Nuclear structure in nonmesonic weak decay of hypernuclei

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed. It involves a partial wave expansion of the emitted nucleon waves, preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak Λ-core coupling formalism. The hypernuclei are grouped having in view their A-1 cores, that is in those with even-even, even-odd and odd-odd cores. It is shown that in all three cases the nuclear structure manifests itself basically through Pauli Principle, and very simple expressions are derived for the neutron and proton induced decays rates Γn and Γp, which does not involve the spectroscopic factors. For the strangeness-Changing weak ΛN → NN transition potential we use the One-Meson-Exchange Model (OMEM), which comprises the exchange of the complete pseudoscalar and vector meson octets (π,η, K, ρ, ω, K*). We evaluate 3H, 4H 4ΛHe, 5ΛHe,11B, 12ΛC, 16ΛO, 17ΛO, and 28ΛSi hypernuclei, with commonly used parametrization for the OMEM, and compare the results with the available experimental information, The calculated rates ΓNM = Γn + Γp are consistent with the data, but the measurements of Γn/p = Γn/Γp are not well accounted for by the theory. It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.Facultad de Ciencias Exacta

Nuclear Structure in Nonmesonic Weak Decay of Hypernuclei

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed.It involves a partial wave expansion of the emitted nucleon waves,preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak Lambda-core coupling formalism. The hypernuclei are grouped having in view their A-1 cores, that is in those with even-even, even-odd and odd-odd cores.It is shown that in all three cases the nuclear structure manifests itself basically through Pauli Principle, and very simple expressions are derived for the neutron and proton induced decays rates, which does not involve the spectroscopic factors

Nuclear structure in nonmesonic weak decay of hypernuclei

A general shell model formalism for the nonmesonic weak decay of the hypernuclei has been developed. It involves a partial wave expansion of the emitted nucleon waves, preserves naturally the antisymmetrization between the escaping particles and the residual core, and contains as a particular case the weak Λ-core coupling formalism. The hypernuclei are grouped having in view their A-1 cores, that is in those with even-even, even-odd and odd-odd cores. It is shown that in all three cases the nuclear structure manifests itself basically through Pauli Principle, and very simple expressions are derived for the neutron and proton induced decays rates Γn and Γp, which does not involve the spectroscopic factors. For the strangeness-Changing weak ΛN → NN transition potential we use the One-Meson-Exchange Model (OMEM), which comprises the exchange of the complete pseudoscalar and vector meson octets (π,η, K, ρ, ω, K*). We evaluate 3H, 4H 4ΛHe, 5ΛHe,11B, 12ΛC, 16ΛO, 17ΛO, and 28ΛSi hypernuclei, with commonly used parametrization for the OMEM, and compare the results with the available experimental information, The calculated rates ΓNM = Γn + Γp are consistent with the data, but the measurements of Γn/p = Γn/Γp are not well accounted for by the theory. It is suggested that, unless additional degrees of freedom are incorporated, the OMEM parameters should be radically modified.Facultad de Ciencias Exacta

Proton-neutron self-consistent quasiparticle random phase approximation within the o(5) model

15 págs.; 7 figs.; 1 tab.; 1 app. ; PACS number(s): 21.60.Jz, 23.40.Hc, 23.90.1wThe self-consistent quasiparticle random phase approximation (SCQRPA) within the O(5) model in the coupled proton-neutron representation is analyzed. The exact vacuum wave function is used to compute all involved matrix elements. A stability analysis of the stationary points is performed. A phase transition from the uncoupled to the coupled stable proton-neutron regime beyond the QRPA breakdown value of the particle-particle strength is evidenced. The excitation energies are close to the lowest stable exact eigenvalues given by the diagonalization procedure for all cases. The conditions for which the Ikeda sum rule is fulfilled for all values of the particle-particle strength are pointed out. ©2000 American Physical SocietyOne of us (D.S.D.) is grateful for the financial support given by CNRS during his stay in Grenoble, where the work was performed.Peer Reviewe

Nuclear moments for the neutrinoless double beta decay II

The recently developed formalism for the evaluation of nuclear form factors in neutrinoless double beta decay is applied to $^{48}Ca$, $^{76}Ge$, $^{82}Se$, $^{100}Mo$, $^{128}Te$ and $^{130}Te$ nuclei. Explicit analytical expressions that follows from this theoretical development, in the single mode model for the decay of $^{48}Ca$, have been worked out. They are useful both for testing the full numerical calculations, and for analytically checking the consistency with other formalisms. Large configuration space calculations are compared with previous studies, where alternative formulations were used. Yet, besides using the G-matrix as residual interaction, we here use a simple $\delta$-force. Attention is paid to the connected effects of the short range nuclear correlations and the finite nucleon size. Constraints on lepton number violating terms in the weak Hamiltonian (effective neutrino Majorana mass and effective right-handed current coupling strengths) are deduced.Comment: 18 pages, latex, minor changes, to appear in Nucl. Phys.

Neutrino capture by r-process waiting-point nuclei

We use the Quasiparticle Random Phase Approximation to include the effects of low-lying Gamow-Teller and first forbidden strength in neutrino capture by very neutron-rich nuclei with N = 50, 82, or 126. For electron neutrinos in what is currently considered the most likely r-process site the capture cross sections are two or more times previous estimates. We briefly discuss the reliability of our calculations and their implications for nucleosynthesis.Comment: 9 pages, 4 figure

Self-consistent random phase approximation within the O(5) model and Fermi transitions

Self-consistent quasiparticle random phase approximation (SCQRPA) is considered in application to the Fermi transitions within the O(5) model. It is demonstrated that SCQRPA improves on renormalized QRPA (RQRPA), a method that has recently become rather popular in this context. The analytical form of the SCQRPA vacuum is used to evaluate all the matrix elements. The SCQRPA results show a general trend similar to the exact solutions. The necessity to change the single particle basis beyond the transition point, and to include the proton-proton and neutron-neutron channels in the QRPA operator, in addition to the proton-neutron one, is pointed out.Facultad de Ciencias Exacta

Majoron emitting neutrinoless double beta decay in the electroweak chiral gauge extensions

Fundamental mechanisms for Majoron emitting neutrinoless double beta decay in SU(3)_C x G_W x U(1) models, for electroweak flavor chiral extensions, G_W = SU(3)_L and SU(4)_L are pointed out. Both kinds of known Majoron emitting processes, charged Majoron emitting where the massless Nambu-Goldstone boson itself carries lepton charge, $L=-2$, and the ordinary Majoron emitting where the boson has a small mass are found possible. PACS numbers: 11.15.Ex, 12.60.Fr, 14.80.CpComment: 18 pages, Revtex, 3 Postscript figures. To be published in Phys.Rev.D(1 May 1998

Nuclear pairing: new perspectives

Nuclear pairing correlations are known to play an important role in various single-particle and collective aspects of nuclear structure. After the first idea by A. Bohr, B. Mottelson and D. Pines on similarity of nuclear pairing to electron superconductivity, S.T. Belyaev gave a thorough analysis of the manifestations of pairing in complex nuclei. The current revival of interest in nuclear pairing is connected to the shift of modern nuclear physics towards nuclei far from stability; many loosely bound nuclei are particle-stable only due to the pairing. The theoretical methods borrowed from macroscopic superconductivity turn out to be insufficient for finite systems as nuclei, in particular for the cases of weak pairing and proximity of continuum states. We suggest a simple numerical procedure of exact solution of the nuclear pairing problem and discuss the physical features of this complete solution. We show also how the continuum states can be naturally included in the consideration bridging the gap between the structure and reactions. The path from coherent pairing to chaos and thermalization and perspectives of new theoretical approaches based on the full solution of pairing are discussed.Comment: 47 pages, 11 figure