arXiv subjects

T. -S. H. Lee

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

At least 109 records · Page 6Linked to original sources

arXiv2003

Spin effects and baryon resonance dynamics in phi-meson photoproduction at few GeV

The diffractive $ϕ$-meson photoproduction amplitude is dominated by the Pomeron exchange process and contains the terms that govern the spin-spin and spin-orbital interactions. We show that these terms are responsible for the spin-flip transitions at forward photoproduction angles and appear in the angular distributions of $ϕ\to K^+K^-$ -decay in reactions with unpolarized and polarized photon beams. At large momentum transfers, the main contribution to the $ϕ$-meson photoproduction is found to be due to the excitation of nucleon resonances. Combined analysis of $ω$ and $ϕ$ photoproduction indicates strong OZI-rule violation in $ϕNN^*$ - couplings. We also show that the spin observables are sensitive to the dynamics of $ϕ$-meson photoproduction at large angles and could help distinguish different theoretical models of nucleon resonances. Predictions for spin effects in $ϕ$-meson photoproduction are presented for future experimental tests.

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arXiv2003

Dynamical Model of Weak Pion Production Reactions

The dynamical model of pion electroproduction has been extended to investigate the weak pion production reactions. The predicted cross sections of neutrino-induced pion production reactions are in good agreement with the existing data. We show that the renormalized(dressed) axial N-$Δ$ form factor contains large dynamical pion cloud effects and this renormalization effects are crucial in getting agreement with the data. We conclude that the N-$Δ$ transitions predicted by the constituent quark model are consistent with the existing neutrino induced pion production data in the $Δ$ region.

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arXiv2002

pi and rho loop corrections to omega photoproduction in the resonance region

One-loop corrections due to the intermediate $πN$ and $ρN$ states are studied in $ω$ photoproduction near threshold. Our results show that the coupled-channel effects should be taken into account in extracting reliable nucleon resonance parameters from the forthcoming vector meson photoproduction data in the resonance region.

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arXiv2002

Quark-Exchange Mechanism of $γd \to np$ Reaction At 2-6 GeV

Within the constituent quark model, we examine the extent to which the deuteron photo-disintegration at 2-6 GeV can be described by the quark-exchange mechanism. With the parameters constrained by the $np$ scattering, the calculated differential cross sections disagree with the data in both magnitude and energy-dependence. The results can be improved if we use a smaller size parameter for quark wavefunctions. We also find that the on-shell approximation used in a previous investigation is not accurate

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arXiv2002

One-loop corrections to omega photoproduction near threshold

One-loop corrections to $ω$ photoproduction near threshold have been investigated by using the approximation that all relevant transition amplitudes are calculated from the tree diagrams of effective Lagrangians. With the parameters constrained by the data of $γN \to πN$, $γN \to ρN$, and $πN \to ωN$ reactions, it is found that the one-loop effects due to the intermediate $πN$ and $ρN$ states can significantly change the differential cross sections and spin observables. The results from this exploratory investigation suggest strongly that the coupled-channel effects should be taken into account in extracting reliable resonance parameters from the data of vector meson photoproduction in the resonance region.

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arXiv2002

Study of Nucleon Resonances with Double Polarization Observables of Pion Photoproduction

The role of the nucleon resonances in the double polarization observables of pion photoproduction is investigated by using the resonance parameters predicted by Capstick and Roberts. As an example, we show that the not-well-determined two-star resonance N$^{-}_{3/2}$(1960) can be examined by performing experiments on beam-recoil polarization at large angles.

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arXiv2002

Eta Meson Production in NN Collisions

Eta meson production in both proton-proton and proton-neutron collisions is investigated within a relativistic meson exchange model of hadronic interactions. It is found that the available cross section data can be described equally well by either the vector or pseudoscalar meson exchange mechanism for exciting the S_{11}(1535) resonance. It is shown that the analyzing power data can potentially be very useful in distinguishing these two scenarios for the excitaion of the S_{11}(1535) resonance.

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arXiv2002

Final state interactions in omega photoproduction near threshold

Vector meson photoproduction and electroproduction have been suggested as a tool to find or confirm the nucleon resonances. In order to extract more reliable informations on the nucleon resonances, understanding the non-resonant background is indispensable. We consider final state interactions in omega photoproduction as a background production mechanism. For the intermediate states, we consider nucleon--vector-meson and nucleon-pion channels. The role of the final state interactions is discussed in omega meson photoproduction near threshold.

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arXiv2001

Dynamical Study of the d(e,e' pi+) reaction

The $d(e,e'π^+)$ reaction in the parallel kinematics has been investigated using a dynamical model of pion electroproduction on the nucleon. A unitary $πNN$ model has been used in order to examine the effects due to the final two-nucleon interactions, pion rescattering from the second nucleon, and the intermediate $NN$ and $NΔ$ interactions. It has been found that these $πNN$ mechanisms are small, but they can have significant contributions to the $d(e,e'π^+)$ cross sections through their interference with the dominant impulse term. For the longitudinal cross sections, the effects due to the interference between the pion pole term and other production mechanisms are also found to be very large. Our findings clearly indicate that these interference effects must be accounted for in any attempt to determine from the $d(e,e'π^+)$ data whether the pion form factor and/or $πNN$ vertex of the pion pole term are modified in the nuclear medium.

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arXiv2001

Coupled-channel study of gamma p --> K+ Lambda

A coupled-channel (CC) approach has been developed to investigate kaon photoproduction on the nucleon. In addition to direct K+ Lambda production, our CC approach accounts for strangeness production including K+ Lambda final state interactions with both pi0 p and pi+ n intermediate states. Calculations for the gamma p --> K+ Lambda reaction have been performed, and compared with the recent data from SAPHIR, with emphasis on the CC effects. We show that the CC effects are significant at the level of inducing 20% changes on total cross sections; thereby, demonstrating the need to include pi N channels to correctly describe the gamma p --> K+ Lambda reaction.

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arXiv2001

Higher and missing resonances in omega photoproduction

We study the role of the nucleon resonances ($N^*$) in $ω$ photoproduction by using the quark model resonance parameters predicted by Capstick and Roberts. The employed $γN \to N^*$ and $N^* \to ωN$ amplitudes include the configuration mixing effects due to the residual quark-quark interactions. The contributions from the nucleon resonances are found to be important in the differential cross sections at large scattering angles and various spin observables. In particular, the parity asymmetry and beam-target double asymmetry at forward scattering angles are suggested for a crucial test of our predictions. The dominant contributions are found to be from $N\frac32^+ (1910)$, a missing resonance, and $N\frac32^- (1960)$ which is identified as the $D_{13}(2080)$ of the Particle Data Group.

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arXiv2001

Meson-exchange $πN$ Models in Three-Dimensional Bethe-Salpeter Formulation

The pion-nucleon scattering is investigated by using several three-dimensional reduction schemes of the Bethe-Salpeter equation for a model Lagrangian involving $π$, $N$, $Δ$, $ρ$, and $σ$ fields. It is found that all of the resulting meson-exchange models can give similar good descriptions of the $πN$ scattering data up to 400 MeV. However they have significant differences in describing the $πNN$ and $πNΔ$ form factors and the $πN$ off-shell t-matrix elements. We point out that these differences can be best distinguished by investigating the near threshold pion production from nucleon-nucleon collisions and pion photoproduction on the nucleon. The consequences of using these models to investigate various pion-nucleus reactions are also discussed.

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arXiv2000

Nucleon resonances in polarized omega photoproduction

The role of the nucleon resonances ($N^*$) in $ω$ photoproduction is investigated by using the resonance parameters predicted by Capstick and Roberts. The contributions from the nucleon resonances are found to be significant in various spin asymmetries. In particular, we found that a crucial test of our predictions can be made by measuring the parity asymmetry and beam-target double asymmetry at forward scattering angles.

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arXiv2000

Nucleon resonances in omega photoproduction

The role of the nucleon resonances ($N^*$) in $ω$ photoproduction is investigated by using the resonance parameters predicted by Capstick and Roberts [Phys. Rev. D {\bf 46}, 2864 (1992); {\bf 49}, 4570 (1994)]. In contrast with the previous investigations based on the ${SU}(6) \times {O}(3)$ limit of the constituent quark model, the employed $N^* \to γN$ and $N^* \to ωN$ amplitudes include the configuration mixing effects due to the residual quark-quark interactions. The contributions from the nucleon resonances are found to be significant relative to the non-resonant amplitudes in changing the differential cross sections at large scattering angles and various spin observables. In particular, we suggest that a crucial test of our predictions can be made by measuring the parity asymmetry and beam-target double asymmetry at forward scattering angles.

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arXiv2000

Phi^{zero}-N Bound State

We show that the QCD van der Waals attractive potential is strong enough to bind a $ϕ^{0}$ meson onto a nucleon inside a nucleus to form a bound state. The direct experimental signature for such an exotic state is proposed in the case of subthreshold $ϕ^{0}$ meson photoproduction from nuclear targets. The production rate is estimated and such an experiment is found to be feasible at the Jefferson Laboratory.

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arXiv2000

Quantum Monte Carlo Calculations of Pion Scattering from Li

We show that the neutron and proton transition densities predicted by recent quantum Monte Carlo calculations for A=6,7 nuclei are consistent with pion scattering from 6Li and 7Li at energies near the Delta resonance. This has provided a microscopic understanding of the enhancement factors for quadrople excitations, which were needed to describe pion inelastic scattering within the nuclear shell model of Cohen and Kurath.

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arXiv2000

Dynamical Study of the Delta Excitation in N(e,e' pi) Reactions

The dynamical model developed in [Phys. Rev. C 54, 2660 (1996)] has been applied to investigate the pion electroproduction reactions on the nucleon. It is found that the model can describe to a very large extent the recent data of p(e,e' pi^0) reaction from Jefferson Laboratory and MIT-Bates. The extracted magnetic dipole(M1), electric dipole(E2), and Coulomb(C2) strengths of the gamma N -> Delta transition are presented. It is found that the C2/M1 ratio drops significantly with Q^2 and reaches about -13% at Q^2=4 (GeV/c)^2, while the E2/M1 ratio remains close to the value \sim -3 % at the Q^2=0 photon point. The determined M1 transition form factor drops faster than the usual dipole form factor of the proton. We also find that the non-resonant interactions can dress the gamma N -> Delta vertex to enhance strongly its strength at low Q^2, but much less at high Q^2. Predictions are presented for future experimental tests. Possible developments of the model are discussed.

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arXiv2000

Pion Production Model - Connection between Dynamics and Quark Models

We discuss the difficulties in testing the hadron models by using the N^* parameters extracted from the empirical amplitude analyses of the pi-N and gamma-N reaction data. As an alternative or perhaps a more advantageous approach, we present a Hamiltonian formulation that can relate the pion production dynamics and the constituent quark models of N^* structure. The application of the approach in investigating the Delta and N^*(S_{11}) excitations is reviewed. It is found that while the Delta excitation can be described satisfactory, the pi-N scattering in S_{11} channel can not be described by the constituent quark models based on either the one-gluon-exchange or one-meson-exchange mechanisms. A phenomenological quark-quark potential has been constructed to reproduce the S_{11} amplitude.

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