37 Matching Results

Search Results

Advanced search parameters have been applied.

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

Intrinsic operators for the electromagnetic nuclear current

Description: The intrinsic electromagnetic nuclear meson exchange charge and current operators arising from a separation of the center-of-mass motion are derived for a one-boson-exchange model for the nuclear interaction with scalar, pseudoscalar and vector meson exchange including leading order relativistic terms. Explicit expressions for the meson exchange operators corresponding to the different meson types are given in detail for a two-nucleon system. These intrinsic operators are to be evaluated between intrinsic wave functions in their center-of-mass frame.
Date: September 1, 1996
Creator: J. Adam, Jr. & Arenhovel, H.
Partner: UNT Libraries Government Documents Department

Relativistic calculation of the triton binding energy and its implications

Description: First results for the triton binding energy obtained from the relativistic spectator or Gross equation are reported. The Dirac structure of the nucleons is taken into account. Numerical results are presented for a family of realistic OBE models with off-shell scalar couplings. It is shown that these off-shell couplings improve both the fits to the two-body data and the predictions for the binding energy.
Date: July 1, 1996
Creator: Stadler, Alfred & Gross, Franz
Partner: UNT Libraries Government Documents Department

Relativistic Few Body Calculations

Description: A modern treatment of the nuclear few-body problem must take into account both the quark structure of baryons and mesons, which should be important at short range, and the relativistic exchange of mesons, which describes the long range, peripheral interactions.A way to model both of these aspects is described.The long range, peripheral interactions are calculated using the spectator model, a general approach in which the spectators to nucleon interactions are put on their mass-shell. Recent numerical results for a relativistic OBE model of the NN interaction, obtained by solving a relativistic equation with one-particle on mass-shell, will be presented and discussed.Two meson exchange models, one with only four mesons (pi, sigma, rho, omega) but with a 25% admixture of gamma^5 coupling for the pion, and a second with six mesons (pi, sigma, rho, omega, delta, eta) but pure gamma^5 gamma^micro pion coupling, are shown to give very good quantitative fits to the NN scattering phase s
Date: January 1, 1988
Creator: Gross, Franz
Partner: UNT Libraries Government Documents Department

Effects of relativistic dynamics in pp {r_arrow} pp{pi}{sup 0} near threshold

Description: The cross-section for threshold {pi}{sup 0} production in proton-proton collisions is evaluated in the framework of the covariant spectator description. The negative energy intermediate states are included non-perturbatively and seen to yield a considerably smaller contribution, when compared to perturbative treatments. A family of OBE-models with different off-shell scalar coupling is considered.
Date: February 1, 1997
Creator: J. Adam, Jr.; Stadler, Alfred; Pena, M.T. & Gross, Franz
Partner: UNT Libraries Government Documents Department

Comment on ''Valence QCD: Connecting QCD to the quark model''

Description: The author criticize certain conclusions about the physics of hadrons drawn from a ``valence QCD'' approximation to QCD. Lattice QCD is not just useful as a technique for calculating strong interaction observables like the proton mass: it can also be used to help understand QCD. This is the goal of the work described in reference 1. Its authors present a field theory which they call valence QCD (vQCD) which they hope can be identified with the valence quark model. The key feature built into vQCD is a form of suppression of Z-graphs, i.e., of quarks propagating backward in time. The authors make sound arguments for the importance of trying to capture the essence of the quark model in a field-theoretic framework, and present some interesting results (both theoretical and numerical) on vQCD. This comment is not directed at the goals of vQCD but rather at certain conclusions about the physics of hadrons which the authors have drawn from their work which the author considers unjustified. Foremost among these is the claim highlighted in their abstract that baryon hyperfine interactions are ``largely attributed to the Goldstone boson exchanges between the quarks'' and not to standard one-gluon-exchange (OGE) forces.
Date: August 1, 1999
Creator: Isgur, Nathan
Partner: UNT Libraries Government Documents Department

Relativistic theory of few body systems and their electromagnetic interactions

Description: Recent results obtained from the manifestly covariant spectator theory are summarized. A recent unpublished best fit to NN scattering using 13 OBE parameters gives x{sup 2}=2.25 and a triton binding energy of {minus}8.491 MeV. This agreement is possible because of the existence of off-shell coupling of the phenomenological, intermediate range effective scalar exchanges (both isoscalar and isovector). The previously published results for the deuteron form factors are reviewed, and new, preliminary results for electrodisintegration of the deuteron are presented.
Date: May 1, 1998
Creator: Gross, Franz
Partner: UNT Libraries Government Documents Department

[Topics in theoretical physics]. Progress report

Description: This report describes the most significant results to emerge from recent work done by the W and M group, and the Ph.D. thesis work of Rick Kahler, a student who just received his degree in September, 1996. These include results from the first complete relativistic calculation of the three body binding energy, a new covariant calculation of the deuteron form factors, and a study of the origin of the effective scalar exchanges which are needed in the OBE models.
Date: November 1, 1998
Partner: UNT Libraries Government Documents Department

Calculations of anti pp. -->. anti yy near threshold

Description: The reactions anti pp ..-->.. anti yy will soon be studied near threshold at LEAR. Comparison of theoretical calculations with the measured differential cross sections, polarizations and spin correlations may help to determine the role of various meson exchange terms and the extent to which quark degrees of freedom are important. Calculations along these lines are in progress and are described briefly.
Date: January 1, 1982
Creator: Eisenstein, R. A.
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

Relativistic scalar-vector models of the N-N and N-nuclear interactions

Description: This paper for the Proceedings of Conference an Anti-Nucleon and Nucleon-Nucleus Interactions summarizes work by the principal investigator and his collaborators on the nucleon-nucleon (N-N) and nucleon-nuclear (N-eta) interactions. It draws heavily on a paper presented at the Many Body Conference in Rome in 1972 but also includes a brief review of our phenomenological N-eta interaction studies. We first summarize our 48-49 generalized scalar-vector meson field theory model of the N-N interactions. This is followed by a brief description of our phenomenological work in the 50's on the N-eta interaction sponsored by the Atomic Energy Commission (the present DOE). This work finally led to strong velocity dependent potentials with spin orbit and isospin terms for shell and optical model applications. This is followed by a section on the Emergence of One-Boson Exchange Models describing developments in the 60's of quantitative generalized one boson exchange potentials (GOBEP) including our purely relativistic N-N analyses. Then follows a section on the application of this meson field model to the N-eta interaction, in particular to spherical closed shell nuclei. This work was sponsored by AFOSR but funding was halted with the Mansfield amendment. We conclude with a discussion of subsequent collateral work by former colleagues and by others who have converged upon scalar-vector relativistic models of N-N, antiN-N, N-eta and antiN-eta interactions and some lessons learned from this extended endeavor. 61 refs.
Date: January 1, 1985
Creator: Green, A.E.S.
Partner: UNT Libraries Government Documents Department

Polarization phenomena in two body systems. [Review]

Description: A review is given of strong interactions at very low, low, intermediate, and high energies over the range 6.14 MeV to 150 GeV/c with regard to polarization phenomena in two-body systems. From the one-pion-exchange model to the theory that can possibly relate to all the phenomena, namely, quantum electrodynamics the review pointed to a unified explanation for the interactions under study. 46 references. (JFP)
Date: January 1, 1978
Creator: Thomas, G.H.
Partner: UNT Libraries Government Documents Department

Energy difference of T=1 and T=0 J[sup [pi]]=0[sup [minus]] states in [sup 16]O: Effects of the tensor interaction, configuration mixing, and density-dependent Dirac spinors

Description: It has been shown in the past that in a shell-model calculation, restricted to l[h bar][omega] lp-lh configurations, the energy splitting between the lowest J[sup [pi]]=0[sup [minus]], T=0 and J[sup [pi]]=0[sup [minus]], T=1 states in [sup 16]O is mainly due to the tensor component of the nucleon-nucleon interaction. In this work, the calculation is extended to include 3[h bar][omega] 2p-2h and 3p-3h configurations. Realistic G matrices derived from various One-Boson-Exchange potentials are used into account the effects of the Dirac-Brueckner-Hartree-Fock approach, i.e., a modification of the Dirac spinors for the nucleons in the medium. It is found that the splitting is rather sensitive to the long-range components of the isovector tensor force. The splitting is reduced considerably by including 2p-2h configurations, in particular.
Date: January 1, 1992
Creator: Zheng, Dao-Chen (Washington Univ., Seattle, WA (United States). Inst. for Nuclear Theory Arizona Univ., Tucson, AZ (United States). Dept. of Physics); Zamick, L. (Rutgers--the State Univ., Piscataway, NJ (United States). Dept. of Physics and Astronomy) & Muether, H. (Tuebingen Univ. (Germany). Inst. fuer Theoretische Physik)
Partner: UNT Libraries Government Documents Department

Decay constants of pseudoscalar mesons containing heavy quarks

Description: The QCD sum-rules of Shifman et al. for n-th order moments are applied to the determination of the decay constants of pseudoscalar mesons containing a heavy quark (c or b). The general case when Q/sup 2/, the squared momentum transfer, is non-zero is considered. The stability of the sum-rules against variations in both Q/sup 2/ and n is discussed.
Date: January 1, 1981
Creator: Mathur, V. S. & Yamawaki, M. T.
Partner: UNT Libraries Government Documents Department

One photon exchange processes and the calibration of polarization of high energy protons

Description: Polarization phenomena in small momentum transfer high energy one-photon exchange processes in the reaction p + A ..-->.. X + A where A is a complex nucleus and X is anything are examined. It is shown that these polarizations can be related directly to photoproduction polarization effects in the reaction ..gamma.. + p ..-->.. X at low energies. Explicit formulae are written for polarization effects in the case where X ..-->.. ..pi../sup 0/ + p.
Date: January 1, 1978
Creator: Margolis, B. & Thomas, G.H.
Partner: UNT Libraries Government Documents Department

Deuteron forward photodisintegration: meson currents and relativity

Description: The few-nucleon problem in nuclear physics and the few-electron problem in atomic physics are shown to possess similarities. Relativistic aspects of the latter are reviewed. The radiative decay of the /sup 3/P/sub 1/ excited state of helium-like ions to the /sup 1/S/sub 0/ ground state is shown to be a theoretical analogue of low-energy deuteron forward photodisintegration. Both have large relativistic components. The extended Siegert's theorem, which permits application of Siegert's technique to arbitrary photon wave lengths, is applied to both transitions. Physical arguments for the two processes are stressed, and the relevance of interaction currents is discussed. 28 references.
Date: January 1, 1983
Creator: Friar, J.L.
Partner: UNT Libraries Government Documents Department

Deep-inelastic lepton scattering by nuclei. The pion-exchange model

Description: Quark constituents of nucleons were discovered in deep-inelastic electron scattering. It is not surprising therefore that the nuclear effects in deep-inelastic lepton scattering first observed by the European Muon Collaboration (EMC) have spawned many suggestions of revolutionary changes in our views of nuclear structure. Perhaps quarks play a far more active role in nuclear dynamics than the conventional theory provides. Before following the natural appeal of radical conclusions we believe it is important to examine in quantitative detail the consequences of the conventional nuclear model, in which the nucleon-nucleon potential features a one-pion exchange tail at large distances, an intermediate range attraction dominated by two-pion exchange and a strong short-range phenomenological repulsion. For this purpose let us assume that quarks are strictly confined to single hadrons, and that the quark structure of the hadrons in a nucleus is not affected by their environment. These assumptions allow a consistent description of deep-inelastic lepton-nucleus scattering, and the quantitative consequences agree well with available data. 15 refs.
Date: January 1, 1985
Creator: Coester, F. & Berger, E.L.
Partner: UNT Libraries Government Documents Department

pi. pi. scattering by pole extrapolation methods. [2. 67 GeV/c, angular distribution, Chew-Low and Durr-Pilkum methods, OPE model, isospin]

Description: A 25-inch hydrogen bubble chamber was used at the Lawrence Berkeley Laboratory Bevatron to produce 300,000 pictures of ..pi../sup +/p interactions at an incident momentum of the ..pi../sup +/ of 2.67 GeV/c. The 2-prong events were processed using the FSD and the FOG-CLOUDY-FAIR data reduction system. Events of the nature ..pi../sup +/p ..-->.. ..pi../sup +/p..pi../sup 0/ and ..pi../sup +/p ..-->.. ..pi../sup +/..pi../sup +/n with values of momentum transfer to the proton of -t less than or equal to 0.238 GeV/sup 2/ were selected. These events were used to extrapolate to the pion pole (t = m/sub ..pi..//sup 2/) in order to investigate the ..pi.. ..pi.. interaction with isospins of both T = 1 and T = 2. Two methods were used to do the extrapolation: the original Chew-Low method developed in 1959 and the Durr-Pilkuhn method developed in 1965 which takes into account centrifugal barrier penetration factors. At first it seemed that, while the Durr-Pilkuhn method gave better values for the total ..pi.. ..pi.. cross section, the Chew-Low method gave better values for the angular distribution. Further analysis, however, showed that if the requirement of total OPE (one-pion-exchange) were dropped, then the Durr-Pilkuhn method gave more reasonable values of the angular distribution as well as for the total ..pi.. ..pi.. cross section.
Date: December 9, 1977
Creator: Lott, F.W. III
Partner: UNT Libraries Government Documents Department

EMC effect: asymptotic freedom with nuclear targets

Description: General features of the EMC effect are discussed within the framework of quantum chromodynamics as expressed via the operator product expansion and asymptotic freedom. These techniques are reviewed with emphasis on the target dependence. 22 references.
Date: January 1, 1984
Creator: West, G.B.
Partner: UNT Libraries Government Documents Department

Study of the reaction. gamma. p. -->. rho/sup -/. delta. /sup + +/ at 20 GeV

Description: A clean sample of ..gamma..p ..-->.. rho/sup -/ ..delta../sup + +/ events has been isolated in a hydrogen bubble chamber experiment having a sensitivity of 2.4 events/nb. The experiment uses a narrow band, 20 GeV photon beam produced by backscattering laser light from the Stanford Linear Accelerator's electron beam. A large lead-glass photon detector was used to measure ..pi../sup 0/'s from the rho/sup -/ ..-->.. ..pi../sup -/..pi../sup 0/ decays. We study the s and t dependence of the ..gamma..p ..-->.. rho/sup -/ ..delta../sup + +/ production process and the spin density matrix elements for the decay of the ..delta../sup + +/. These data show that this reaction is not mediated by pure one pion exchange. This result brings in doubt previous measurements of the rho ..-->.. ..pi gamma.. decay width which use a one pion exchange analysis of the ..gamma..p ..-->.. rho/sup -/ ..delta../sup + +/ reaction. 14 references.
Date: June 26, 1984
Partner: UNT Libraries Government Documents Department

Nucleon-antinucleon interaction

Description: The current status of our understanding of the low energy nucleon-antinucleon (N anti N) interaction is reviewed. We compare several phenomenological models which fit the available N anti N cross section data. The more realistic of these models employ an annihilation potential W(r) which is spin, isospin and energy dependent. The microscopic origins for these dependences are discussed in terms of quark rearrangement and annihilation processes. It is argued that the study of N anti N annihilation offers a powerful means of studying quark dynamics at short distances. We also discuss how one may try to isolate coherent meson exchange contributions to the medium and long range part of the N anti N potential. These pieces of the N anti N interaction are calculable via the G-parity transformation from a model for the NN potential; their effects are predicted to be seen in N anti N spin observables, to be measured at LEAR. The possible existence of quasi-stable bound states or resonances of the anti N plus one or more nucleons is discussed, with emphasis on few-body systems. 42 references.
Date: January 1, 1983
Creator: Dover, C.B.
Partner: UNT Libraries Government Documents Department