Spin-Parity Decomposition of Spin Dipole Resonances and Tensor Interaction Effects. Tomotsugu Wakasa. Department of Physics, Kyushu University

Transcription

1 Spin-Parity Decomposition of Spin Dipole Resonances and Tensor Interaction Effects Tomotsugu Wakasa Department of Physics, Kyushu University

2 Outline Residual interaction effects of spin-isospin responses GT strength (as an introduction) SD strength Tensor force effects New data and analysis of 208 Pb(p,n) Spin-parity decomposition (J π =0 -, 1 -, 2 - ) of SD strengths Tensor force effects on SD strengths Softening for 1 - by triplet-even tensor force Softening for 0 - by triplet-odd tensor force Summary

3 Giant Resonances Collective motion Many nucleons participate coherently Classified by Multipolarity : L Spin : S Isospin : T IsoVector (IV) Spin-flip Dipole (SD) ΔS = 1 and ΔT=1 Macroscopic picture Dipole oscillation of p (p ) against n (n ) Information from GR Resonance peak Residual interaction Total strength Quark degrees of freedom Neutron skin thickness IVSM(GT) SDR

4 Spin-Isospin Modes and Sum Rule Spin-isospin transition operators [GR = Coherent 1p1h excitations] IV Spin-scalar IV Spin-vector Model-independent sum-rule : scalar neutron proton : vector GT sum-rule (Ikeda s sum-rule) Gamow-Teller (L=0, ΔS=1, ΔT=1) 50% quenching of GTR Config. mix. (2p2h) Δ (quark) Exp./Theor. Quenching 50%

5 Residual interaction effects on GTR Landau-Migdal interaction at q=0 repulsion between particle and hole (ph) coupling between ph and Δh repulsive LM parameter g NN Determine the p-h repulsion Larger g NN Stronger repulsion Peak shifts to higher ω Collectivity becomes large LM parameter g NΔ Determine the coupling to Δ Larger g NΔ Stronger coupling Strength becomes small (quenched) [Strength moves to Δ region] repulsive

6 Comparison with experimental data g NN dependence on GTR GTGR peak position Strongly depends on g NN Weak g NΔ dependence g NΔ depdendence on GT quenching Q Q = 0.86 ± 0.07 from MD analysis 2p2h effects are dominant Q evaluated in RPA repulsive Strongly depends on g NΔ How about other modes (SD resonances) Distribution (information on effective interaction) Tensor force effects Quenching? K. Yako et al., PLB 615, 193 (2006).

7 K. Yako et al., PRC 74, (R) (2006). SD strength distributions Exp. strength 1 - Extends up to 50 MeV Configuration mix. SIngle bump HF+RPA (1p1h) Underestimation at Ex > 25 MeV 2p2h is important Three bumps SD strength (fm 2 /MeV) 2 - Ex(2 - ) > Ex(1 - ) Second-order RPA Reasonably reproduce in whole region Excitation energy (MeV) Three bumps Each ΔJ π (0 -, 1 -, 2 - ) distributions Inconsistent (tensor correlation?)

8 Tensor force effects in nuclei Shell-structure due to tensor force 0 Proton s.p energy of Sb Due to tensor correlations, excess neutrons (j>) Pull-down proton orbit with j< Experimental data for Sb are well reproduced Tensor effects in Skyrme int. Energy (MeV) 1g7/2 1h11/2 Additional contribution to normal spin-orbit pot. central ex. tensor central ex. tensor Neutron # T. Otsuka et al.,prl 95,232502(2005). Tensor terms depend on central-ex. terms Negative α values are also proposed Further exp. informations are important E(1h11/2)-E(1g7/2) (MeV) Proton s.p energy of Sn no tensor with tensor Weaker LS by ΔWp Neutron # D.M. Brink and FL. Stancu, PRC 75, (2007).

9 Tensor force effects on SD strengths HF+RPA prediction for 208 Pb HF : Tensor forces hardly change LS splitting [ (nn/pp contribution) + (np contribution) 0 ] RPA : Tensor effects depend on J π U > 0 T > 0 triplet-even triplet-odd T > T>0 (βt>0) U>0 (αt>0) U<0 (αt<0) hardening softening hardening softening insensitive insensitive Separated SD strengths would constrain both T and U (αt and βt)

10 This experimental work for 208 Pb(p,n) New data and analysis for 208 Pb(p,n) Cross sections and analyzing powers at θ = (11 angles) Complete sets of polarization transfers at θ = (5 angles) Goal Spin-parity J π separated SD strengths for 208 Pb Tools Distribution of separated SD strengths Tensor correlation effects on SD strengths Quenching of separated SD strengths J π dependence (c.f. total SD strength is not quenched for 90 Zr) Polarization transfer Dij Sensitive to ΔJ π (0 -, 1 -, 2 - ) Multipole decomposition analysis (MDA) with Dij Based on reliable DWIA+RPA calculations

11 Ring Cyclotron RCNP, Osaka Beam Swinger System Ring Cyclotron AVF Cyclotron BLP1 & BLP2 100m TOF tunnel NPOL3 SOL1 & SOL2 300 MeV polarized protons Smallest distortion Beam polarization Controlled by two solenoids Measured by two BLPs (p+p) Beam swinger Θ = 0-10 Neutron measurement NPOL3 with 70m TOF Dij measurement with NSR

12 DWIA+RPA calculations Computer code: crdw Developed by Ichimura group DWIA Global optical potentials for 208 Pb GT, Ex=0 MeV Proton: Hama et al. Neutron: Shen et al. NN t-matrix Franey and Love t-matrix RPA π+ρ+g p-h interaction g NN = 0.60 ± 0.10 g NΔ = 0.35 ± Zr(p,n) data QES, q=1.7 fm -1 Exp. DWIA+RPA No free parameter Well reproduce pionic modes for 12 C(p,n) in wide momentum-transfer region Absolute values are reliable

13 Results ーPolarized cross sectionー Separate into 3 components using Dij scalar longitudinal transverse IDL : unnatural parity (0- and 2- for SD) IDT : both parities (1-, 2- for SD) 0- contributes to IDL only (special case) Comparison with DWIA+RPA at 4 Exp: Narrow bump in both IDL and IDT Theory : Broad bump [ω(2-) < ω(1-) ω(0-)] MDA should be performed Example at 0

14 Separation of SDR into each J π Separation of SDR (L=1) into 0 -, 1 -, 2 - is important Tensor effects depends on J π DWIA prediction Spin-longitudinal (π) Normal multipole decomposition Separate into each L component Works very well to extract GT (L=0) Could NOT separate into J π with same L Angular distributions are governed by L Spin-transverse (ρ) Idea to separate SDR into each J π Polarization observables are sensitive to J π Separate c.s. into longitudinal (π) - transverse (ρ) 0 - : Spin-longitudinal (π) only 1 - : Spin-transverse (ρ) only 2 - : Both Multipole decomposition for longitudinal (π) and transverse (ρ) c.s. Can separate/specify not only L, but also J π

15 Results of multipole decomposition L=0 (GT) contribution Spin-longitudinal (π) Spin-transverse (ρ) Large contribution up to 50 MeV Configuration mixing (2p2h) IVSM contribution L=1 SD (0-, 1-, 2-) contributions SD cross sections at 4 Theory : ω(2-) < ω(1-) ω(0-) Exp. : ω(2-) ω(1-) < ω(0-) Softening on 1- Multipole (Jπ) decomposition is successful Jπ dependence on SD resonances could not be reproduced Signature of tensor force effects?

16 SD unit cross section and B(SD) SD unit cross section Maximum cross section at 4 Proportionality relation DWIA calculations ( ) 12 C, 16 O, 48 Ca, 90 Zr, 208 Pb SD excitation at ω=0 MeV A-dependence Proper description for A-dep. Reproduce DWIA results within 15% Experimental SD strength Exp MDA RPA

17 Comparison with RPA and self-consistent HF+RPA RPA (used in DWIA+RPA calc.) π+ρ+g residual interaction Systematically lower than HF+RPA Different mean field Self-consistent HF+RPA SII (w/o tensor) Reproduce 2 - strength c.f. Tensor correlations are insensitive to 2 - Significantly higher for 1 - Softening effect by T(TE)? Roughly consistent for 0 - Hardening effect by T(TE) Cancelled by softening effect by U(TO) > 0?

18 Tensor force effects on SDR Tensor force effects on SDR triplet-even triplet-odd T>0 U>0 U<0 hardening softening hardening softening insensitive insensitive T(TE) > 0 tensor interaction Softening for 1 - : ω calc. ω exp. Hardening for 0 - : ω calc. > ω exp. T(TE) > 0 + U(TO) > 0 tensor interaction Softening for 0 - : ω calc. ω exp. T(TE) > 0 + U(TO) < 0 tensor interaction Hardening for 0 - : ω calc. >> ω exp. U > 0 T > 0 T > 0 U < 0 Softening on 1 - is reproduced by T>0 Tensor effect on 0 - is weak Hardening by T>0 should be cancelled by softening by U>0 T(TE) 650 MeV fm 5 U(TO) 200 MeV fm 5

19 Integrated SD strengths Integrated strength Total strength (ω < 50 MeV) Exp. : (1.00 ± 0.05) 10 3 fm 2 Theory : ( ) 10 3 fm 2 Quenching fac.: 0.84 ± 0.04 Systematic uncertainty 15% Each J π strength 0 - and 2 - : 70% of RPA prediction 1 - : 100% of RPA prediction Systematic uncertainty 30% (Correlation between each strength) 84 ± 4% of total SD strength is found Quenching might depend on J π Not conclusive (Large uncertainties)

20 Summary New experimental data for 208 Pb(p,n) Cross sections and analyzing powers at θ = (11 angles) Complete sets of polarization transfers at θ = (5 angles) Extended multipole decomposition (MD) analysis Polarization observables were used, for the first time, in MD analysis Reasonable agreement with experimental data Successful separation into individual J π components SD strength for 208 Pb Softening effect for 1 - T(TE) 650 MeV fm 5 (βt200 MeV fm 5 ) Small effect for 0 - (= hardening effect by T(TE) is NOT observed) Hardening effect by T should be cancelled by softening effect by U>0 U(TO) 200 MeV fm 5 (αt 100 MeV fm 5 ) Similar to βt=238 MeV fm 5 and αt=135 MeV fm 5 by low-q limit of G-matrix calc. First exp./theor. findings for tensor force effects in nuclear spin excitations