9151156
Effect of differences in proton and neutron density distributions on fission barriers
The neutron and proton density distributions obtained in constrained Hartree-Fock-Bogolyubov calculations with the Gogny force along the fission paths of 232Th, 236U, 238U, and 240Pu are analyzed. Significant differences in the multipole deformations of neutron and proton densities are found. The effect on potential energy surfaces and on barrier heights is studied under an additional constraint by imposing similar spatial distributions to neutrons and protons, as assumed in macroscopic-microscopic models.
38420080
Limits on the lifetime of the shape isomer of 238U.
119216511
Microscopic transport theory of nuclear processes
Abstract We formulate a microscopic theory of the decay of a compound nucleus through fission which generalizes earlier microscopic approaches of fission dynamics performed in the framework of the adiabatic hypothesis. It is based on the constrained Hartree–Fock–Bogoliubov procedure and the Generator Coordinate Method, and requires an effective nucleon–nucleon interaction as the only input quantity. The basic assumption is that the slow evolution of the nuclear shape must be treated explicitly, whereas the rapidly time-dependent intrinsic excitations can be treated by statistical approximations. More precisely, we introduce a “reference density” ρ P which represents the slow evolution of the nuclear shape by a reduced density matrix and the state of intrinsic excitations by a canonical distribution at each given shape of the nucleus. The shape of the nuclear density distribution is described by parameters (“generator coordinates” q ), not by “superabundant” degrees of freedom introduced in addition to the complete set of nucleonic degrees of freedom. We first derive a rigorous equation of motion for the reference density ρ P and, subsequently, simplify this equation on the basis of the “Markov approximation”. The temperature T which appears in the canonical distribution is determined by the requirement that, at each time t , the reference density should correctly reproduce the mean excitation energy at given values of the shape parameters q . The resulting equation for the “local” temperature T ( q , t ) must be solved together with the equations of motion obtained for the reduced density matrix R ( q 1 , q 2 ; t ) .
121250172
Microscopic analysis of collective dynamics in low energy fission
Abstract A microscopic description based on the density-dependent Hartree-Fock-Bogolyubov approach with several kinds of constraints is used to extract the characteristic features of the collective dynamics of the fission process at low energy. The dynamical behaviour of the nucleus beyond the second saddle point and a mechanism for scission are deduced. They are analyzed in the framework of a quantum-mechanical treatment of collective dynamics taking into account the variations of the collective inertia.
122686083
FISSION COLLECTIVE DYNAMICS IN A MICROSCOPIC FRAMEWORK
A time-dependent microscopic approach of the fission process based on the constrained HFB approach with the Gogny force and the Hill and Wheeler configuration mixing method is described. Recent results concerning fragment distributions (mass, kinetic and deformation energies) and other fragment observables in 238U obtained with such an approach are presented and discussed.
118436812
Low-lying spectroscopy of a few even-even silicon isotopes investigated by means of the multiparticle-multihole Gogny energy density functional
A multiconfiguration microscopic method has been applied with the Gogny effective interaction to the calculation of low-lying positive-parity states in even-even Si isotopes. The aim of the study is to compare the results of this approach with those of a standard method of GCM type and to get insight into the predictive power of multiconfiguration methods employed with effective nucleonnucleon force taylored to mean-field calculations. It is found that the multiconfiguration approach leads to an excellent description of the low-lying spectroscopy of Si, Si and Si, but gives a systematic energy shift in Si. A careful analysis of this phenomenon shows that this discrepancy originates from too large proton-neutron matrix elements supplied by the Gogny interaction at the level of the approximate resolution of the multiparticle-multihole configuration mixing method done in the present study. These proton-neutron matrix elements enter in the definition of both singleparticle orbital energies and coupling matrix elements. Finally, a statistical analysis of highly excited configurations in Si is performed, revealing exponential convergence in agreement with previous work in the context of the shell model approach. This latter result provides strong arguments towards an implicit treatment of highly excited configurations.
706059
Calculations of prompt-photon production in QCD.
Cross sections for prompt-photon production at collider energies are not strictly inclusive because an isolation requirement is imposed whereby the photon is detected only if accompanied by a restricted amount of hadronic energy. In this paper we provide a consistent treatment of the isolated prompt-photon cross section in QCD perturbation theory. The isolation requirement reduces sensitivity of the cross section to poorly known long-distance fragmentation contributions. However, the normal cancellation of infrared singularities is upset by the isolation cutoffs, rendering the short-distance perturbative calculation highly nontrivial. In our solution we address both the short- and the long-distance issues in detail, showing that well-behaved predictions can be derived for a wide range of isolation parameters. Calculated cross sections are presented at several energies and scale dependence of the cross section is examined.
31181789
Measurement of the B semileptonic branching fraction with lepton tags.
We have used the CLEO II detector and 2.06fb^(-1) of ϒ(4S) data to measure the B-meson semileptonic branching fraction. The B→Xeν momentum spectrum was obtained over nearly the full momentum range by using charge and kinematic correlations in events with a high-momentum lepton tag and an additional electron. We find B(B→Xeν) = (10.49±0.17±0.43)%, with overall systematic uncertainties less than those of untagged single-lepton measurements. We use this result to calculate the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V_(cb) and to set an upper limit on the fraction of ϒ(4S) decays to final states other than BB .
16573131
Breakdown of conventional factorization for isolated photon cross sections.
Using {ital e{sup +}e{sup -} {r_arrow} {gamma} + X} as an example, we show that the conventional factorization theorem of perturbative QCD breaks down for isolated photon cross sections in a specific part of phase space. Implications are discussed.
45469707
Prompt photon production with a polarized deuteron target.
Predictions are presented of the polarization asymmetry in prompt photon production at large transverse momentum in the scattering of longitudinally polarized protons from longitudinally polarized deuteron targets. Data would provide important constraints on the polarization asymmetry of the gluon density in a nucleon. The experiments could take advantage of recent advances in the technology of polarized {sup 6}LiD.
10820318
Implications of bottom-quark cross section data at hadron collider energies.
In next-to-leading-order {ital O}({alpha}{sub {ital s}}{sup 3}) quantum chromodynamics, gluon-gluon interactions are the dominant mechanism for the production of bottom quarks at CERN and Fermilab hadron collider energies at values of momentum transfer currently accessible. There is a notable discrepancy between the data from the Collider Detector at Fermilab and existing {ital O}({alpha}{sub {ital s}}{sup 3}) theoretical calculations. Using these data, in conjunction with data from deep-inelastic lepton scattering, we show that it is possible to determine a new gluon density whose shape differs substantially from that derived from previous fits to data. Other implications of the data are discussed.