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T. -S. H. Lee

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

At least 55 records · Page 3Linked to original sources
arXiv2011

Unitary coupled-channels model for three-mesons decays of heavy mesons

A unitary coupled-channels model is presented for investigating the decays of heavy mesons and excited meson states into three light pseudoscalar mesons. The model accounts for the three-mesons final state interactions in the decay processes, as required by both the three-body and two-body unitarity conditions. In the absence of the Z-diagram mechanisms that are necessary consequences of the three-body unitarity, our decay amplitudes are reduced to a form similar to those used in the so-called isobar-model analysis. We apply our coupled-channels model to the three-pions decays of a1(1260), pi2(1670), pi2(2100), and D0 mesons, and show that the Z-diagram mechanisms can contribute to the calculated Dalitz plot distributions by as much as 30% in magnitudes in the regions where f0(600), rho(770), and f2(1270) dominate the distributions. Also, by fitting to the same Dalitz plot distributions, we demonstrate that the decay amplitudes obtained with the unitary model and the isobar model can be rather different, particularly in the phase that plays a crucial role in extracting the CKM CP-violating phase from the data of B meson decays. Our results indicate that the commonly used isobar model analysis must be extended to account for the final state interactions required by the three-body unitarity to reanalyze the three-mesons decays of heavy mesons, thereby exploring hybrid or exotic mesons, and signatures of physics beyond the standard model.

hep-ph↗
arXiv2011

On the Determination of \bar{d}/\bar{u} Ratios from Proton-Proton and Proton-Deuteron Drell-Yan Processes

By using the convolution formula derived within the framework of relativistic quantum mechanics, we have examined the Fermi Motion effects on the ratios R_{pd/pp}= sigma^{pd}/(2 sigma^{pp}) between the proton-proton (p-p) and proton-deuteron (p-d) Drell-Yan cross sections. We have found that in the small x_2 < 0.3 region, the Fermi Motion effect is less than 1% and our results for the ratios R_{pd/pp} = sigma^{pd}/(2 sigma^{pp}) agree well with the data at 800 GeV. In the large Bjorken x_2 > about 0.4 region, the p-d Drell-Yan cross sections can be influenced strongly by the Fermi motion effect. At 120 GeV the predicted Fermi Motion effect can enhance the ratios R_{pd/pp} by about 20% at x_2 \sim 0.6 and about a factor of 2.5 at x_2 -> 1.0. Our results suggest that the Fermi motion effect, along with other possible nuclear effects, must be included, in extracting the \bar{d}/\bar{u} ratios in the proton from the experiments on p-p and p-d Drell-Yan processes at large x_2.

hep-ph↗
arXiv2011

Determining pseudoscalar meson photo-production amplitudes from complete experiments

We review the development of the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple-polarization terms involving 16 spin-dependent observables. We examine the different conventions that have been used by different authors, and we present expressions that allow the direct numerical calculation of any pseudoscalar meson photo-production observables with arbitrary spin projections from the Chew-Goldberger-Low-Nambu amplitudes. We use this numerical tool to clarify apparent sign differences that exist in the literature. We also present analytic expressions that determine the recoil baryon polarization, together with examples of their potential use with quasi-4pi detectors to deduce observables. As an illustration of the use of the consistent machinery presented in this review, we carry out a multipole analysis of the gamma p -> K^+ Lambda reaction and examine the impact of recently published polarization measurements. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow chi^2 valley pitted with a very large number of local minima. This results in broad bands of multipole solutions that are experimentally indistinguishable. While these bands have been noticeably narrowed by the inclusion of new polarization measurements, many of the multipoles remain very poorly determined, even in sign, despite the inclusion of data on 8 different observables. We have compared multipoles from recent PWA codes with our model-independent solution bands. The potential accuracy of amplitudes that could be extracted from measurements of all 16 polarization observables has been studied with mock data using the statistical variations that are expected from ongoing experiments.

nucl-th↗
arXiv2011

Results from the N* program at JLab

We discuss the results on the fundamental degrees of freedom underlying the nucleon excitation spectrum and how they evolve as the resonance transitions are investigated with increasingly better space-time resolution of the electromagnetic probe. The search for undiscovered but predicted states continues to be pursued with a vigorous experimental program. While recent data from JLab and elsewhere provide intriguing hints of new states, final conclusions will have to wait for the results of the broad experimental effort currently underway with CLAS, and subsequent analyses involving the EBAC at JLab.

nucl-ex↗
arXiv2010

Calculations of Polarization Observables in Pseudoscalar Meson Photo-production Reactions

In preparation for the extraction of pseudoscalar meson photo-production amplitudes from a new generation of complete experiments, we assemble the relations between experimental observables and the Chew-Goldberger-Low-Nambu amplitudes. We present expressions that allow the direct calculation of matrix elements with arbitrary spin projections and uses these to clarify sign differences that exist in the literature. Comparing to the MAID and SAID analysis codes, we have found that the implied definitions of six double-polarization observables are the negative of what has been used in comparing to recent experimental data.

nucl-th↗
arXiv2010

Extraction of P11 Resonance from pi-N Data and Its Stability

An important question about resonance extraction is how much resonance poles and residues extracted from data depend on a model used for the extraction, and on the precision of data. We address this question with the dynamical coupled-channel (DCC) model developed in Excited Baryon Analysis Center (EBAC) at JLab. We focus on the P11 pi-N scattering. We examine the model-dependence of the poles by varying parameters to a large extent within the EBAC-DCC model. We find that two poles associated with the Roper resonance are fairly stable against the variation. We also develop a model with a bare nucleon, thereby examining the stability of the Roper poles against different analytic structure of the P11 amplitude below pi-N threshold. We again find a good stability of the Roper poles.

nucl-th↗
arXiv2010

Medium Modifications of the Rho Meson in Nuclear Photoproduction

We extend our recent study of dilepton invariant-mass spectra from the decays of rho mesons produced by photon reactions off nuclei. We specifically focus on experimental spectra as recently measured by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility using carbon and iron nuclei. Building on our earlier work we broaden our description to a larger set of observables in order to identify sensitivities to the medium effects predicted by microscopic calculations of the rho spectral function. We compute mass spectra for several target nuclei and study the spectral shape as a function of the 3-momentum of the outgoing lepton pair. We also compute the so-called nuclear transparency ratio which provides an alternative means (and thus consistency check) of estimating the inelastic rho width in the cold nuclear medium.

nucl-th↗
arXiv2010

Extraction of P11 resonances from pi N data

We show that two P11 nucleon resonance poles near the pi Delta threshold, obtained in several analyses, are stable against large variations of parameters within a dynamical coupled-channels analysis based on meson-exchange mechanisms. By also performing an analysis based on a model with a bare nucleon state, we find that this two-pole structure is insensitive to the analytic structure of the amplitude in the region below pi N threshold. Our results are M_pole = (1363^{+9}_{-6} -i79^{+3}_{-5}) MeV and (1373^{+12}_{-10} -i114^{+14}_{-9}) MeV. We also demonstrate that the number of poles in the 1.5 GeV < W < 2 GeV region could be more than one, depending on how the structure of the single-energy solution of SAID is fitted. For three-pole solutions, our best estimated result of a pole near N(1710) listed by Particle Data Group is (1829^{+131}_{-65} -i192^{+88}_{-110}) MeV which is close to the results of several previous analyses. Our results indicate the need of more accurate pi N reaction data in the W > 1.6 GeV region for high precision resonance extractions.

nucl-th↗
arXiv2010

Extraction of Electromagnetic Transition Form Factors for Nucleon Resonances within a Dynamical Coupled-Channels Model

We explain the application of a recently developed analytic continuation method to extract the electromagnetic transition form factors for the nucleon resonances ($N^*$) within a dynamical coupled-channel model of meson-baryon reactions.Illustrative results of the obtained $N^*\rightarrow γN$ transition form factors, defined at the resonance pole positions on the complex energy plane, for the well isolated $P_{33}$ and $D_{13}$, and the complicated $P_{11}$ resonances are presented. A formula has been developed to give an unified representation of the effects due to the first two $P_{11}$ poles, which are near the $πΔ$ threshold, but are on different Riemann sheets. We also find that a simple formula, with its parameters determined in the Laurent expansions of $πN \rightarrow πN$ and $γN \rightarrowπN$ amplitudes, can reproduce to a very large extent the exact solutions of the considered model at energies near the real parts of the extracted resonance positions. We indicate the differences between our results and those extracted from the approaches using the Breit-Wigner parametrization of resonant amplitudes to fit the data.

nucl-th↗
arXiv2010

Disentangling the dynamical origin of P11 Nucleon Resonances

We show that two almost degenerate poles near the $πΔ$ threshold and the next higher mass pole in the $P_{11}$ partial wave of $πN$ scattering evolve from a single bare state through its coupling with $πN$, $ηN$ and $ππN$ reaction channels. This finding provides new information on understanding the dynamical origins of the Roper $N^*(1440)$ and $N^*(1710)$ resonances listed by Particle Data Group. Our results for the resonance poles in other $πN$ partial waves are also presented.

nucl-th↗
arXiv2009

Double and single pion photoproduction within a dynamical coupled-channels model

Within a dynamical coupled-channels model which has already been fixed from analyzing the data of the pi N -> pi N and gamma N -> pi N reactions, we present the predicted double pion photoproduction cross sections up to the second resonance region, W< 1.7 GeV. The roles played by the different mechanisms within our model in determining both the single and double pion photoproduction reactions are analyzed, focusing on the effects due to the direct gamma N -> pi pi N mechanism, the interplay between the resonant and non-resonant amplitudes, and the coupled-channels effects. The model parameters which can be determined most effectively in the combined studies of both the single and double pion photoproduction data are identified for future studies.

nucl-th↗
arXiv2009

Extraction of Transition Form Factors for Nucleon Resonances within a Coupled-Channels Model

We explain how an analytic continuation we have developed recently is applied to determine the residues of the nucleon resonance poles within a dynamical coupled-channel model of meson-baron reactions. A procedure for evaluating the electromagnetic N-N^* transition form factors at resonance poles is developed. Illustrative results of the obtained N^* -> pi N, gamma N transition form factors for P_{11}, P_{33}, and D_{13} nucleon resonances are presented and compared with previous results.

nucl-th↗
arXiv2009

Dynamical Model of Coherent Pion Production in Neutrino-Nucleus Scattering

We study coherent pion production in neutrino-nucleus scattering in the energy region relevant to neutrino oscillation experiments of current interest. Our approach is based on a combined use of the Sato-Lee model of electroweak pion production on a nucleon and the Delta-hole model of pion-nucleus reactions. Thus we develop a model which describes pion-nucleus scattering and electroweak coherent pion production in a unified manner. Numerical calculations are carried out for the case of the 12C target. All the free parameters in our model are fixed by fitting to both total and elastic differential cross sections for pi-12C scattering. Then we demonstrate the reliability of our approach by confronting our prediction for the coherent pion photo-productions with data. Finally, we calculate total and differential cross sections for neutrino-induced coherent pion production, and some of the results are (will be) compared with the recent (forthcoming) data from K2K, SciBooNE and MiniBooNE. We also study effect of the non-locality of the Delta-propagation in the nucleus, and compare the elementary amplitudes used in different microscopic calculations.

nucl-th↗
arXiv2009

Theory Support for the Excited Baryon Program at the Jlab 12 GeV Upgrade

This document outlines major directions in theoretical support for the measurement of nucleon resonance transition form factors at the JLab 12 GeV upgrade with the CLAS12 detector. Using single and double meson production, prominent resonances in the mass range up to 2 GeV will be studied in the range of photon virtuality $Q^2$ up to 12 GeV$^2$ where quark degrees of freedom are expected to dominate. High level theoretical analysis of these data will open up opportunities to understand how the interactions of dressed quarks create the ground and excited nucleon states and how these interactions emerge from QCD. The paper reviews the current status and the prospects of QCD based model approaches that relate phenomenological information on transition form factors to the non-perturbative strong interaction mechanisms, that are responsible for resonance formation.

nucl-th↗
arXiv2009

Electromagnetic Nucleon-to-Delta Transition in Holographic QCD

We study nucleon-to-delta electromagnetic transition form factors and relations between them within the framework of holographic dual model of QCD proposed by Sakai and Sugimoto. In this setup, baryons appear as topological solitons of the five dimensional holographic gauge theory that describes a tower of mesons and their interactions. We find a relativistic extension of the nucleon-delta-vector meson interaction vertices and use these to calculate transition form factors from holographic QCD. We observe that at low momentum transfer, magnetic dipole, electric and Coulomb quadrupole form factors and their ratios follow the patterns expected in the large N_c limit. Our results at this approximation are in reasonable agreement with experiment.

hep-ph↗
arXiv2009

Dynamical coupled-channels analysis of 1H(e,e'pi)N reactions

We have performed a dynamical coupled-channels analysis of available p(e,e'pi)N data in the region of W < 1.6 GeV and Q^2 < 1.45 (GeV/c)^2. The channels included are gamma^* N, pi N, eta N, and pi pi N which has pi Delta, rho N, and sigma N components. With the hadronic parameters of the model determined in our previous investigations of pi N --> pi N, pi pi N reactions, we have found that the available data in the considered W < 1.6 GeV region can be fitted well by only adjusting the bare gamma^* N --> N^* helicity amplitudes for the lowest N^* states in P33, P11, S11 and D13 partial waves. The sensitivity of the resulting parameters to the amount of data included in the analysis is investigated. The importance of coupled-channels effect on the p(e,e' pi)N cross sections is demonstrated. The meson cloud effects, as required by the unitarity conditions, on the gamma^* N --> N^* form factors are also examined. Necessary future developments, both experimentally and theoretically, are discussed.

nucl-th↗
arXiv2009

Dynamical Model of Coherent Electroweak Pion Production Reactions on Nuclei

We have developed a dynamical model for a unified description of the pion-nucleus scattering and photo- and neutrino-induced coherent pion production on nuclei. Our approach is based on a combined use of the Sato-Lee model for the electroweak pion production on a single nucleon and the Delta-hole model of pion-nucleus scattering. Numerical calculations are carried out for the case of the C12 target. After testing our model with the use of the pion photo-production data, we confront our predictions of the neutrino-induced coherent pion production reactions with the recent data from K2K and MiniBooNE.

nucl-th↗