Andivahis et al. (Phys. Rev. D 50, 5491 (1994)) are analyzed. However, substantial discrepancy remains if the data of Qattan et al. (nucl-ex/0610006) are used. For the ratio R ± between e ± p scatterings, our predictions appear to be in satisfactory agreement with the preliminary data from VEPP-3. The agreement between our model predictions and the recent measurements on single spin asymmetry, transverse and longitudinal recoil proton polarizations ranges from good to poor.
Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the singularity structur... more Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the singularity structure of the πN scattering amplitudes has been investigated. For all partial waves up to F waves and c.m. energies up to W ∼ 2 GeV, the T-matrix poles have been calculated by three different techniques: analytic continuation into the complex energy plane, speed-plot and the regularization method. For all 4-star resonances, we find a perfect agreement between the analytic continuation and the regularization method. We also find resonance poles for resonances that are not so well established, but in these cases the pole positions and residues obtained by analytic continuation can substantially differ from the results predicted by the speed-plot and regularization methods.
Journal of Physics G: Nuclear and Particle Physics, 2000
The violation of the scaling law for the electric and magnetic form factors of the proton is exam... more The violation of the scaling law for the electric and magnetic form factors of the proton is examined within the cloudy bag model. We find that the suppression of the ratio of the electric and magnetic form factors is natural in the bag model. The pion cloud plays a moderate role in understanding the recent data from TJNAF.
A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the... more A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N , ∆, ρ, and σ fields, is extended to energies up to 2 GeV by including the ηN channel and all the four stars πN resonances up to the F−waves. The effects of other 2π channels are taken into account phenomenologically. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F−waves. However, a few of the extracted resonance parameters differ considerably from the PDG values.
We show that recent experiment data for the ratios E 1+/M 1+ and S 1+/M 1+ can be explained in a ... more We show that recent experiment data for the ratios E 1+/M 1+ and S 1+/M 1+ can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare γ*NΔ form factors. Within our model we find that the bare Δ is almost spherical and the electric E2 and Coulomb C2 quardrupole excitations of the physical Δ are nearly saturated by pion cloud contribution in Q2 ≤ 4.0 GeV 2.
A meson-exchange model for pion-nucleon scattering was previously constructed using a three-dimen... more A meson-exchange model for pion-nucleon scattering was previously constructed using a three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N , ∆, ρ, and σ fields. We thereby extend our previous work by including the ηN channel and all the πN resonances with masses ∼ 2 GeV, up to the F waves. The effects of the ππN channels are taken into account by introducing an effective width in the resonance propagators. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F waves and for energies below 2 GeV. We present a new scheme to extract the properties of overlapping resonances. The predicted values for the resonance masses and widths as well as resonance pole positions and residues are compared to the listing of the Particle Data Group.
A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the... more A meson-exchange πN model, previously constructed using three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving π, η, N, ∆, ρ, and σ fields, is extended to energies up to 2 GeV by including the ηN channel and all the four stars πN resonances up to the F −waves. The effects of other 2π channels are taken into account phenomenologically. The extended model gives an excellent fit to both πN phase shifts and inelasticity parameters in all channels up to the F −waves. However, a few of the extracted resonance parameters differ considerably from the PDG values.
Journal of Physics G: Nuclear and Particle Physics, 1998
... Mark I Gorenstein§, Walter Greiner and Shin Nan Yang Institute for Theoretical Physics... more ... Mark I Gorenstein§, Walter Greiner and Shin Nan Yang Institute for Theoretical Physics, Goethe University, Frankfurt, Germany Department of Physics, National Taiwan University, Taipei, Taiwan 10617, Republic of China § School of Physics and ... m2T 2π2 K2 (m T ...
Pion photo- and electroproduction has been studied at threshold and in the resonance region below... more Pion photo- and electroproduction has been studied at threshold and in the resonance region below W=2GeV. At threshold pi^0 production can be very well explained within a dynamical model derived from an effective chiral Lagrangian. The final state interaction is nearly saturated by single charge exchange rescattering. In the resonance region new electroproduction data at Q^2=1GeV^2 has been analyzed with MAID and longitudinal and transverse photon helicity amplitudes have been determined for different resonances. A detailed study of the E/M and S/M ratios of the N->Delta transition shows a zero crossing of REM near Q^2=4GeV^2, whereas the RSM becomes increasingly negative at large Q^2.
The Dubna-Mainz-Taipei dynamical model for pion electromagnetic production, which can describe we... more The Dubna-Mainz-Taipei dynamical model for pion electromagnetic production, which can describe well the existing data from threshold up to 1 GeV photon lab energy, is presented and used to analyze the recent precision data in the ∆ region. We find that, within our model, the bare ∆ is almost spherical while the physical ∆ is oblate. The deformation is almost saturated by the pion cloud effects. We further find that up to Q 2 = 4.0(GeV/c) 2, the extracted helicity amplitude A 3/2 and A 1/2 remain comparable with each other, implying that hadronic helicity is not conserved at this range of Q 2. The ratio E1+/M 1 + obtained show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and to become positive with increasing Q 2. This is a possible indication of a very slow approach toward the pQCD region. Finally, we find that the bare helicity amplitude A 1/2 and S 1/2, but not A 3/2, starts exhibiting the scaling behavior at about Q 2 ≥ 2.5(G...
The $S$- and $D$-state components of the deuteron-triton overlap integral and of the form factor ... more The $S$- and $D$-state components of the deuteron-triton overlap integral and of the form factor required for a distorted-wave Born-approximation analysis of ($d$,$t$) reactions have been calculated for a number of variational triton wave functions corresponding to realistic nucleon-nucleon potentials. It is found that the calculated overlap functions decay too rapidly for large $n\ensuremath{-}d$ separations, and this has important consequences for the corresponding distorted-wave Born-approximation form factors. In particular, the parameter ${D}_{2}$, which measures the relative importance of the $D$-state form factor in the low-momentum limit, is found to be more than a factor of 2 smaller than the best experimental value. A more direct calculation of the form factors leads to a much larger value of ${D}_{2}$.
Current status of the two-boson exchange contributions to elastic electron-proton scattering, bot... more Current status of the two-boson exchange contributions to elastic electron-proton scattering, both for parity conserving and parity-violating, is briefly reviewed. How the discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments can be understood, in large part, by the two-photon exchange corrections is discussed. We also illustrate how the measurement of the ratio between positron-proton and electron-proton scattering can be used to differentiate different models of two-photon exchange. For the parity-violating electron-proton scattering, the interest is on how the two-boson exchange (TBE), γZ-exchange in particular, could affect the extraction of the long-sought strangeness form factors. Various calculations all indicate that the magnitudes of effect of TBE on the extraction of strangeness form factors is small, though can be large percentage-wise in certain kinematics.
We review the description of the lowest-energy nucleon excitation-the ∆(1232)-resonance. Much of ... more We review the description of the lowest-energy nucleon excitation-the ∆(1232)-resonance. Much of the recent experimental effort has been focused on the precision measurements of the nucleon-to-∆ transition by means of electromagnetic probes. We confront the results of these measurements with the state-of-the-art calculations based on chiral effective-field theories (EFT), lattice QCD, large-N c relations, perturbative QCD, and QCD-inspired models. We also discuss the link of the nucleon-to-∆ form factors to generalized parton distributions (GPDs). Some of the theoretical approaches are reviewed in detail, in particular, recent dynamical and unitary-isobar models of pion electroproduction, which are extensively used in the interpretation of experiments. Also, the novel extension of chiral EFTs to the energy domain of the ∆-resonance is described in detail. The two-photon exchange effects in the electroexcitation of the ∆-resonance are addressed here as well.
We evaluate the corrections of the two-photon exchange (TPE) process on the γ * N ∆ transition fo... more We evaluate the corrections of the two-photon exchange (TPE) process on the γ * N ∆ transition form factors. The contributions of the TPE process to the eN → e∆(1232) → eN π are calculated in a hadronic model with the inclusion of only the elastic nucleon intermediate states, to estimate its effects on the multipoles M (3/2) 1 + , E (3/2) 1 + , S (3/2) 1 + at the ∆ peak. We find that TPE effects on G * M is very small. G * E , and G * C are also little affected at small Q 2. For G * E , the TPE effects reach about 3 − 8% near Q 2 ∼ 4 GeV 2 , depending on the model, MAID or SAID, used to emulate the data. For G * C , the TPE effects decrease rapidly with increasing ǫ while growing with increasing Q 2 to reach ∼ 6 − 15% with Q 2 ∼ 4 GeV 2 at ǫ = 0.2. Sizeable TPE corrections to G * E and G * C found here points to the need of including TPE effects in the multipole analysis in the region of high Q 2 and small ǫ. The TPE corrections to REM and RSM obtained in our hadronic calculation are compared with those obtained in a partonic calculation for moderate momentum transfer of 2 < Q 2 < 4 GeV 2 .
We revisit the question of the contributions of two-photon exchange with ∆(1232) excitation to th... more We revisit the question of the contributions of two-photon exchange with ∆(1232) excitation to the electron-proton scattering in a hadronic model. Three improvements over the previous calculations are made, namely, correct vertex function for γN → ∆, realistic γN∆ form factors, and coupling constants. The discrepancy between the values of R ≡ µ p G E /G M extracted from Rosenbluth technique and polarization transfer method can be reasonably accounted for if the data of Andivahis et al. (Phys. Rev. D 50, 5491 (1994)) are analyzed. However, substantial discrepancy remains if the data of Qattan et al. (nucl-ex/0610006) are used. For the ratio R ± between e ± p scatterings, our predictions appear to be in satisfactory agreement with the preliminary data from VEPP-3. The agreement between our model predictions and the recent measurements on single spin asymmetry, transverse and longitudinal recoil proton polarizations ranges from good to poor.
Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the singularity structur... more Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the singularity structure of the πN scattering amplitudes has been investigated. For all partial waves up to F waves and c.m. energies up to W ∼ 2 GeV, the T-matrix poles have been calculated by three different techniques: analytic continuation into the complex energy plane, speed-plot and the regularization method. For all 4-star resonances, we find a perfect agreement between the analytic continuation and the regularization method. We also find resonance poles for resonances that are not so well established, but in these cases the pole positions and residues obtained by analytic continuation can substantially differ from the results predicted by the speed-plot and regularization methods.
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Papers by Shin-Nan Yang