10,026 research outputs found

    Clustering aspects and the shell model

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    In this talk I shall discuss the clustering aspect and the shell model. I shall first discuss the α\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-jj 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

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    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

    An alternative marginal likelihood estimator for phylogenetic models

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    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

    Exchange Currents for Hypernuclear Magnetic Moments

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    The meson(K and π\pi) exchange currents for the hypernuclear magnetic moments are calculated using the effective Lagrangian method. The seagull diagram, the mesonic diagram and the Σ0\Sigma^0-excitation diagram are considered. The Λ\Lambda-N exchange magnetic moments for the Λ5He{}^5_{\Lambda}He, Λ6He{}^6_{\Lambda}He and Λ6Li{}^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 Λ5He{}^5_{\Lambda}He. The π\pi exchange current, induced only in the Σ0\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

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

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    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 03+_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-jj fermions with JJ-pairing interaction

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

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    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+0^+ ground states in even-even many-body systems

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    For single-jj shells with j=7/2,9/2j={7/2}, {9/2} and 11/2, we relate the large probability of I+I^+ ground states to the largest (smallest) coefficients αI(vβ)J=<nvβI∣\alpha^J_{I(v \beta)} = <nv \beta I | AJ†⋅AJ∣nvβI>A^{J \dagger} \cdot A^J | n v\beta I>, where nn is the particle number, vv is the seniority, β\beta is an additional quantum number, and II is the angular momentum of the state. Interesting regularities of the probabilities of I+I^+ ground states are noticed and discussed for 4-particle systems. Several counter examples of the 0+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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