Etat de lieux de la QRPA = state of the art of the QRPA calculations

1. Etat de lieux de la QRPA = state of the art of the QRPA calculations Espace de Structure Nucléaire Théorique SPhN, Saclay, January 11-12, 2005 G. Colò / E. Khan

2. N,Z N+2,Z N+1,Z-1 E*, S(E*) inelastic cross sections Pairing vibrations, 2n transfer cross sections b half-lives, GT strengths, charge exchange cross sections QRPA = Quasi-particle RPA • Method known since 40 years in nuclear physics • Strong peak of activity since ~ year 2000. Why ? Study nuclear transitions on the whole chart ! (isotopic chains, open shells, drip-line nuclei, …)

3. QRPA inputs • 1) Single quasiparticle (mean field) wave functions and energies : WS, HF+BCS, HFB ? • 2) Interaction : separable, self-consistency, pairing ? • Necessity to have a microscopic approach

4. Single quasiparticle spectrum e E Single particle states Single quasiparticle states E -l D 0 e -l -50 l 50 l 2l 0 HF HFB

5. As already said, the QRPA equations were derived by M. Baranger many years ago. However, most of the calculations done in earlier times were NOT self-consistent. The single quasiparticle states and the interaction were often the result of an empirical choice. On the other hand, there is recent interest in trying to define a microscopic, universal “Energy Functional” for nuclei and nuclear matter. Within this framework, the HF (or HFB) define the minimum of the functional, while the self-consistent RPA (or QRPA) equations describe the small oscillations around this minimum. Non-relativistic framework : this talk RMF : cf. D. Vretenar

6. - Meaning of self-consistency within RPA: the restoring force which governs the nuclear oscillations is derived from the energy functional. Veff(1,2) → E[ρ] = < H > = ∫ ρ(1) Veff(1,2)ρ(2) , building the density ρas a combination of independent particles wavefunctions h[ρ] = δE / δρ= 0 defines the mean field; the restoring force is: δ2E / δρ2 If pairing is introduced, the energy functional depends on both the usual density ρ=<ψ+(r)ψ(r)> and the abnormal density κ=<ψ(r)ψ(r)> (κ=<ψ+(r)ψ+(r)>).

7. R0 How to derive the QRPA equations ? Generalised density TDHFB: External field : with U= (UV)

8. Matrix elements which enter the QRPA equations When we evaluate the quantities that we have called V, and which are the second derivatives of the energy functional, matrix elements between quasi-particle configurations are obtained. They are written in terms of particle-hole (p-h) and particle-particle (p-p) matrix elements. There is no compulsory reason for the effective interaction to be the same in the p-h and p-p channel.

9. Effective interactions The Gogny force is designed so that it allows treating the p-h and p-p channels on the same footing. When the Skyrme force is used in the p-h channel, a popular choice is to complement it with a zero-range density dependent pairing force (which is usually fitted on the ground-state pairing in the nucleus, or in the mass region, of interest).

10. Pairing, divergence & cutoff • Pairing : • Zero range UV divergence in the pp channel • Prescription (V0 , Ecutoff) : D stable

11. Advantages and disadvantages of self-consistency: it allows testing a functional on known cases, and making extrapolations for unknown situations (e.g., predictions for exotic nuclei) without introducing free parameters. … disadvantages : heavy treatment more difficult (not impossible) to carry out large-scale calculations, including deformed systems, and to go beyond QRPA

12. Examples of recent non self-consistent QRPA • Macro/Micro (Möller) : WS for the single particle spectrum, constant G pairing, and separable GT residual interaction • Interest : large-scale b rates calculations • FR-QRPA (Faessler) : takes into account the Pauli principle and allows the Ikeda sum rule to be verified (b decay) • Interest : go beyond the quasi-boson approximation

13. Advantages of QRPA compared to other methods (e.g., shell model): • simplicity, also from the computational point of view; • there is no “core” (that is, no need of effective charges); • it is possibile to study highly excited states. • provides densities and transition densities • Disadvantage: • not all the many-body correlations are taken into account. • By definition, states which include components like four quasi-particles cannot be described within the framework of standard QRPA.

14. Discrete vs. continuum : • Continuum is important close to, or at, the drip lines. Continuum calculations are by definitions more complete than discrete calculations, and require a coordinate space formalism for the QRPA equations • Discrete calculations have the advantage that they more directly provide the information about the wavefunctions on a quasi-particle basis (important for, e.g., the particle decay). • Conclusion: there is some degree of complementarity !

15. Microscopic calculations on the market… • Skyrme: • HFBCS (WS) continuum K.Hagino,H.Sagawa, NPA695 (2001) 82 • HFB (WS) continuum M.Matsuo, NPA696 (2001) 371 • HFB continuum E.Khan et al., PRC66 (2002) 024309 • HFBCS discrete G.Colò et al., NPA722 (2003) 111c • HFB discrete M.Yamagami,N.Van Giai, PRC69 (2004) • HFB discrete J.Terasaki et al. (unpublished, see nucl-th) • Gogny: • HFB discrete G.Giambrone et al., NPA726 (2003) 6 • HFB discrete S. Peru et al. Most of the calculations drop the residual Coulomb and spin-orbit force

16. Charge-exchange calculations • Skyrme: • BCS discrete P.Sarriguren et al., PRC67 (2003) 044313 • HFB discrete (canonical basis) J.Engel et al., PRC60 (1999) 014302 • M.Bender et al., PRC65 (2002) 054322 • HFBCS discrete S.Fracasso,G.Colò • DF-Fayans: • HFB continuum I.Borzov et al., PRC67 (2003) 025802

17. Some results…

18. QRPA gives, as a rule, reasonable results as far as the energy location of Giant Resonances is concerned (within 1 MeV or so). In fact, pairing is relatively unimportant for high-lying states and QRPA is almost equivalent to RPA. Width !! (exp. ~4.9 MeV) Need to go beyond QRPA S. Kamerdzhiev et al.

19. S. Goriely et al. Pigmy dipole appears in microscopic HFB+QRPA calculations Necessity for truly deformed QRPA

20. Results for the neutron-rich oxygen isotopes E. Khan et al. • difficulties for 18O : deformed ? • pairing decreases in the heavier 22,24O • sensivity of the low-lying states on the pairing interaction • Matsuo et al. : • E2 = 2.5 2.7 2.9 3.3 [MeV] • B(E2) = 17 18 20 18 e2.fm4

21. Gogny QRPA G. Giambrone et al.

22. 32Mg 36S 30Ne 38Ar 34Si M.Yamagami,N.Van Giai In the N=20 isotones self-consistent QRPA performs well D.T.Khoa et al. In the sulfur isotopes QRPA input in a folding model calculation reproduces the inelastic (p,p’) cross sections

23. Sn isotopes D. Sarchi et al. 120Sn K.Hagino and H.Sagawa

24. Astrophysics : (n,g) rates QRPA/Hybrid Discrepancy pheno/micro QRPA/QRPA Agreement HF+BCS QRPA / HFB QRPA T=1.5 109 K Deviation up to a factor 10

25. Charge-exchange and ν-induced reactions GT C. Volpe et al. 12C Σkr2k Y2(Ωk) t+ full = QRPA, dashed = SM used for calculation of (νe,e)(νμ,μ) Important to extend these calculations to exotic nuclei

26. Short conclusions • It is difficult to assess in general the reliability of QRPA for the low-lying states. These are sensitive to the details of the shell-structure around the Fermi energy. • The neglected terms in the calculations (e.g., the two-body spin-orbit residual force) may play a role; • the pairing force used is not universal, is simply fitted to the g.s. We need to systematically study its influence; • anharmonicities ? • Continuum treatment can affect GR in drip-line nuclei.