41501121
Can four-quark states be easily detected in baryon-antibaryon scattering?
We attempt to explain the experimental sparsity of diquonia candidates given the theoretical abundance of such states. We do this by investigating the lowest-order contributions of such states as intermediates in {ital p{bar p}} scattering into exclusive baryon-antibaryon final states. We find that the contributions depend on the partial widths for the meson-meson decays of the diquonia, and that resonant effects can be easily made to disappear. We conclude that if the meson-meson widths of diquonia are larger than about 50 MeV, most of these states will be extremely difficult to observe in {ital p{bar p}} scattering, for instance. We note that diquonia may offer a convenient means of describing some aspects of the dynamics of baryon-antibaryon scattering.
117025439
On the Monopole Model of Form Factors for Heavy Meson Decays
A pole model for the form factors describing heavy to light meson transitions is constructed, and the results compared with the predictions of the heavy quark effective theory (HQET). While most of the HQET-predicted relationships among form factors hold at the maximum value of q{sup 2}, many are violated, suggesting that the simplest version of the pole model of meson form factors should be used with caution, especially when combined with HQET.
119191937
Photoproduction of meson pairs: First measurement of the polarization observable I**s
Abstract The polarization observable I s , a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. Results for the reaction γ → p → p π 0 η are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry I c . The comparably large asymmetries demonstrate a high sensitivity of I s to the dynamics of the reaction. The sensitivity of these new polarization observables to the contributing partial waves is demonstrated by fits using the Bonn–Gatchina partial wave analysis.
119193204
Strangeness -2 and -3 baryons in a constituent quark model.
We apply a quark model developed in earlier work to the spectrum of baryons with strangeness -2 and -3. The model describes a number of well-established baryons successfully, and application to cascade baryons allows the quantum numbers of some known states to be deduced. [JLAB-THY-07-728].
120605148
Partial-wave analysis of lepton-pair annihilation into two photons
The partial-wave (Jp) composition of the process l+l− → γγ is analyzed in lowest order quantum electrodynamics. The results for low-momentum leptons are as expected, the partial wave with Jp = 0− being dominant. At high energies, the partial wave with Jp = 2+ has the largest single partial cross section. At all energies, it is found that lepton pairs in the triplet state give rise exclusively to positive-parity final states, whereas pairs in the singlet state give rise to negative-parity final states.Some features of the differential cross section are also explained in terms of the spin state of the annihilating lepton pair. Possible implications for e+e− → γγ → hadrons and hadrons are briefly discussed.
125011745
Heavy quark effective theory
After a brief introduction, the results of application of the heavy quark effective theory to semileptonic decays of beta mesons are discussed. A nearly-model-independent extraction of {vert_bar}V{sub cb}{vert_bar} from data is described. Application of the effective theory to inclusive semileptonic decays of heavy mesons, in semileptonic decays of heavy baryons, and to the nonleptonic decays of hadrons are outlined. The authors conclude by mentioning other areas of research in this very active field.
11657024
Critical coupling for dynamical chiral-symmetry breaking.
We study chiral-symmetry breaking in a model of quantum chromodynamics (QCD). In our study we make a critical analysis of an approximation that is often used in QCD and other theories to simplify the Schwinger-Dyson equation for the fermion self-energy. Solving the Schwinger-Dyson equation numerically, without employing the approximation, we find that chiral symmetry is restored when the running coupling constant at zero momentum transfer falls below 0.890.
8084451
Renormalization and chiral symmetry breaking in quenched QED in arbitrary covariant gauges.
We extend a previous Landau-gauge study of subtractive renormalization of the fermion propagator Dyson-Schwinger equation in strong-coupling, quenched four-dimensional QED to {ital arbitrary} covariant gauges. We use the fermion-photon proper vertex proposed by Curtis and Pennington with an additional correction term included to compensate for the small gauge dependence induced by the ultraviolet regulator. We discuss the chiral limit and the onset of dynamical chiral symmetry breaking in the presence of nonperturbative renormalization. We extract the critical coupling in several different gauges and find evidence of a small residual gauge dependence in this quantity. {copyright} {ital 1996 The American Physical Society.}
32389654
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator?
We have studied a model Dyson-Schwinger equation for the quark propagator, constructed using an Ansatz for the gluon propagator of the form D(q) {approximately} q{sup 2}/[(q{sup 2}){sup 2} + b{sup 4}] and two Ansatze for the quark-gluon vertex: the minimal Ball-Chiu and the modified form suggested by Curtis and Pennington. The aim was to determine whether such a form of the gluon propagator, which was suggested by a number of authors and which recent lattice simulations of QCD suggest may be plausible, can support dynamical chiral symmetry breaking and ensure quark confinement. The form of the gluon propagator at small space-like momenta is crucial to the nature of the strong interaction spectrum but is presently unknown and information gathered in such studies is invaluable in supporting or invalidating given hypotheses. It was found that there is a critical value of b = b{sub c} such that the model does not support dynamical chiral symmetry breaking for b > b{sub c}. Further, it was shown that this form of gluon propagator cannot confine quarks. As a consequence this form represents a physically unreasonable model. In addition, these results formed the basis for an invited presentation at a workshop on quantum infraredmore » physics and will be published in the proceedings.« less
12221906
Measurements of discrete nuclear reactions induced by a radioactive 8Li beam.
Energy- and angle-resolved nuclear reactions, primarily one-nucleon stripping, have been observed for targets of CD{sub 2}, Be, and C, using a radioactive {sup 8}Li beam ({ital T}{sub 1/2}=842 ms) at {ital E}=14.3 MeV. The ({sup 8}Li,{sup 7}Li) neutron transfer reaction populating discrete final states was identified and angular distributions were obtained. This direct-reaction channel has large cross sections (10--100 mb/sr) due, in part, to the unique nuclear properties of {sup 8}Li. Implications for astrophysical abundances of Li isotopes may be significant.
2502593
Dependence of the cross section for inclusive pion double charge exchange on nuclear mass and charge.
Total reaction cross sections for inclusive pion double charge exchange have been obtained at incident pion energies of 180 and 240 MeV for /sup 4/He, /sup 6.7/Li, /sup 9/Be, /sup 12/C, /sup 16/O, /sup 40/Ca, /sup 103/Rh, and /sup 208/Pb. An explanation of the dependence of these cross sections upon the mass and charge of the target nucleus and the charge of the incident pion is suggested in terms of a phenomenological model in which successive single-charge-exchange processes compete with more probable processes such as quasifree scattering.