539 research outputs found
Structure of the vacuum states in the presence of isovector and isoscalar pairing correlations
The long standing problem of proton-neutron pairing and, in particular, the
limitations imposed on the solutions by the available symmetries, is revisited.
We look for solutions with non-vanishing expectation values of the proton, the
neutron and the isoscalar gaps. For an equal number of protons and neutrons we
find two solutions where the absolute values of proton and neutrons gaps are
equal but have the same or opposite sign. The behavior and structure of these
solutions differ for spin saturated (single l-shell) and spin unsaturared
systems (single j-shell). In the former case the BCS results are checked
against an exact calculation.Comment: 19 pages, 5 postscript figure
Triaxial quadrupole deformation dynamics in sd-shell nuclei around 26Mg
Large-amplitude dynamics of axial and triaxial quadrupole deformation in
24,26Mg, 24Ne, and 28Si is investigated on the basis of the quadrupole
collective Hamiltonian constructed with use of the constrained
Hartree-Fock-Bogoliubov plus the local quasiparticle random phase approximation
method. The calculation reproduces well properties of the ground rotational
bands, and beta and gamma vibrations in 24Mg and 28Si. The gamma-softness in
the collective states of 26Mg and 24Ne are discussed. Contributions of the
neutrons and protons to the transition properties are also analyzed in
connection with the large-amplitude quadrupole dynamics.Comment: 16 pages, 18 figures, submitted to Phys. Rev.
Anomalous rotational-alignment in N=Z nuclei and residual neutron-proton interaction
Recent experiments have demonstrated that the rotational-alignment for the
nuclei in the mass-80 region is considerably delayed as compared to the
neighboring nuclei. We investigate whether this observation can be
understood by a known component of nuclear residual interactions. It is shown
that the quadrupole-pairing interaction, which explains many of the delays
known in rare-earth nuclei, does not produce the substantial delay observed for
these nuclei. However, the residual neutron-proton interaction which is
conjectured to be relevant for nuclei is shown to be quite important in
explaining the new experimental data.Comment: 4 pages, 3 figures, final version accepted by Phys. Rev. C as a Rapid
Communicatio
Microscopic description of large-amplitude shape-mixing dynamics with inertial functions derived in local quasiparticle random-phase approximation
On the basis of the adiabatic self-consistent collective coordinate method,
we develop an efficient microscopic method of deriving the five-dimensional
quadrupole collective Hamiltonian and illustrate its usefulness by applying it
to the oblate-prolate shape coexistence/mixing phenomena in proton-rich
68,70,72Se. In this method, the vibrational and rotational collective masses
(inertial functions) are determined by local normal modes built on constrained
Hartree-Fock-Bogoliubov states. Numerical calculations are carried out using
the pairing-plus-quadrupole Hamiltonian including the quadrupole-pairing
interaction. It is shown that the time-odd components of the moving mean-field
significantly increase the vibrational and rotational collective masses in
comparison with the Inglis-Belyaev cranking masses. Solving the collective
Schroedinger equation, we evaluate excitation spectra, quadrupole transitions
and moments. Results of the numerical calculation are in excellent agreement
with recent experimental data and indicate that the low-lying states of these
nuclei are characterized as an intermediate situation between the
oblate-prolate shape coexistence and the so-called gamma unstable situation
where large-amplitude triaxial-shape fluctuations play a dominant role.Comment: 17 pages, 16 figures, Submitted to Phys. Rev.
Correlation energy contribution to nuclear masses
The ground state correlation energies associated with collective surface and
pairing vibrations are calculated for Pb- and Ca-isotopes. It is shown that
this contribution, when added to those predicted by one of the most accurate
modern nuclear mass formula (HFBCS MSk7 mass formula), reduces the associated
rms error by an important factor, making mean field theory, once its time
dependence is taken into account, a quantitative predictive tool for nuclear
masses.Comment: 4 pages, 2 figures, RevTeX
Parametrization of the octupole degrees of freedom
A simple parametrization for the octupole collective variables is proposed
and the symmetries of the wave functions are discussed in terms of the
solutions corresponding to the vibrational limit. [PACS: 21.60Ev, 21.60.Fw,
21.10.Re]Comment: 14 page
Anharmonic properties of double giant dipole resonance
A systematic microscopic study of the anharmonic properties of the double
giant dipole resonance (DGDR) has been carried out, for the first time, for
nuclei with mass number spanning the whole mass table. It is concluded that
the corrections of the energy centroid of the and
components of the DGDR from its harmonic limit are negative, have a value of
the order of few hundred keV and follow an dependence.Comment: 4 pages, 2 figure
Hadronic Contributions to the Photon Vacuum Polarization and their Role in Precision Physics
I review recent evaluations of the hadronic contribution to the shift in the
fine structure constant and to the anomalous magnetic moment of the muon.
Substantial progress in a precise determination of these important observables
is a consequence of substantially improved total cross section measurement by
the CMD-2 and BES II collaborations and an improved theoretical understanding.
Prospects for further possible progress is discussed.Comment: 17 pages 7 figures 2 tables, update: incl. CMD-2 data, reference
Lowest-lying Tetra-Quark Hadrons in Anisotropic Lattice QCD
We present a detailed study of lowest-lying hadrons in
quenched improved anisotropic lattice QCD. Using the and
diquark-antidiquark local and smeared operators, we attempt to isolate the
signal for and states in two flavour
QCD. In the chiral limit of light-quark mass region, the lowest scalar
state is found to have a mass, MeV, which is slightly
lower than the experimentally observed . The results from our
variational analysis do not indicate a signature of a tetraquark resonance in
I=1 and I=2 channels. After the chiral extrapolation the lowest
state is found to have a mass, MeV. We analysed the
static potential extracted form a tetraquark Wilson loop and illustrated
the behaviour of the state as a bound state, unbinding at some critical
diquark separation. From our analysis we conclude that scalar system
appears as a two-pion scattering state and that there is no spatially-localised
state in the light-quark mass region.Comment: 9 pages, 10 figure
Rotational states in deformed nuclei: An analytic approach
The consequences of the spontaneous breaking of rotational symmetry are
investigated in a field theory model for deformed nuclei, based on simple
separable interactions. The crucial role of the Ward-Takahashi identities to
describe the rotational states is emphasized. We show explicitly how the rotor
picture emerges from the isoscalar Goldstone modes, and how the two-rotor model
emerges from the isovector scissors modes. As an application of the formalism,
we discuss the M1 sum rules in deformed nuclei, and make connection to
empirical information.Comment: 19 pages, 9 figure
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