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T. -S. H. Lee

Publications and source records attributed to T. -S. H. Lee.

At least 91 records · Page 5Linked to original sources
arXiv2006

Dynamical coupled-channel model of meson production reactions in the nucleon resonance region

A dynamical coupled-channel model is presented for investigating the nucleon resonances in the meson production reactions induced by pions and photons. The model is based on an energy-independent Hamiltonian which is derived from a set of Lagrangians by using a unitary transformation method. By applying the projection operator techniques,we derive a set of coupled-channel equations which satisfy the unitarity conditions within the channel space spanned by the considered two-particle meson-baryon states and the three-particle $ππN$ state. We present and explain in detail a numerical method based on a spline-function expansion for solving the resulting coupled-channel equations which contain logarithmically divergent one-particle-exchange driving terms resulted from the $ππN$ unitarity cut. We show that this driving term can generate rapidly varying structure in the reaction amplitudes associated with the unstable particle channels. It also has large effects in determining the two-pion production cross sections. Our results indicate that cautions must be taken to interpret the $N^*$ parameters extracted from using models which do not include $ππN$ cut effects.

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arXiv2006

Dynamical coupled-channel approach to hadronic and electromagnetic production of kaon-hyperon on the proton

A dynamical coupled-channel formalism for processes $πN \to KY$ and $γN \to KY$ is presented which provides a comprehensive investigation of recent data on the $γp \to K^+ Λ$ reaction. The non-resonant interactions within the subspace $KY\oplusπN$ are derived from effective Lagrangians, using a unitary transformation method. The calculations of photoproduction amplitudes are simplified by casting the coupled-channel equations into a form such that the empirical $γN \to πN$ amplitudes are input and only the parameters associated with the $KY$ channel are determined by performing $χ^2$-fits to all of the available data for $π^- p \to K^\circΛ, K^\circΣ^\circ$ and $γp \to K^+Λ$. Good agreement between our models and those data are obtained. In the fits to $πN \to KY$ channels, most of the parameters are constrained within $\pm 20%$ of the values given by the Particle Data Group and/or quark model predictions, while for $γp \to K^+ Λ$ parameters, ranges compatible with broken $SU(6)\otimes O(3)$ symmetry are imposed. The main reaction mechanisms in $K^+ Λ$ photoproduction are singled out and issues related to newly suggested resonances $S_{11}$, $P_{13}$, and $D_{13}$ are studied. Results illustrating the importance of using a coupled-channel treatment are reported. Meson cloud effects on the $γN \to N^*$ transitions are also discussed.

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arXiv2005

Coupled channel study of $K^+Λ$ photoproduction

A coupled channel model with $γN$, $KY$ and $πN$ channels has been used to analyze the recent data of $γp \to K^+ Λ$. The non-resonant interactions within the subspace $KY \oplus πN$ are derived from effective Lagrangians using a unitary transformation method. The direct photoproduction reaction is obtained from a chiral constituent quark model with $SU(6)\otimes O(3)$ breaking. Missing baryon resonances issues are briefly discussed.

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arXiv2005

Photo-excitation of hyperons and exotic baryon in gamma N --> K Anti-K N

We investigate the reaction of $γN \to K \bar{K} N$ focusing on the photoproduction of $Λ(1520)$ and putative exotic $Θ(1540)$. We consider various background production mechanisms including the production of vector mesons, tensor mesons, and other $Λ$ and $Σ$ hyperons. We discuss the angular distribution of $Λ(1520)$ photoproduction cross section and the radiative decays of $Λ(1520)$. We also discuss what we expect for the invariant mass distributions if the $Θ(1540)$ is formed in the reaction, with the parameters studied so far. We find that the peak in the $KN$ invariant mass distribution, if confirmed, can hardly come from the kinematic reflections, especially, due to the tensor meson backgrounds.

hep-ph↗
arXiv2005

Pentaquark Theta^+(1540) production in gamma N --> K Anti-K N

The recent developments in the search of exotic pentaquark hadrons are briefly reviewed. We then focus on investigating how the exotic pentaquark $Θ(1540)$ baryon production can be identified in the $γN \to K \bar{K} N$ reactions, focusing on the influence of the background (non-$Θ$ production) mechanisms. By imposing the SU(3) symmetry and using various quark model predictions, we are able to fix the coupling constants for evaluating the kaon backgrounds, the $K\bar{K}$ production through the intermediate vector meson and tensor meson photoproduction, and the mechanisms involving intermediate $Λ(1116)$, $Λ(1405)$, $Λ(1520)$, $Σ(1193)$, $Σ(1385)$, and $Δ(1232)$ states. The total cross section data of $γp \to K^+K^- p$ is then used to fix the form factors which regularize the background amplitudes so that the signal of $Θ(1540)$ in $γn \to K^+K^-n$ and $γp \to K^0\bar{K}^0p$ cross sections can be predicted. We find that the predicted $K^+K^-$ and $K^+ n$ invariant mass distributions of the $γn \to K^+ K^- n$ reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted $Θ(1540)$ peak can not be identified unambiguously with the data. High statistics experiments are needed to resolve the problem. We also find that an even-parity $Θ$ is more likely to be detected, while it will be difficult to identify an odd-parity $Θ$, even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions.

hep-ph↗
arXiv2005

Quark-Hadron Duality and Parity Violating Asymmetry of Electroweak Reactions in the Delta Region

A dynamical model of electroweak pion production reactions in the Delta(1232) region has been extended to include the neutral current contributions for examining the local Quark-Hadron Duality in neutrino-induced reactions and for investigating how the axial N-Delta form factor can be determined by the parity violating asymmetry of N(\vec{e},e') reactions. We first show that the recent data of (e,e') structure functions F_1 and F_2, which exhibit the Quark-Hadron Duality, are in good agreement with our predictions. For possible future experimental tests, we then predict that the structure functions F_1, F_2, and F_3 for (ν,e) and (ν,ν') processes also show the similar Quark-Hadron Duality. The spin dependent structure functions g_1 and g_2 of (e,e') have also been calculated from our model. It is found that the local Quark-Hadron Duality is not seen in the calculated g_1 and g_2, while our results for g_1 and some polarization observables associated with the exclusive p(\vec{e},e' pi) and \vec{p}(\vec{e},e' pi) reactions are in reasonably good agreement with the recent data. In the investigation of parity violating asymmetry A of N(\vec{e},e') reactions, it is found that the non-resonant contribution is small at the Delta peak and a measurement of A can be used to distinguish two previously determined axial N-Delta transition form factors. The predicted asymmetry A are also compared with the Parton Model predictions for future experimental investigations of Quark-Hadron Duality.

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arXiv2005

Dynamical coupled-channel study of K+ Lambda photoproduction

Results for the reaction gamma p --> K+ Lambda, studied within a constituent quark model and a dynamical coupled-channel approach, are presented and compared with recent data. Issues related to the search for missing baryon resonances are briefly discussed and the role played by a third S_{11} resonance is underlined.

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arXiv2004

Dynamical Model of Electroweak Pion Production Reaction

The dynamical model of pion electroproduction developed by the authors has been extended to study the pion weak production reaction. The axial vector N-Delta transition form factor G_A^{NΔ} obtained from the analysis of existing neutrino reaction data and the role of dynamical pion cloud on G_A^{NΔ} are discussed.

hep-ph↗
arXiv2004

Leading-order Corrections of a Dynamical Model for Electromagnetic Pion Production Reactions

By applying a unitary transformation method, we have derived the leading-order corrections on the effective Hamiltonian of a dynamical model developed in Phys. Rev. C54, 2660 (1996) for electromagnetic pion production reactions. The resulting energy-independent one-loop corrections on the baryon masses and the gamma N -> Delta vertex interaction are associated with the structure of the nucleon and Delta and have been calculated within a constituent quark model. We find that the one-loop corrections on the magnetic M1 transition of the gamma N -> Delta are very small, while their contributions to the electric E2 and Coulomb C2 transitions are found to be in opposite signs of that due to pion cloud effects associated with the scattering states. Our results further indicate that the determination of the nonspherical L=2 components of the constituent quark wavefunctions of N and Delta from the extracted empirical E2 and C2 form factors requires a rigorous and complete calculation of meson cloud effects. We also find that the one-loop corrections on the non-resonant pion production operator can resolve the difficulty in describing the near threshold gamma p -> pi^0 p reaction. Possible future developments are discussed.

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arXiv2004

Models of Meson-Baryon Reactions in the Nucleon Resonance Region

It is shown that most of the models for analyzing meson-baryon reactions in the nucleon resonance region can be derived from a Hamiltonian formulation of the problem. An extension of the coupled-channel approach to include $ππN$ channel is briefly described and some preliminary results for the $N^*(1535)$ excitation are presented.

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arXiv2004

Dynamical coupled-channel model of kaon-hyperon interactions

The pi N --> KY and KY --> KY reactions are studied using a dynamical coupled-channel model of meson-baryon interactions at energies where the baryon resonances are strongly excited. The channels included are: pi N, K Λ, and KΣ. The resonances considered are: N^* [S_{11}(1650), P_{11}(1710), P_{13}(1720),D_{13}(1700)]; Δ^* [S_{31}(1900), P_{31}(1910), P_{33}(1920)]; Λ^* [S_{01}(1670), P_{01}(1810)] Σ^* [P_{11}(1660), D_{13}(1670)]; and K^*(892). The basic non-resonant πN --> KY and KY --> KY transition potentials are derived from effective Lagrangians using a unitary transformation method. The dynamical coupled-channel equations are simplified by parametrizing the pi N -->pi N amplitudes in terms of empirical pi N partial-wave amplitudes and a phenomenological off-shell function. Two models have been constructed. Model A is built by fixing all coupling constants and resonance parameters using SU(3) symmetry, the Particle Data Group values, and results from a constituent quark model. Model B is obtained by allowing most of the parameters to vary around the values of model A in fitting the data. Good fits to the available data for pi^- p to K^0 Λ, K^0 Σ^0 have been achieved. The investigated kinematics region in the center-of-mass frame goes from threshold to 2.5 GeV. The constructed models can be imbedded into associated dynamical coupled-channel studies of kaon photo- and electro-production reactions.

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arXiv2003

Rho meson photoproduction at low energies

The $σ$-exchange and $f_2$-exchange mechanisms for $ρ$ meson photoproduction are re-examined. Then the commonly employed $σ$-exchange amplitude is revised by using the recent information from the analyses on the $ρ\to π^0π^0γ$ decay and the $σNN$ coupling constant from Bonn potential. Instead of relying on the Pomeron-$f$ proportionality assumption, the $f_2$ meson exchange amplitude is established from an effective Lagrangian which is constructed from the tensor structure of the $f_2$ meson. Phenomenological information together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ coupling constants. As a first step to improve the current theoretical models, we have also explored the effects due to the un-correlated $2π$ exchange amplitude with $πN$ intermediate state. This leading-order $2π$ exchange amplitude can be calculated using the coupling constants determined from the study of pion photoproduction and the empirical width of $ρ\to ππ$ decay. In comparing with the existing differential cross section data, we find that a model with the constructed $2π$, $σ$, and $f_2$ exchanges is comparable to the commonly used $σ$ exchange model in which the $σ$ coupling parameters are simply adjusted to fit the experimental data. We suggest that experimental verifications of the predicted single and double spin asymmetries in the small $|t|$ ($ < 2$ GeV$^2$) region will be useful for distinguishing the two models and improving our understanding of the non-resonant amplitude of $ρ$ photoproduction. Possible further improvements of the model are discussed.

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arXiv2003

f_2, sigma, and 2-pi exchanges in rho meson photoproduction

The $σ$ meson exchange model for $ρ$ photoproduction at low energies is re-examined and a new model is developed by considering explicit two-pion exchange and the $f_2$ tensor meson exchange. The $f_2$ exchange model, which is motivated by the low energy proton-proton elastic scattering, is constructed by fully taking into account of the tensor structure of the $f_2$ meson interactions. Phenomenological informations together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ meson's coupling constants. For $2π$ exchange, the loop terms including intermediate $πN$ and $ωN$ channels are calculated using the coupling constants determined from the study of pion photoproduction. It is found that our model with $f_2$ and $2π$ exchanges can successfully replace the commonly used $σ$ exchange model that suffers from the big uncertainty of the coupling constants. We found that the two models can be distinguished by examining the single and double spin asymmetries.

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