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/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

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

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

S = {minus}1 and S = {minus}2 few-body hypernuclei

Description: The S = {minus}1 and S = {minus}2 few-body bound states are the focus of this discussion. Implications for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not found in conventional, non-strange nuclei are highlighted. TJNAF wave function tests for the S = {minus}1 sector are noted. The need to identify S = {minus}2 hypernuclei to explore the S = {minus}2 strong interaction and to test model predictions is emphasized.
Date: November 1997
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

The {pi}{sup +} Decay of Light Hypernuclei

Description: The observed {pi}{sup +} emission from the weak decay of the {sup 4}{sub {Lambda}}He hypernucleus has been an intriguing puzzle for more than 30 years, because the Lambda decays in free space only by emission of a {pi}{sup {minus}} or a {pi}{sup 0}. We re-examine this puzzling weak decay with our focus upon a decay mechanism involving the {Sigma}{sup +}N {r_arrow} {pi}{sup +}nN decay of a virtual {Sigma}{sup +}, stemming from {Lambda}N to {Sigma}N conversion (mixing) within the hypernucleus. We emphasize the observed energy distribution of the observed {pi}{sup +}s compared to that of {pi}{sup -}s in standard mesonic decay as well as the isotropic angular distribution of the {pi} {sup +}s. Competing suggestions to explain the positive pion weak decay have been offered. A possible search for {pi}{sup +} decay from the other {Lambda} hypernuclei is explored as means to test our hypothesis.
Date: February 19, 1999
Creator: Gibson, B.F.
Partner: UNT Libraries Government Documents Department

The {pi}{sup +}-emission puzzle in 4 over Lambda He decay

Description: The observed {pi}{sup +} emission from the weak decay of 4 over {Lambda} He has long been an intriguing puzzle. Experimentally, the {pi}{sup +} to {pi}{sup {minus}} ratio for 4 over {Lambda} He decay is about 5%. Because mesonic decay modes of the free {Lambda} ({yields} p + {pi}{sup {minus}}, n + {pi}{sup 0}) produce no {pi}{sup +}s, more complicated mechanisms must be responsible for the {pi}{sup +} decay of 4 over {Lambda} He. Dalitz and von Hippel explored two-body decay processes of the type: (1) {Lambda} {yields} {pi}{sup 0} + n decay followed by a {pi}{sup 0} + p {yields} {pi}{sup +} + n charge-exchange reaction, and (2) {Sigma}{sup +} {yields} {pi}{sup +} + n decay following a {Lambda} + p {yields} {Sigma}{sup +} + n conversion. They concluded that neither process could account for even a 1% {pi}{sup +} + n decay as a p-wave process ruled out the promising explanation coming from von Hippel`s calculations, which had found that s-wave {Sigma}{sup +} decay might yield a sufficiently high rate. Cieply and Gal re-examined the charge-exchange contribution and concluded that, although up-to-date input parameters yield a 1.2% branching ratio, the charge-exchange mechanism cannot account for the experimental value of about 5%.
Date: October 1, 1997
Creator: Gibson, B.F. & Timmermans, R.
Partner: UNT Libraries Government Documents Department

Overview of strangeness nuclear physics

Description: Novel as well as puzzling aspects of strangeness (S = {minus}1 and S = {minus}2) nuclear physics are highlighted. Opportunities to gain new insights into hypernuclear spectroscopy, structure, and weak decays and to contribute to the continuing effort to understand the fundamental baryon-baryon force are outlined. Connections to strangeness in heavy-ion reactions and astrophysics are noted.
Date: December 1998
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

Baryon-baryon mixing in hypernuclei

Description: Implications of few-body hypernuclei for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not present in conventional, non strange nuclei are the focus. The need to identify strangeness {minus}2 hypernuclei to test model predictions is emphasized.
Date: May 1998
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

The Lambda Lambda interaction and the reaction Xi plus deuteron goes to neutron plus Lambda plus Lambda{dagger}

Description: Interest in the {Lambda}{Lambda} interaction is partly due to the presence of quark-model predictions for an S = {minus}2 dibaryon, and partly to the interest in the role of the coupling between the {Lambda}{Lambda} and {Xi}{Nu} channels in {Lambda}{Lambda} hypernuclei. This latter effect is expected to be substantially more important than the coupling of the {Nu}{Nu} to the {Nu}{Delta} channel in the S = 0 sector, since the difference in threshold between the {Lambda}{Lambda} and {Xi}{Nu} is only {approx}25 MeV. In the absence of any direct measurement of the {Lambda}{Lambda} amplitude, one must resort to either {Lambda}{Lambda} hypernuclei, or to a reaction with a {Lambda}{Lambda} final-state interaction to determine the {Upsilon}{Upsilon} interaction. In this report the authors present results of a theoretical study of the hypernucleus {sub {Lambda}{Lambda}}{sup 6}He and the reaction {Xi}d {r_arrow} n{Lambda}{Lambda} whereby they examine the sensitivity of the calculations to details of the {Lambda}{Lambda} potential, and the coupling between the {Lambda}{Lambda} and the {Xi}{Nu} channels.
Date: July 1, 1997
Creator: Carr, S.B.; Afnan, I.R. & Gibson, B.F.
Partner: UNT Libraries Government Documents Department