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Physics of few-body lambda hypernuclei

Description: The energies of the particle-stable states in few-body ..lambda.. hypernuclei are summarized. Other topics reviewed include: the role of the hypertriton in determining the spin dependence of the ..lambda..N force, the role of the hypertriton in three-body force investigations, the effect of medium modifications upon ..lambda..N-..sigma..N coupling in the ..lambda..-4 isodoublet and the spin dependence of the ..lambda..N force, the importance of exact equation formalisms in interpreting precision data, and the need for a renewed effort to identify and measure the masses of ..lambda lambda.. hyperfragments. 40 refs., 4 figs., 3 tabs.
Date: January 1, 1987
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

/sub. lambda. //sup 5/He: hadrons or quarks

Description: Nuclear physics has dealt with nuclei and their interactions at interparticle distances which correspond to conditions that might be described as two bags of quarks barely overlapping. Here, where the asymptotically free theories of QCD have yet to provide a calculable picture of observed phenomena, the nuclear physicist has found a modicum of success and simplification in terms of a picture based upon the physically observable baryons and mesons. However, our understanding is far from complete. Because of this and our desire to learn where the transition to the quark-gluon matter picture occurs, we seek measurable effects due to the quark substructure. To that end, we must first define the limits of validity for describing nuclear phenomena in terms of the observed hadrons before evidence for quark-gluon degrees of freedom can be critically evaluated.
Date: January 1, 1985
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Kaon interactions with very light nuclei. [Review, cross sections]

Description: Low energy kaon interactions (both K and anti K) with very light nuclei are reviewed. Limitations upon present K-nucleus studies due to uncertainties in the K-N amplitudes are emphasized along with promising uses. A brief review of some of the many interesting aspects of anti K-nucleus scattering is given. Comparison of the limited anti K-d and anti K-/sup 4/He elastic data with theory is made. The anti Kd ..-->.. ..pi lambda..p reaction is discussed including the possible ..sigma..N virtual bound state. 49 references.
Date: January 1, 1979
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Three-body force in the three-nucleon system

Description: A brief summary of the symposium is presented. Three-nucleon force models are discussed, including the two-pion exchange potential, NN-..delta..N coupled-channels model, and phenomenological parametrization. Relevant experimental data and model calculations are discussed including form factors, binding energies, charge radii, and charge density for /sup 3/H and /sup 3/He. A calculation of the EMC effect for /sup 3/He is also made using Sasakawa's wave function and compared to experimental data obtained at SLAC. The paper ends with discussions of proton-deuteron scattering, investigations at intermediate energies, and QCD efforts to understand the three-body problem. (DWL)
Date: January 1, 1986
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Decay of hypernuclei

Description: The experimental and theoretical situation regarding the mesonic and nonmesonic decays of light hypernuclei is reviewed. Nuclear matter and shell model estimates for nonmesonic decays are compared within the one-boson-exchange model approximation. The importance of strangeness exchange is explored as well as the spin-isospin dependence of the nonmesonic weak decays. 30 refs., 3 tabs.
Date: January 1, 1988
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Hypernuclear physics

Description: A brief overview of progress made in the study of hypernuclear physics is presented. The use of ..lambda..-hypernuclei to study properties of conventional (nonstrange) nuclei is explored. Our knowledge of the hyperon-nucleon force is reviewed. Anecdotal examples of interesting hypernuclear phenomena are discussed. The status of ..sigma..-hypernuclei is considered along with a search for the ''H'' dibaryon. 30 refs., 10 figs.
Date: January 1, 1987
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Hyperon and hypernuclear physics with intense beams

Description: A brief examination of progress in the study of hypernuclear physics and the hyperon-nucleon interaction is presented. The use of ..lambda..-hypernuclei in the study of conventional (nonstrange) nuclei is explored. The status of the hyperon-nucleon force problem is reviewed. Anecdotal results are discussed for baryon numbers 4 and 13. ..sigma..-hypernuclei are discussed. Production of S = -2 hypernuclei is mentioned.
Date: January 1, 1983
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Calculation of electromagnetic observables in few-body systems

Description: An introduction to the calculation of electromagnetic observables in few-body systems is given by studying two examples in the trinucleon system: (1) the elastic electron scattering charge form factor in configuration space and momentum space and (2) the two-body photodisintegration of /sup 3/H leading to a neutron-deuteron final state in a separable potential formalism. In the discussion of charge form factor calculations, a number of related topics are touched upon: the relation of structure in Psi to the properties of simple NN forces, the Faddeev and Schroedinger solution to the harmonic oscillator problem, the Rosenbluth formula for electron scattering from a spin-1/2 nuclear target (e.g., the proton or /sup 3/H), and the charge density operator. Formulae for /sup 3/He and /sup 3/H charge form factors in a central force approximation are given in configuration and momentum space. The physics of these form factors is discussed in light of results from realistic nucleon-nucleon potential model calculations, including the effects of two-pion-exchange three-body force models. Topics covered are the rms charge densities, and the Coulomb energy of /sup 3/He. In the discussion of the /sup 3/H photodisintegration, the Siegert form of the electric dipole operator (in the long wave length limit) is derived as are the separable potential equations which describe the off-shell transition amplitudes which connect nucleon-plus-corrected-pair states. Expressions for the Born amplitudes required to complete the two-body photodisintegration amplitude calculation are given. Numerical results for a model central force problem are discussed and compared with an approximate calculation. Comparisons with /sup 3/H(..gamma..,n)d and /sup 3/He(..gamma..,p)d data are made, and the significant features of the exact theoretical calculation are outlined. 61 refs., 26 figs.
Date: October 1, 1986
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Lambda N-. sigma. N coupling in S-shell Lambda-hypernuclei

Description: The inclusion of Lambda N-..sigma..N coupling in the A = 4 Lambda-hypernuclei is shown to be required in order to obtain proper ordering of the 0/sup +/ ground state and 1/sup +/ spin-flip excited state, when exact four-body equations are solved. It is argued that suppression of the off-diagonal Lambda N-..sigma..N coupling in the A = 5 system, similar to that in the A = 4 isodoublet, should account for the anomalously small binding of /sup 5/He. An alternative quark model explanation is also considered.
Date: January 1, 1984
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

AN HISTORICAL OVERVIEW OF THE IMPORTANCE OF THE WEAK DECAY OF HYPERNUCLEI

Description: Weak decay of hypernuclei, first cousin to the {beta}-decay of conventional, nonstrange nuclei, was initially observed in the 1950s. Pionic decay rates have proven a challenge--to reconcile nuclear decay rates with that of free {lambda} decay. Pauli blocking of the decay nucleon plays an important role. Nonmesonic decay provides our only practical means of exploring the four-fermion, strangeness-changing N{Lambda} {yields} NN weak interaction. The N{Lambda}{rho} vertex can be investigated in no other way. The large momentum transfer in the nonmesonic decay process suggests a means to probe short distance aspects of the interaction, possibly revealing baryon substructure effects. Whether the {Delta}I = 1/2 rule, which governs free {Lambda} decay, also applies to the nonmesonic decay process remains an open question. The free {Lambda} does not decay by emission of a {pi}{sup +}; the {pi}{sup +} decay of {sup 4}He is a puzzle. Finally, the weak decay of strangeness -2 hypernuclei is an important topic, because the pionic decay process is central to current efforts to seek and identify {Lambda}{Lambda} hypernuclei.
Date: January 9, 2001
Creator: GIBSON, B. F.
Partner: UNT Libraries Government Documents Department

Hypernuclear physics: A brief past and bright future

Description: A primary reason for investigating the structure and reactions of baryon systems is to achieve an understanding the fundamental baryon-baryon force in the realm of non-perturbative QCD. Few-baryon systems play an essential role, because one can calculate complete solutions to test a particular baryon-baryon interaction ansatz. Hypernuclei, exotic nuclei containing one or more hyperons (Y = {Lambda}, {Sigma}, or {Xi}) are crucial to this investigation, because they permit one to probe models based upon our experience in the nonstrange sector; they lie outside of the conventional world where our models were developed. That is, we can test whether our sophisticated models of the nucleon-nucleon (NN) interaction extrapolate successfully beyond the zero strangeness region in which the parameters were determined, or whether the models merely interpolate. The presence of the strangeness degree of freedom (flavor) adds a new dimension to our evolving picture of nuclear physics. We shall see that the physics of hypernuclei is both novel and puzzling, stretching our intuition and analysis capability beyond that developed during the more than half century that we have explored conventional nuclear physics. The hypernuclear sector of hadronic physics is not just a simple extension of zero-strangeness phenomena.
Date: October 1, 2000
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

A = 4 0/sup +/ - 1/sup +/ binding-energy difference

Description: The A = 4 ..lambda..-hypernuclei provide a rich source of information about the s-wave properties of the fundamental hyperon-nucleon (YN) force as well as offer a unique opportunity to investigate the complications that arise in calculations of the properties of bound systems in which one baryon (here the ..lambda..) with a given isospin couples strongly to another (the ..sigma..) with a different isospin. The ..lambda../sup 4/H - ..lambda../sup 4/He isodoublet ground-state energies are not consistent with a charge symmetry hypothesis for the YN interaction. The (spin-flip) excitation energies are quite sensitive to the ..lambda..N - ..sigma..N coupling of the YN interaction. In particular, when one represents the free YN interaction in terms of one-channel effective ..lambda..N potentials, the resulting 0/sup +/ (ground) state and 1/sup +/ (excited) spin-flip state are inversely ordered in terms of binding energies, the 1/sup +/ state being more bound. It is the ..sigma.. suppression that results from the reduced strength of the ..lambda..N - ..sigma..N off-diagonal coupling potential when the trinucleon core is restricted to isospin-1/2 which we study here. We find this spin-isospin suppression of the ..lambda..-..sigma.. conversion, which is due to the composite nature of the nuclear cores of the ..lambda../sup 4/H and ..lambda../sup 4/He hypernuclei, to be a significant factor in understanding the 0/sup +/ - 1 /sup +/ binding energy relationship.
Date: January 1, 1982
Creator: Gibson, B. F. & Lehman, D. R.
Partner: UNT Libraries Government Documents Department

The trinucleons: Physical observables and model properties

Description: Our progress in understanding the properties of {sup 3}H and {sup 3}He in terms of a nonrelativistic Hamiltonian picture employing realistic nuclear forces is reviewed. Trinucleon model properties are summarized for a number of contemporary force models, and predictions for physical observables are presented. Disagreement between theoretical model results and experimental results are highlighted.
Date: January 1, 1992
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Nonmesonic decay of the. lambda. in nuclear matter

Description: As a step towards the theoretical investigation of the ratio of nonmesonic to free decay rate of hypernuclei, the pion exchange has been recalculated including consideration of the contribution from rho exchange. (GHT)
Date: January 1, 1982
Creator: McKellar, B.H.J. & Gibson, B.F.
Partner: UNT Libraries Government Documents Department

Remarks on the s-shell. lambda. -hypernuclei

Description: The complexities of the s-shell ..lambda..-hypernuclei (A less than or equal to 5) are explored. Difficulties associated with attempts to describe the hyperon-nucleon (YN) interaction in all such ..lambda..-hypernuclei by simple, effective ..lambda..p and ..lambda..n potentials are examined. The explicit A dependence of the effective YN interaction due to ..lambda..N-..sigma..N coupling and isospin differences among the nuclear core states is investigated. The necessity of using exact four-body theory to calculate small charge-symmetry-breaking effects (in the A = 4 system) using ..lambda..N potentials fitted to free ..lambda..N scattering data is emphasized. Possible use of s-shell hypernuclear binding energies to help distinguish among candidate YN potential parameterizations is discussed.
Date: January 1, 1981
Creator: Gibson, B.F. & Lehman, D.R.
Partner: UNT Libraries Government Documents Department

Few-body hypernuclear constraints

Description: Since the discovery of the first hyperfragment in a balloon flown emulsion stack some two score years ago, physicists have worked to understand how the addition of the strangeness degree of freedom alters the picture of nuclei and the baryon-baryon force. Because the {Lambda} and {Sigma} masses differ markedly from that of the proton and neutron, SU (3) symmetry is broken. How it is broken is a question of importance to the fundamental understanding of the baryon-baryon interaction. New dynamical symmetries, forbidden by the Pauli principle in conventional nuclei, appear. Three-body forces play a more significant role. A binding anomaly in A {equals} 5 as well as a possible spin inversion between ground and excited states in A {equals} 4 appear. Surprisingly narrow structure near the threshold for {Sigma} production has been reported in the {sup 4}He (K{sup {minus}}, {pi}{sup {minus}}) spectrum while no corresponding structure is observed in the companion {sup 4}He(K{sup {minus}}, {pi}{sup +}) spectrum; this has been interpreted as evidence for a {sub {Sigma}}{sup 4}He bound state. Finally, the reported observation of {Lambda}{Lambda}-hypernuclei, in particular {sub {Lambda}{Lambda}}{sup 6}He, bears directly upon the possibilities for the prediction of a bound H particle--the S {equals} {minus}2 dibaryon. Although it is not feasible to invert the analysis and determine the interaction from the data on few-body systems, it is possible to utilize these data to constrain the models, provided one is careful. The author will explore briefly the constraints which the few-body data impose and the level of understanding that has been achieved.
Date: December 31, 1993
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department