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Generalized models of pairing in non-degenerate orbits

Generalized models of pairing in non-degenerate orbits. J. Dukelsky, IEM, Madrid, Spain D.D. Warner, Daresbury, United Kingdom A. Frank, UNAM, Mexico P. Van Isacker, GANIL, France. Symmetries of pairing models Generalized pairing models Deuteron transfer. The nuclear shell model.

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Generalized models of pairing in non-degenerate orbits

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  1. Generalized models of pairingin non-degenerate orbits J. Dukelsky, IEM, Madrid, Spain D.D. Warner, Daresbury, United Kingdom A. Frank, UNAM, Mexico P. Van Isacker, GANIL, France Symmetries of pairing models Generalized pairing models Deuteron transfer Generalized pairing models, Saclay, June 2005

  2. The nuclear shell model • Mean field plus residual interaction (between valence nucleons). • Assume a simple mean-field potential: • Contains • Harmonic-oscillator potential with constant . • Spin-orbit term with strength ls. • Orbit-orbit term with strength ll. Generalized pairing models, Saclay, June 2005

  3. Shell model for complex nuclei • Solve the eigenvalue problem associated with the matrix (n active nucleons): • Methods of solution: • Diagonalization (Lanczos): d~109. • Monte-Carlo shell model: d~1015. • Density Matrix Renormalization Group: d~10120? Generalized pairing models, Saclay, June 2005

  4. Symmetries of the shell model • Three bench-mark solutions: • No residual interaction  IP shell model. • Pairing (in jj coupling)  Racah’s SU(2). • Quadrupole (in LS coupling)  Elliott’s SU(3). • Symmetry triangle: Generalized pairing models, Saclay, June 2005

  5. Racah’s SU(2) pairing model • Assume pairing interaction in a single-j shell: • Spectrum 210Pb: Generalized pairing models, Saclay, June 2005

  6. Solution of the pairing hamiltonian • Analytic solution of pairing hamiltonian for identical nucleons in a single-j shell: • Seniority  (number of nucleons not in pairs coupled to J=0) is a good quantum number. • Correlated ground-state solution (cf. BCS). G. Racah, Phys. Rev. 63 (1943) 367 Generalized pairing models, Saclay, June 2005

  7. Nuclear “superfluidity” • Ground states of pairing hamiltonian have the following correlated character: • Even-even nucleus (=0): • Odd-mass nucleus (=1): • Nuclear superfluidity leads to • Constant energy of first 2+ in even-even nuclei. • Odd-even staggering in masses. • Smooth variation of two-nucleon separation energies with nucleon number. • Two-particle (2n or 2p) transfer enhancement. Generalized pairing models, Saclay, June 2005

  8. Two-nucleon separation energies • Two-nucleon separation energies S2n: (a) Shell splitting dominates over interaction. (b) Interaction dominates over shell splitting. (c) S2n in tin isotopes. Generalized pairing models, Saclay, June 2005

  9. Integrability of pairing hamiltonian • Pair operators (several shells): • The pairing hamiltonian for degenerate shells • … is solvable by virtue of an underlying SU(2) “quasi-spin” symmetry: A.K. Kerman, Ann. Phys. (NY)12 (1961) 300 Generalized pairing models, Saclay, June 2005

  10. Generalized pairing model • Hamiltonian for pairing interaction in non-degenerate shells: • Is the pairing model with non-degenerate orbits integrable? Generalized pairing models, Saclay, June 2005

  11. Richardson-Gaudin models • Algebraic structure: • The Gaudin operators • …commute if Xij and Yij are antisymmetric and satisfy the equations • Any combination of Ri is integrable. R.W. Richardson, Phys. Lett.5 (1963) 82 M. Gaudin, J. Phys. (Paris) 37 (1976) 1087. Generalized pairing models, Saclay, June 2005

  12. Pairing with non-degenerate orbits • If we choose •  A hamiltonian for pairing in non-degenerate shells is integrable! Solution: J. Dukelsky et al., Phys. Rev. Lett.87 (2001) 066403 Generalized pairing models, Saclay, June 2005

  13. Pairing with neutrons and protons • For neutrons and protons two pairs and hence two pairing interactions are possible: • Isoscalar (S=1,T=0): • Isovector (S=0,T=1): Generalized pairing models, Saclay, June 2005

  14. Neutron-proton pairing hamiltonian • A hamiltonian with two pairing interactions • …has an SO(8) algebraic structure. • Vpairing is integrable and solvable (dynamical symmetries) for g0=0,g0=0and g0=g0. Generalized pairing models, Saclay, June 2005

  15. SO(8) “quasi-spin” formalism • A closed algebra is obtained with the pair operators S± with in addition • This set of 28 operators forms the Lie algebra SO(8) with subalgebras B.H. Flowers & S. Szpikowski, Proc. Phys. Soc.84 (1964) 673 Generalized pairing models, Saclay, June 2005

  16. Solvable limits of the SO(8) model • Pairing interactions can expressed as follows: • Symmetry lattice of the SO(8) model: • Analytic solutions for g0=0,g0=0 &g0=g0. Generalized pairing models, Saclay, June 2005

  17. Quartetting in N=Z nuclei • T=0 and T=1 pairing has a quartet structure with SO(8) symmetry. Pairing ground state of an N=Z nucleus: •  Condensate of “-like” objects. • Observations: • Isoscalar component in condensate survives only in N~Z nuclei, if anywhere at all. • Spin-orbit term reduces isoscalar component. Generalized pairing models, Saclay, June 2005

  18. Generalized neutron-proton pairing • Hamiltonian for pairing interactions in non-degenerate shells: • Solution techniques: • Richardson-Gaudin for SO(8) model. • Boson mappings: • requiring same commutation relations; • associating state vectors. Generalized pairing models, Saclay, June 2005

  19. Generalized pairing models • Pairing in degenerate orbits between identical particles has SU(2) symmetry. • Richardson-Gaudin models can be generalized to higher-rank algebras: J. Dukelsky et al., to be published Generalized pairing models, Saclay, June 2005

  20. Example: SO(5) pairing • Hamiltonian: • “Quasi-spin” algebra is SO(5) (rank 2). • Example: 64Ge in pfg9/2 shell (d~91014). S. Dimitrova, unpublished Generalized pairing models, Saclay, June 2005

  21. Model with L=0 vector bosons • Correspondence: • Algebraic structure is U(6). • Symmetry lattice of U(6): • Boson mapping is exact in the symmetry limits [for fully paired states of the SO(8)]. P. Van Isacker et al., J. Phys. G 24 (1998) 1261 Generalized pairing models, Saclay, June 2005

  22. Masses of N~Z nuclei • Neutron-proton pairing hamiltonian in non-degenerate shells: • HF maps into the boson hamiltonian: • HB describes masses of N~Z nuclei. E. Baldini-Neto et al., Phys. Rev. C 65 (2002) 064303 Generalized pairing models, Saclay, June 2005

  23. Two-nucleon transfer • Amplitude for two-nucleon transfer in the reaction A+aB+b: • Nuclear-structure information contained in GN(L,S,J) which for L=0 transfer reduces to N.K. Glendenning,Direct Nuclear Reactions Generalized pairing models, Saclay, June 2005

  24. Deuteron transfer • Overlap of uncorrelated pair: • Bosons correspond to correlated pairs: • Scale property: P. Van Isacker et al., Phys. Rev. Lett. 94 (2005) 162502 Generalized pairing models, Saclay, June 2005

  25. Deuteron transfer with bosons • Correspondence does not take account of Pauli principle. • The following correspondence is shown to be exact [in the Wigner limit]: • Even-even  odd-odd • Odd-odd  even-even Generalized pairing models, Saclay, June 2005

  26. Masses of pf-shell nuclei • Boson hamiltonian: • Rms deviation is 306 (or 254) keV. • Parameter ratio: b/a5. Generalized pairing models, Saclay, June 2005

  27. Deuteron transfer in N=Z nuclei • Deuteron-transfer intensity cT2 calculated in sp-boson IBM based on SO(8). • Ratio b/a fixed from masses in lower half of 28-50 shell. Generalized pairing models, Saclay, June 2005

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