7,093 research outputs found
A see-saw scenario of an flavour symmetric standard model
A see-saw scenario for an flavour symmetric standard model is
presented. The latter, compared with the standard model, has an extended field
content adopting now an additional symmetry structure (along with the
standard model symmetry). As before, the see-saw mechanism can be realized in
several models of different types depending on different ways of neutrino mass
generation corresponding to the introduction of new (heavy in general) fields
with different symmetry structures. In the present paper, a general description
of all these see-saw types is made with a more detailed investigation on type-I
models, while for type-II and type-III models a similar strategy can be
followed. As within the original see-saw mechanism, the symmetry structure of
the standard model fields decides the number and the symmetry structure of the
new fields. In a model considered here, the scalar sector consists of three
standard-model-Higgs-like iso-doublets (-doublets) forming together an
-triplet, and three iso-singlets transforming as three singlets (1,
and ) of . In the lepton sector, the three left-handed lepton
iso-doublets form an -triplet, while the three right-handed charged
leptons are either -singlets in one version of the model, or components of
an -triplet in another version. To generate neutrino masses through, say,
the type-I see-saw mechanism, it is natural to add four right-handed neutrino
multiplets, including one -triplet and three -singlets. For an
interpretation, the model is applied to deriving some physics quantities such
as neutrinoless double beta decay effective mass , CP
violation phase and Jarlskog parameter , which can be
verified experimentally.Comment: LaTeX, 31 pages, 12 figures, 6 tables. V3: some parts modifie
Energies of the ground state and first excited state in an exactly solvable pairing model
Several approximations are tested by calculating the ground-state energy and
the energy of the first excited state using an exactly solvable model
with two symmetric levels interacting via a pairing force. They are the BCS
approximation (BCS), Lipkin - Nogami (LN) method, random-phase approximation
(RPA), quasiparticle RPA (QRPA), the renormalized RPA (RRPA), and renormalized
QRPA (RQRPA). It is shown that, in the strong-coupling regime, the QRPA which
neglects the scattering term of the model Hamiltonian offers the best fit to
the exact solutions. A recipe is proposed using the RRPA and RQRPA in
combination with the pairing gap given by the LN method. Applying this recipe,
it is shown that the normal-superfluid phase transition is avoided, and a
reasonably good description for both of the ground-state energy and the energy
of the first excited state is achieved.Comment: 18 pages, 4 figure
Superfluid-normal phase transition in finite systems and its effect on damping of hot giant resonances
Thermal fluctuations of quasiparticle number are included making use of the
secondary Bogolyubov's transformation, which turns quasiparticles operators
into modified-quasiparticle ones. This restores the unitarity relation for the
generalized single-particle density operator, which is violated within the
Hartree-Fock-Bogolyubov (HFB) theory at finite temperature. The resulting
theory is called the modified HFB (MHFB) theory, whose limit of a constant
pairing interaction yields the modified BCS (MBCS) theory. Within the MBCS
theory, the pairing gap never collapses at finite temperature T as it does
within the BCS theory, but decreases monotonously with increasing T. It is
demonstrated that this non-vanishing thermal pairing is the reason why the
width of the giant dipole resonance (GDR) does not increase with T up to T
around 1 MeV. At higher T, when the thermal pairing is small, the GDR width
starts to increase with T. The calculations within the phonon-damping model
yield the results in good agreement with the most recent experimental
systematic for the GDR width as a function of T. A similar effect, which causes
a small GDR width at low T, is also seen after thermal pairing is included in
the thermal fluctuation model.Comment: Invited lecture at the Predeal international summer school in nuclear
physics on ``Collective motion and phase transitions in nuclear systems'', 28
August - 9 September, 2006, Predeal, Romania; 18 pages, 3 figures; to be
published by World Scientific in the proceedings of this schoo
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