Dipole Polarizability and the neutron skin thickness

Transcription

1 Dipole Polarizability and the neutron skin thickness Xavier Roca-Maza Università degli Studi di Milano and INFN MITP Scientific Program Neutron Skins of Nuclei May 17th-27th

2 Table of contents: Introduction: Energy Density Functionals (EDFs) Dipole polarizability Statistic uncertainties in EDFs Example on the dipole polarizability Sistematic uncertainties in EDFs Example on the dipole polarizability Conclusions 2

3 INTRODUCTION 3

4 Nuclear Energy Density Functionals (EDFs): Based on effective interactions solved at the HF level, EDFs are successful in the description of ground and excited state properties such as m, r 2 1/2 or GR along the nuclear chart Main types of EDFs: Relativistic mean-field models (RMF), based on Lagrangians where effective mesons carry the interaction: L int = ΨΓ σ ( Ψ,Ψ)ΨΦ σ + ΨΓ δ ( Ψ,Ψ)τΨΦ δ ΨΓω ( Ψ,Ψ)γµ ΨA (ω)µ ΨΓρ ( Ψ,Ψ)γµ τψa (ρ)µ Non-relativistic mean-field models (NRMF), based on Hamiltonians where ef f. interactions are proposed and tested: VNucl eff = Vlong range attractive +V short range repulsive +V SO EDFs are phenomenological not directly connected to any NN (or NNN) interaction in the vacuum -EDFs derived from a Mean-Field we expect bulk properties more accurate as heavier is the nucleus 4

5 Dipole polarizability: definition From a macroscopic perspective The electric polarizability measures the tendency of the nuclear charge distribution to be distorted electric dipole moment external electric field From a microscopic perspective The electric polarizability is proportional to the inverse energy weighted sum rule (IEWSR) of the electric dipole response in nuclei = 8π 9 e2 B(E1) E or = hc σph. abs. (E) 2π 2 E 2 de 5

6 In more detail (from theory)... The linear response or dynamic polarizability of a nuclear system excited from its g.s., 0, to an excited state, ν, due to the action of an external isovector oscillating field (dipolar in our case) of the form (Fe iwt +F e iwt ): A F JM = r J Y JM (ˆr)τ z (i) ( L = 1 Dipole) i is proportional to the static polarizability for small oscillations α = (8π/9)e 2 m 1 = (8π/9)e 2 ν F 0 2 /E where m 1 is the ν inverse energy weighted moment of the strength function The dielectric theorem establishes that them 1 moment can be computed from the expectation value of the Hamiltonian in the constrained ground state H = H+λD. 6

7 STATISTIC UNCERTAINTIES AND CORRELATIONS IN EDFs Example on the dipole polarizability 7

8 Covariance analysis: χ 2 test ObservablesOused to calibrate the parameters p (e.g. of an EDF) χ 2 1 m ( O theo. ı O ref. ) 2 ı (p) = m n p 1 O ref. ı ı=1 Assuming that the χ 2 can be approximated by an hyper-parabola around the minimum p 0, χ 2 (p) χ 2 (p 0 ) 1 n (p ı p 0ı ) pı pj χ 2 (p j p 0j ) 2 ı,j where M 1 2 p ı pj χ 2 (curvature m.) and E M 1 (error m.). errors between predicted observables A A = n pı AE ıı pı A correlations between predicted observables, C AB c AB CAA C BB ı where, C AB = (A(p) A)(B(p) B) n pı AE ıj pj B ıj 8

9 Example: Fitting protocol of SLy5-min (NON-Rel) and DD-ME-min1 (Rel) SLy5-min: Binding energies of 40,48 Ca, 56 Ni, 130,132 Sn and 208 Pb with a fixed adopted error of 2 MeV the charge radius of 40,48 Ca, 56 Ni and 208 Pb with a fixed adopted error of 0.02 fm the neutron matter Equation of State calculated by Wiringa et al. (1988) for densities between 0.07 and 0.40 fm 3 with an adopted error of 10% the saturation energy (e(ρ 0 ) = 16.0±0.2 MeV) and density (ρ 0 = 0.160±0.005 fm 3 ) of symmetric nuclear matter. DD-ME-min1: binding energies, charge radii, diffraction radii and surface thicknesses of 17 even-even spherical nuclei, 16 O, 40,48 Ca, 56,58 Ni, 88 Sr, 90 Zr, 100,112,120,124,132 Sn, 136 Xe, 144 Sm and 202,208,214 Pb. The assumed errors of these observables are 0.2%, 0.5%, 0.5%, and 1.5%, respectively. 9

10 Covariance analysis: SLy5-min (NON-Rel) and DD-ME-min1 (Rel) J. Phys. G: Nucl. Part. Phys (2015). The neutron skin is correlated with L in both models but NOT with. (I will come back on that latter) 10

11 Covariance analysis: SLy5-min (NON-Rel) and DD-ME-min1 (Rel) SLy5-min DDME-min1 A A 0 σ(a 0 ) A 0 σ(a 0 ) units SNM ρ ± ± fm 3 e(ρ 0 ) ± ± 0.03 MeV m /m ± ± J ± ± 1.7 MeV K ± ± 23 MeV L 47.5 ± ± 16 MeV 208 Pb E ISGMR x ± ± 0.49 MeV E ISGQR x ± ± 1.76 MeV r np ± ± 0.03 fm E IVGDR x 13.9 ± ± 0.38 MeV m IVGDR ± ± 0.28 MeV 1 fm 2 E IVGQR x 21.6 ± ± 2.05 MeV Statistical uncertainties depend on the fitting protocol, that is on the data (or pseudo-data) and associated errors used for the fits: Let us see an example... 11

12 Covariance analysis: modifying the χ 2 SLy5-a: χ 2 as in SLy5-min except for the neutron EoS (relaxed the required accuracy=increasing associated error). SLy5-b: χ 2 as in SLy5-min except the neutron EoS (not employed) and used instead a tight constraint on the r np in 208 Pb J. Phys. G: Nucl. Part. Phys (2015). When a constraint on a property is relaxed, correlations of other observables with such a property should become larger SLy5-a: is now better correlated with r np When a constraint on a property is enhanced artificially or by an accurate experimental measurement correlations of other observables with such a property should become small SLy5-b: r np is not correlated with any other observable 12

13 SISTEMATIC UNCERTAINTIES AND CORRELATIONS IN EDFs Example on the dipole polarizability 13

14 Dipole polarizability: macroscopic approach The dielectric theorem establishes that the m 1 moment can be computed from the expectation value of the Hamiltonian in the constrained ground state H = H+λD. Adopting the Droplet Model (m 1 ): m 1 A r2 1/2 ( ) J 48J 4 Q A 1/3 within the same model, connection with the neutron skin thickness: A r e r np + 2 Z 5 70J rsurface np 12J 2 r 2 1/2 (I I C ) Is this correlation appearing also in EDFs? 14

15 Isovector Giant Dipole Resonance in 208 Pb: Dipole polarizability: microscopic results HF+RPA (fm 3 ) r=0.62 FSU NL3 (a) DD-ME Skyrme SV SAMi TF r np (fm) 10 2 J (MeV fm 3 ) r=0.97 FSU NL3 DD-ME Skyrme SV SAMi TF (b) r np (fm) X. Roca-Maza, et al., Phys. Rev. C 88, (2013). J is linearly correlated with r np and no alone within EDFs 15

16 Warnings: RPA versus experiment Cumulative sum of (fm 3 ) Pb A. Tamii et al., PRL 107, (2011) SAMi-J27 SAMi SAMi-J29 SAMi-J30 SAMi-J31 SAMi (CPVC) Cumulative sum of EWSR / TRK J (MeV fm 3 ) Sn DD-MEd J=36 MeV SAMi-J31 J=31 MeV SkM* J=30 MeV KDE0-J32 J=32 MeV E x (MeV) 250 Empty symbols include pairing E x (MeV) A - Important to take into account the full energy range to compare with RPA results. (we expect RPA to be quantitative for excitation energy and sum rules but not in details of the response function) - RPA do not reproduce the resonance width, maximum possible Γ error: 2 < 2% in 208 Pb 4E 2 x - Including pairing correlations for Sn: r np and tend to be smaller by few % (0-8% in 120 Sn and studied models). - quasi-deuteron contributions should be substracted from exp. 16

17 Isovector Giant Dipole Resonance in 68 Ni: What about other nuclei? Dipole polarizability: microscopic results HF+RPA (fm 3 ) 4,8 4,6 4,4 4,2 4,0 3,8 3,6 (a) r = 0.65 Skyrmes SAMi-J KDE0-J FSUΛ NL3Λ TAMU-FSU DD-ME 0,15 0,2 0,25 0,3 r np (fm) J (MeV fm 3 ) (b) 68 Ni r = ,15 0,2 0,25 0,3 r np (fm) X. Roca-Maza et al. Phys. Rev. C 92, (2015) Experimental dipole polarizability = 3.40±0.23 fm 3 D. M. Rossi et al., PRL 111, (GSI). = 3.88±0.31 fm 3 full response D. M. Rossi, T. Aumann, and K. Boretzky. 17

18 208 Pb vs 68 Ni: Dipole polarizability: microscopic results HF+RPA ( 208 Pb) (fm 3 ) KDE0-J FSUΛ NL3Λ TAMU-FSU DD-ME SAMi-J Skyrmes RCNP 18 GSI r = 0.96 (a) 3,4 3,6 3,8 4 4,2 4,4 4,6 4,8 ( 68 Ni) (fm 3 ) X. Roca-Maza et al. Phys. Rev. C 92, (2015) Just as an indication DM would predict: (A = 208)/ (A = 68) (208/68) 5/3 18

19 Can we use this correlation to predict the polarizability in other nuclei? 2,8 (a) 6,8 (b) ( 48 Ca) (fm 3 ) 2,6 2,4 2,2 2 RCNP r = 0.82 Skyrmes SAMi-J KDE0-J FSU DD-ME ( 208 Pb) (fm 3 ) ( 90 Zr) (fm 3 ) 6,4 6 5,6 5,2 RCNP r = ( 208 Pb) (fm 3 ) X. Roca-Maza et al. Phys. Rev. C 92, (2015) Nucleus r np (fm) (fm 3 ) 48 Ca (0.16 ±0.01) (2.30 ±0.14) 90 Zr (0.067±0.008) (5.65±0.23) Table: Estimates for the neutron skin thickness and electric dipole polarizability of 48 Ca and 90 Zr from models that predict α exp in 68 Ni, 132 Sn and 208 Pb. 19

20 Constraints of this analysis on the J L plane A r2 12J [ L J EDFs ρ 0 ρ A 3ρ 0 ] where S(ρ A ) a sym (A) J (MeV) From exp ( 68 Ni) From exp ( 120 Sn) From exp ( 208 Pb) L (MeV) X. Roca-Maza et al. Phys. Rev. C 92, (2015) J = (24.9±2.0)+(0.19±0.02)L for 68 Ni J = (25.4±1.1)+(0.17±0.01)L for 120 Sn J = (24.5±0.8)+(0.168±0.007)L for 208 Pb For S(< ρ > ρ 0 ) J L (ρ 0 < ρ >) 3ρ 20

21 CONCLUSIONS 21

22 Conclusions: We have studied theoretically how sensitive is the isovector channel of the interaction to a measurement of the dipole polarizability in a heavy nucleus such as 208 Pb. we have proposed a physically meaningful correlation between the polarizability and the properties of the effective interaction: J vs r np and not alone. Our results for 208 Pb can be extended to other nuclei such as the exotic 68 Ni. Within our approach, we have derived three bands in the J L plane consistent with the recent measurements of the polarizability in 68 Ni, 120 Sn and 208 Pb The slope shown by the derived bands in thej L is not strictly followed by the models used for the analysis Subset of models that reproduce simultaneously the measured polarizabilities are employed to predict J = MeV, L = MeV; and the values for r np in 68 Ni, 120 Sn, and 208 Pb are in the ranges: fm, fm, and fm 22

23 EXTRA MATERIAL 23

24 r np in 208 Pb: r np of 208 Pb (fm) MSk7 Total fit: r=0.992, slope=1.6 fm/gev Bulk fit: r=0.993, slope=1.4 fm/gev Surface fit: r=0.602, slope=0.2 fm/gev Sk-T6 D1S D1N HFB-8 FSUGold Ska Sk-Rs DD-ME2 SkM* SkSM* SkMP HFB-17 SLy4 SGII NL1 NL-Z NL3* NL-SH NL3 G2 NLC Sk-T4 NL-RA1 TM L (MeV) 24

25 J (MeV fm 3 ) (a) 120 Sn r = 0.95 Skyrmes SAMi-J KDE0-J DD-ME 0,09 0,12 0,15 0,18 0,21 r np (fm) 25

26 ( 208 Pb) J (MeV fm 3 ) r = 0.99 r = 0.99 (a) ( 68 Ni) J ( 120 Sn) J ( 120 Sn) J (MeV fm 3 ) Skyrmes SAMi-J KDE0-J DD-ME 250 r = ( 68 Ni) J (MeV fm 3 ) (b) 26

27 ( 208 Pb) (fm 3 ) KDE0-J FSUΛ NL3Λ TAMU-FSU DD-ME SAMi-J Skyrmes RCNP 18 GSI r = 0.96 (a) 3,4 3,6 3,8 4 4,2 4,4 4,6 4,8 ( 68 Ni) (fm 3 ) ( 208 Pb) (fm 3 ) RCNP 18 RCNP r = 0.96 (b) 8 8,5 9 9, ,5 ( 120 Sn) (fm 3 ) 27

28 J (MeV fm 3 ) (c) 48 Ca 60 r = ,12 0,16 0,2 0,24 r np (fm) J (MeV fm 3 ) (d) 90 Zr 150 r = ,04 0,06 0,08 0,1 0,12 r np (fm) 28