arXiv subjects

T. -S. H. Lee

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

At least 127 records · Page 7Linked to original sources

arXiv1999

Baryon Resonance Extraction from piN Data using a Unitary Multichannel Model

A unitary multi-channel approach, first developed by the Carnegie-Mellon- Berkeley group, is applied to extract the pole positions, masses, and partial decay widths of nucleon resonances. Input used are the partial wave amplitudes for the transitions from piN to eight possible final baryon-meson final states. Because the model preserves analyticity, resonances are found as poles in the T matrix. This approach is designed to handle inelastic threshold openings with minimal model dependence. Although many of the known states have the same properties as in previous analyses, others change significantly. We pay particular attention to S11(1535) which is very close to the eta threshold.

nucl-th↗

arXiv1999

Dynamical Test of Constituent Quark Models with $πN$ Reactions

A dynamical approach is developed to predict the $πN$ scattering amplitudes starting with the constituent quark models. The first step is to apply a variational method to solve the three-quark bound state problem. The resulting wave functions are used to calculate the $N^* \to πN, ηN, πΔ$ vertex functions by assuming that the $π$ and $η$ mesons couple directly to quarks. These vertex functions and the predicted baryon bare masses then define a Hamiltonian for $πN$ reactions. We apply a unitary transformation method to derive from the constructed Hamiltonian a multi-channel and multi-resonance reaction model for predicting the $πN$ scattering amplitudes up to $W = 2$ GeV. With the parameters constrained by the $Δ(1232$) excitation, we have examined the extent to which the $πN$ scattering in $S_{11}$ channel can be described by the constituent quark models based on the one-gluon-exchange or one-meson-exchange mechanisms. It is found that the data seem to favor the spin-spin interaction due to one-meson-exchange and the tensor interaction due to one-gluon-exchange. A phenomenological quark-quark potential has been constructed to reproduce the $S_{11}$ amplitude.

nucl-th↗

arXiv1999

Evidence for the fourth P11 resonance predicted by the constituent quark model

It is pointed out that the third of five low-lying P11 states predicted by a constituent quark model can be identified with the third of four states in a solution from a three-channel analysis by the Zagreb group. This is one of the so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb group to the pi N -> eta N data is the crucial element in finding this fourth resonance in the P11 partial wave.

nucl-th↗

arXiv1999

Strange hadron matter and SU(3) symmetry

We calculate saturation curves for strange hadron matter using recently constructed baryon-baryon potentials which are constrained by SU(3) symmetry. All possible interaction channels within the baryon octet (consisting of $N$, $Λ$, $Σ$, and $Ξ$) are considered. It is found that a small $Λ$ fraction in nuclear matter slightly increases binding, but that larger fractions ($>10%$) rapidly cause a decrease. Charge-neutral ${N,Λ,Ξ}$ systems, with equal densities for nucleons and cascades, are only very weakly bound. The dependence of the binding energies on the strangeness per baryon, $f_s$, is predicted for various ${N,Λ,Ξ}$ and ${N,Λ,Σ,Ξ}$ systems. The implications of our results in relativistic heavy-ion collisions and the core of a dense star are discussed. We also discuss the differences between our results and previous hadron matter calculations.

nucl-th↗

arXiv1998

An isotopic effect in phi photoproduction at a few GeV

A distinct isotopic effect in phi photoproduction at 2-5 GeV region is identified by examining the production amplitudes due to Pomeron-exchange and meson-exchange mechanisms. This effect is mainly caused by the pi-eta interference constrained by SU(3) symmetry and the isotopic structure of the gamma NN coupling in the direct phi-radiation amplitude. It can be tested experimentally by measuring differences in the polarization observables between the gamma-p and gamma-n reactions.

nucl-th↗

arXiv1998

Non-perturbative chiral approach to Kp -->gamma Y reactions

A recently developed nonperturbative chiral approach to describe the S=-1 meson-baryon reactions has been extended to investigate the near threshold $K^- p \to γΛ, γΣ^0$ reactions. With the parameters governed by chiral SU(3) symmetry, we show that the predicted branching ratios $Γ_{K^- p \to γΛ}/Γ_{K^- p \to all}$ and $Γ_{K^-p\to γΣ^0}/Γ_{K^- p \to all}$ are close to the experimental values. The coupling with the $η$ channels, which was shown to be important in the $S=-1$ meson-baryon reactions, is also found to be significant here. Our results are consistent with the interpretation of the $Λ(1405)$ as a quasi-bound meson-baryon state as found in other similar chiral approaches.

nucl-th↗

arXiv1997

The isoscalar S-wave pi-N scattering length a+ from pi-deuteron scattering

We consider constraints on the isoscalar S-wave pi-N scattering length a+ from pi-deuteron scattering, to third order in small momenta and pion masses in chiral perturbation theory. To this order, the pi-deuteron scattering length is determined by a+ together with three-body corrections that involve no undetermined parameters. We extract a novel value for a combination of dimension two low-energy constants which is in agreement with previous determinations.

nucl-th↗

arXiv1997

Chiral Perturbation Theory and the pp -> pp pi0 Reaction Near Threshold

A chiral-perturbative consideration of the near-threshold pp -> pp pi0 reaction indicates that the pion-rescattering term has a substantial energy and momentum dependence. The existing calculations that incorporate this dependence give pion rescattering contributions significantly larger than those of the conventional treatment, and this enhanced rescattering term interferes destructively with the one-body impulse term, leading to theoretical cross sections that are much smaller than the observed values. However, since the existing calculations are based on coordinate-space representation, they involve a number of simplifying assumptions about the energy-momentum flow in the rescattering diagram, even though the delicate interplay between the one-body and two-body terms makes it desirable to avoid these kinematical assumptions. We carry out here a momentum-space calculation that retains the energy-momentum dependence of the vertices as predicted by chiral perturbation theory. Our improved treatment increases the rescattering amplitude by a factor of 3 over the value obtained in the r-space calculations. The pp -> pp pi0 transition amplitude, which is now dominated by the rescattering term, leads to the cross section much larger than what was reported in the approximate r-space calculations. Thus, the extremely small cross sections obtained in the previous chiral perturbative treatments of this reaction should be considered as an accidental consequence of the approximations employed rather than a general feature.

nucl-th↗

arXiv1997

Update of the pi N --> eta N and eta N --> eta N partial-wave amplitudes

A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the $πN \rightarrow ηN$ and $ηN \rightarrow ηN$ amplitudes using as input the latest data for the dominant $S_{11}$ $πN$ elastic scattering partial wave following suggestions of Prof. G. Höhler. The sign error in the numerical evaluation of the dispersion integral in the original publication is eliminated. The remaining weighted data set for the $πN \rightarrow ηN$ total and differential cross sections is used as in the original publication. The correction of the numerical error influences the $ηN$ cusp effect and improves the quality of the fit to the input data. However, our new result for the $ηN$ scattering length, $a_{ηN }= (0.717 \pm 0.030) + i (0.263 \pm 0.025)$ fm, is a sole consequence of the correction of the $S_{11}$ input and suggests that the $ηd$ system is unbound or loosely bound.

nucl-th↗

arXiv1997

Exclusive diffractive processes and the quark substructure of mesons

Exclusive diffractive processes on the nucleon are investigated within a model in which the quark-nucleon interaction is mediated by Pomeron exchange and the quark substructure of mesons is described within a framework based on the Dyson-Schwinger equations of QCD. The model quark-nucleon interaction has four parameters which are completely determined by high-energy $πN$ and $K N$ elastic scattering data. The model is then used to predict vector-meson electroproduction observables. The obtained $ρ$- and $ϕ$-meson electroproduction cross sections are in excellent agreement with experimental data. The predicted $q^2$ dependence of $J/ψ$-meson electroproduction also agrees with experimental data. It is shown that confined-quark dynamics play a central role in determining the behavior of the diffractive, vector-meson electroproduction cross section. In particular, the onset of the asymptotic $1/q^4$ behavior of the cross section is determined by a momentum scale that is set by the current-quark masses of the quark and antiquark inside the vector meson. This is the origin of the striking differences between the $q^2$ dependence of $ρ$-, $ϕ$- and $J/ψ$-meson electroproduction cross sections observed in recent experiments.

nucl-th↗

arXiv1997

Neutral pion photoproduction on deuterium in baryon chiral perturbation theory to order q^4

Threshold neutral pion photoproduction on the deuteron is studied in the framework of baryon chiral perturbation theory beyond next-to--leading order in the chiral expansion. To fourth order in small momenta, the amplitude is finite and a sum of two- and three-body interactions with no undetermined parameters. With accurate theoretical and experimental input from the single nucleon sector for the proton amplitude, we investigate the sensitivity of the threshold cross section to the elementary $γn \to π^0 n$ amplitude. A precise measurement of the threshold cross section for $γd \to π^0 d$ is called for.

hep-ph↗

arXiv1996

Pion Absorption Cross Section for ^2H and ^3He in the Delta-Isobar Region: A Phenomenological Connection

The absorption of π^+ on ^3He in the $Δ$-region is evaluated with exact inclusion of the final state interaction among the three emerging protons. The absorption is described by a $πN \to Δ$ vertex and a $NΔ- NN$ transition t-matrix which are calculated from a phenomenological model for NN and πd reactions. In a calculation where the initial pion scattering effects are neglected, the predicted peaks of the pion absorption cross sections for ^2H and ^3He lie too high in energy in relation to the data. The effect of the final state three-nucleon interaction turns out to be too small for changing the magnitude and shifting the peak position of the total absorption cross section for ^3He. We demonstrate that the adjustment of the peak position for the deuteron cross section by small modifications of the $Δ$-parameters, automatically leads to the correct peak position in ^3He.

nucl-th↗

arXiv1996

Pomeron exchange and exclusive electroproduction of rho-mesons in QCD

A Pomeron-exchange model of exclusive electroproduction of $ρ$-mesons is examined using a dressed-quark propagator. It is shown that by representing the photon-$ρ$-meson-Pomeron coupling by a nonperturbative, confined-quark loop, one obtains predictions for $ρ$-meson electroproduction that are in good agreement with experiment.

nucl-th↗

arXiv1996

Near threshold eta production in proton-proton collisions

The total cross section for the near threshold eta production in proton-proton collisions has been investigated with the assumption that the production mechanism is due to the emission of a meson x (pi, eta, sigma) from one of the nucleons followed by a x N --> eta N transition on the second one. The x N --> eta N amplitudes are generated from the unitary multi-channel multi-resonance model which has recently been constructed in analyzing the pi N scattering and pi N --> eta N reaction. The initial and final pp distortions are calculated from a coupled-channel pi-NN model which describes the NN scattering data up to about 2 GeV. With the x-NN vertex functions taken from the Bonn potential, the predicted total cross sections of threshold pp --> eta pp reaction are in good agreement with the data. In contrast to previous works, we find that the eta-exchange plays an important role. The effect of the two-pion exchange, simulated by sigma- exchange, is found to be significant but not as dominant as the vector meson-exchange introduced in previous works.

nucl-th↗

arXiv1996

Medium Effects on Binary Collisions with the Delta Resonance

To facilitate the relativistic heavy-ion calculations based on transport equations, the binary collisions involving a $Δ$ resonance in either the entrance channel or the exit channel are investigated within a Hamiltonian formulation of $πNN$ interactions. An averaging procedure is developed to define a quasi-particle $Δ^*$ and to express the experimentally measured $NN\rightarrow πNN$ cross section in terms of an effective $NN\rightarrow NΔ^\ast$ cross section. In contrast to previous works, the main feature of the present approach is that the mass and the momentum of the produced $Δ^*$'s are calculated dynamically from the bare $Δ\leftrightarrow πN$ vertex interaction of the model Hamiltonian and are constrained by the unitarity condition. The procedure is then extended to define the effective cross sections for the experimentally inaccessible $NΔ^\ast \rightarrow NN$ and $NΔ^\ast \rightarrow NΔ^\ast$ reactions. The predicted cross sections are significantly different from what are commonly assumed in relativistic heavy-ion calculations. The $Δ$ potential in nuclear matter has been calculated by using a Bruckner-Hartree-Fock approximation. By including the mean-field effects on the $Δ$ propagation, the effective cross sections of the $NN\rightarrow NΔ^\ast$, $NΔ^\ast \rightarrow NN$ and $NΔ^\ast \rightarrow NΔ^\ast$ reactions in nuclear matter are predicted. It is demonstrated that the density dependence is most dramatic in the energy region close to the pion production threshold.

nucl-th↗

arXiv1995

Determination of the $πNN$ form factor from the threshold pion production of $pp$ scattering

It is shown that the threshold productions of $π^0 pp$, $π^+ np$ and $π^+ d$ from $pp$ collisions can be consistently described by a model consisting of a pion s-wave rescattering and $N\bar N$ pair-terms of heavy-meson exchanges. The large difference between $σ^{tot} (pp\rightarrowπ^+ d)$ and $σ^{tot} (pp\rightarrowπ^+ np)$ is understood from the orthogonality of the deuteron and the $np$ scattering wave functions. In a calculation using the Paris potential to account for the initial and final $NN$ interactions, it is found that the data can be best reproduced by using a soft $πNN$ form factor with $Λ_π =650 $ MeV for a monopole form. This is consistent with an earlier study of pion production in the $Δ$ excitation region.

nucl-th↗

arXiv1994

Proton-proton bremsstrahlung below and above pion-threshold: the influence of the $Δ$-isobar

The proton-proton bremsstrahlung is investigated within a coupled-channel model with the $Δ$ degree of freedom. The model is consistent with the $NN$ scattering up to 1 GeV and the $γNΔ$ vertex determined in the study of pion photoproduction reactions. It is found that the $Δ$ excitation can significantly improve the agreements with the $pp \rightarrow ppγ$ at $E_{lab}=280$ MeV. Predictions at $E_{lab}=550$ and $800$ MeV are presented for future experimental tests.

nucl-th↗

arXiv1994

Two-body Pion Absorption on $^3He$ at Threshold

It is shown that a satisfactory explanation of the ratio of the rates of the reactions $^3He(π^-,nn)$ and $^3He(π^-,np)$ for stopped pions is obtained once the effect of the short range two-nucleon components of the axial charge operator for the nuclear system is taken into account. By employing realistic models for the nucleon-nucleon interaction in the construction of these components of the axial charge operator, the predicted ratios agree with the empirical value to within 10-20\%.

nucl-th↗