465 research outputs found
Centroids of Gamow-Teller transitions at finite temperature in fp-shell neutron-rich nuclei
The temperature dependence of the energy centroids and strength distributions
for Gamow-Teller (GT) excitations in several fp-shell nuclei is studied.
The quasiparticle random phase approximations (QRPA) is extended to describe GT
states at finite temperature. A shift to lower energies of the GT strength
is found, as compared to values obtained at zero temperature.Comment: 12 pages, contains 3 tables. E-mail: [email protected],
[email protected]
The Friedrichs-Model with fermion-boson couplings II
In this work we present a formal solution of the extended version of the
Friedrichs Model. The Hamiltonian consists of discrete and continuum bosonic
states, which are coupled to fermions. The simultaneous treatment of the
couplings of the fermions with the discrete and continuous sectors of the
bosonic degrees of freedom leads to a system of coupled equations, whose
solutions are found by applying standard methods of representation of bound and
resonant states.Comment: 13 page
Single- and double-beta decay Fermi-transitions in an exactly solvable model
An exactly solvable model suitable for the description of single and
double-beta decay processes of the Fermi-type is introduced. The model is
equivalent to the exact shell-model treatment of protons and neutrons in a
single j-shell. Exact eigenvalues and eigenvectors are compared to those
corresponding to the hamiltonian in the quasiparticle basis (qp) and with the
results of both the standard quasiparticle random phase approximation (QRPA)
and the renormalized one (RQRPA). The role of the scattering term of the
quasiparticle hamiltonian is analyzed. The presence of an exact eigenstate with
zero energy is shown to be related to the collapse of the QRPA. The RQRPA and
the qp solutions do not include this zero-energy eigenvalue in their spectra,
probably due to spurious correlations. The meaning of this result in terms of
symmetries is presented.Comment: 29 pages, 9 figures included in a Postsript file. Submitted to
Physcal Review
Double beta decay to the excited states: experimental review
A brief review on double beta decay to excited states of daughter nuclei is
given. The ECEC(ov) transision to the excited states are discussed in
association with a possible enhancement of the decay rate by several orders of
magnitude.Comment: 5 pages; talk at MEDEX'07 ("Matrix Elements for the Double-beta-decay
Experiments"; Prague, June 11-14, 2007
Giant dipole resonance with exact treatment of thermal fluctuations
The shape fluctuations due to thermal effects in the giant dipole resonance
(GDR) observables are calculated using the exact free energies evaluated at
fixed spin and temperature. The results obtained are compared with Landau
theory calculations done by parameterizing the free energy. The Landau theory
is found to be insufficient when the shell effects are dominating.Comment: 5 pages, 2 figure
A schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a
Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector,
gluon pairs with spin zero are treated as elementary bosons. The model
restricts to mesons with spin zero and to few baryonic states. The
corresponding energy spectrum is discussed. We show that ground state
correlations are essential to describe physical properties of the spectrum at
low energies. Phase transitions are described in an effective manner, by using
coherent states. The appearance of a Goldstone boson for large values of the
interaction strength is discussed, as related to a collective state. The
formalism is extended to consider finite temperatures. The partition function
is calculated, in an approximate way, showing the convenience of the use of
coherent states. The energy density, heat capacity and transitions from the
hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure
Generalized rotational hamiltonians from nonlinear angular momentum algebras
Higgs algebras are used to construct rotational Hamiltonians. The
correspondence between the spectrum of a triaxial rotor and the spectrum of a
cubic Higgs algebra is demonstrated. It is shown that a suitable choice of the
parameters of the polynomial algebra allows for a precise identification of
rotational properties. The harmonic limit is obtained by a contraction of the
algebra, leading to a linear symmetry.Comment: 3 figures, 6 pages, 15 references. Phys. Rev. C (in press, ms
CZ10038
Competition between standard and exotic double beta decays
We discuss the contributions of higher order terms in weak Hamiltonian to the
standard two-neutrino double beta decay. The formalism for the unique first
forbidden transitions has been developed, and it is shown that they can alter
the two-electron energy spectrum. Yet, their effect is too small to screen the
detection of exotic neutrinoless double beta decays, which are candidates for
testing the physics beyond the standard model.Comment: 9 pages, latex, 1ps figures, minor changes, to appear in Phys. Lett.
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