10,014 research outputs found

### Clustering aspects and the shell model

In this talk I shall discuss the clustering aspect and the shell model. I
shall first discuss the $\alpha$-cluster aspects based on the shell model
calculations. Then I shall discuss the spin zero ground state dominance in the
presence of random interactions and a new type of cluster structure for
fermions in a single-$j$ shell in the presence of only pairing interaction with
the largest multiplicity.Comment: 10 pages and 3 figures. International symposium in Nara, to appear in
Nuclear Physics

### Excitation Spectrum in the Friedberg-Lee Model

The excitation spectrum of the nucleon with the spin 1/2 is examined by using
the Friedberg-Lee model containing the constituent quark and the scalar meson.
An appropriate way of quantization for the non-linear meson field is employed
by taking account of the non-topological soliton existed in the classical
level.
Our model space for the nucleon resonances includes the three-quark plus
one-meson state in addition to the pure three-quark state.
The excitation spectrum in this model space shows that the positive parity
state appears as the first excited state associated with the 0s-excitation of
the scalar meson.
The meson excitation also generates the additional negative parity state
apart from the well-known 0p-excitation of the quark.Comment: 12 pages including 4 figures(eps). Prog. Theor. Phys. in pres

### Exchange Currents for Hypernuclear Magnetic Moments

The meson(K and $\pi$) exchange currents for the hypernuclear magnetic
moments are calculated using the effective Lagrangian method. The seagull
diagram, the mesonic diagram and the $\Sigma^0$-excitation diagram are
considered. The $\Lambda$-N exchange magnetic moments for the
${}^5_{\Lambda}He$, ${}^6_{\Lambda}He$ and ${}^6_{\Lambda}Li$ are calculated
employing the harmonic oscillator shell model. It is found that the two-body
correction is about -9% of the single particle value for ${}^5_{\Lambda}He$.
The $\pi$ exchange current, induced only in the $\Sigma^0$-excitation diagram,
is found to give dominant contribution for the isovector magnetic moments of
hypernuclei with A=6.Comment: 11pp, LaTeX, 7 EPS figures, uses epsf.st

### An alternative marginal likelihood estimator for phylogenetic models

Bayesian phylogenetic methods are generating noticeable enthusiasm in the
field of molecular systematics. Many phylogenetic models are often at stake and
different approaches are used to compare them within a Bayesian framework. The
Bayes factor, defined as the ratio of the marginal likelihoods of two competing
models, plays a key role in Bayesian model selection. We focus on an
alternative estimator of the marginal likelihood whose computation is still a
challenging problem. Several computational solutions have been proposed none of
which can be considered outperforming the others simultaneously in terms of
simplicity of implementation, computational burden and precision of the
estimates. Practitioners and researchers, often led by available software, have
privileged so far the simplicity of the harmonic mean estimator (HM) and the
arithmetic mean estimator (AM). However it is known that the resulting
estimates of the Bayesian evidence in favor of one model are biased and often
inaccurate up to having an infinite variance so that the reliability of the
corresponding conclusions is doubtful. Our new implementation of the
generalized harmonic mean (GHM) idea recycles MCMC simulations from the
posterior, shares the computational simplicity of the original HM estimator,
but, unlike it, overcomes the infinite variance issue. The alternative
estimator is applied to simulated phylogenetic data and produces fully
satisfactory results outperforming those simple estimators currently provided
by most of the publicly available software

### Description of even-even triaxial Nuclei within the Coherent State and the Triaxial Rotation-Vibration Models

The coherent state model (CSM) and the triaxial rotation-vibration model
(TRVM) are alternatively used to describe the ground, gamma and beta bands of
228Th. CSM is also applied to the nuclei 126Xe and 130Ba, which were recently
considered in TRVM. The two models are compared with respect to both their
underlying assumptions and to their predicted results for energy levels and E2
branching ratios. Both models describe energies and quadrupole transitions of
228Th equally well and in good agreement with experiment, if the 0$_3^+$ level
at 1120 keV is interpreted as the head of the beta band. The other two 0$^+$
levels at 832 and 939 keV are most likely not of a pure quadrupole vibration
nature as has already been pointed out in the literature.Comment: 31 pages, RevTeX, 6 figure

### Classification of states of single-$j$ fermions with $J$-pairing interaction

In this paper we show that a system of three fermions is exactly solvable for
the case of a single-$j$ in the presence of an angular momentum-$J$ pairing
interaction. On the basis of the solutions for this system, we obtain new sum
rules for six-$j$ symbols. It is also found that the "non-integer" eigenvalues
of three fermions with angular momentum $I$ around the maximum appear as
"non-integer" eigenvalues of four fermions when $I$ is around (or larger than)
$J_{\rm max}$ and the Hamiltonian contains only an interaction between pairs of
fermions coupled to spin $J=J_{\rm max}=2j-1$. This pattern is also found in
five and six fermion systems. A boson system with spin $l$ exhibits a similar
pattern.Comment: to be published in Physical Review

### General pairing interactions and pair truncation approximations for fermions in a single-j shell

We investigate Hamiltonians with attractive interactions between pairs of
fermions coupled to angular momentum J. We show that pairs with spin J are
reasonable building blocks for the low-lying states. For systems with only a J
= Jmax pairing interaction, eigenvalues are found to be approximately integers
for a large array of states, in particular for those with total angular momenta
I le 2j. For I=0 eigenstates of four fermions in a single-j shell we show that
there is only one non-zero eigenvalue. We address these observations using the
nucleon pair approximation of the shell model and relate our results with a
number of currently interesting problems.Comment: a latex text file and 2 figures, to be publishe

### Towards understanding the probability of $0^+$ ground states in even-even many-body systems

For single-$j$ shells with $j={7/2}, {9/2}$ and 11/2, we relate the large
probability of $I^+$ ground states to the largest (smallest) coefficients
$\alpha^J_{I(v \beta)} = <nv \beta I |$ $A^{J \dagger} \cdot A^J | n v\beta
I>$, where $n$ is the particle number, $v$ is the seniority, $\beta$ is an
additional quantum number, and $I$ is the angular momentum of the state.
Interesting regularities of the probabilities of $I^+$ ground states are
noticed and discussed for 4-particle systems. Several counter examples of the
$0^+$ ground state (0GS) predominance are noticed for the first time.Comment: 5 pages, 1 figure. Phys. Rev. C64, in pres

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