Dipole Polarizability, parity violating asymmetry and the neutron skin thickness Xavier Roca-Maza Università degli Studi di Milano and INFN

Transcription

1 Dipole Polarizability, parity violating asymmetry and the neutron skin thickness Xavier Roca-Maza Università degli Studi di Milano and INFN Department of Physics. Peking University. January 19th

2 Table of contents: Introduction The Nuclear Many-Body Problem Nuclear Energy Density Functionals Symmetry energy Statistic (theoretical) uncertainties in EDFs Dipole polarizability Parity violating electron scattering Conclusions 2

3 INTRODUCTION 3

4 Complex Based di«ective The Nuclear Many-Body Problem: Nucleus: from few to more than 200 strongly interacting and self-bound fermions. Underlying interaction is not perturbative at the (low)energies of interest for the study of masses, radii, deformation, giant resonances,... systems: spin, isospin, pairing, deformation,... Many-body calculations based on NN scattering data in the vacuum are not conclusive yet: erent nuclear interactions in the medium are found depending on the approach EoS and (only very recently) few groups in the world are able to perform extensive calculations for light and medium mass nuclei on e«interactions, Nuclear Energy Density Functionals are successful in the description of masses, nuclear sizes, deformations, Giant Resonances,... 4

5 e«ective Nuclear Energy Density Functionals: Main types of successful EDFs for the description of masses, deformations, nuclear distributions, Giant Resonances,... Relativistic mean-field models, based on Lagrangians where mesons carry the interaction: L int = ΨΓ σ ( Ψ,Ψ)ΨΦ σ + ΨΓ δ ( Ψ,Ψ)τΨΦ δ ΨΓ ω ( Ψ,Ψ)γ µ ΨA (ω)µ ΨΓ ρ ( Ψ,Ψ)γ µ τψa (ρ)µ e Ψ ˆQγ µ ΨA (γ)µ Non-relativistic mean-field models, based on Hamiltonians where e«ective interactions are proposed and tested: VNucl = Vlong range e«attractive +V short range repulsive +V SO +V pair Fitted parameters contain (important) correlations beyond the mean-field Nuclear energy functionals are phenomenological not directly connected to any NN (or NNN) interaction 5

6 The Nuclear Equation of State: Infinite System neutron matter e(ρ,δ=1) e ( MeV ) 10 0 Saturation (0.16 fm 3, 16.0 MeV) S(ρ)~ -10 e(ρ,δ=0) symmetric matter ρ ( fm 3 ) E A (ρ,β) = E A (ρ,β = 0) + S(ρ)β2 +O(β 4 ) Nuclear Matter [ β = ρ ] n ρ p ρ 6

7 The Nuclear Equation of State: Infinite System neutron matter e(ρ,δ=1) e ( MeV ) 10 0 Saturation (0.16 fm 3, 16.0 MeV) S(ρ)~ -10 e(ρ,δ=0) symmetric matter ρ ( fm 3 ) E A (ρ,β) = E A (ρ,β = 0) + S(ρ)β2 +O(β 4 ) Nuclear Matter Symmetric Matter [ β = ρ ] n ρ p ρ 7

8 The Nuclear Equation of State: Infinite System neutron matter e(ρ,δ=1) e ( MeV ) 10 0 Saturation (0.16 fm 3, 16.0 MeV) S(ρ)~ -10 e(ρ,δ=0) symmetric matter ρ ( fm 3 ) E A (ρ,β) = E A (ρ,β = 0) + S(ρ)β2 +O(β 4 ) Nuclear Matter Symmetric Matter Symmetry energy [ β = ρ ] n ρ p ρ 8

9 The Nuclear Equation of State: Infinite System neutron matter e(ρ,δ=1) e ( MeV ) 10 0 Saturation (0.16 fm 3, 16.0 MeV) S(ρ)~ -10 e(ρ,δ=0) symmetric matter ρ ( fm 3 ) E A (ρ,β) = E A (ρ,β = 0) + S(ρ)β2 +O(β 4 ) = E (J+Lx+ A (ρ,β = 1 ) 0)+β2 2 K symx 2 +O(x 3 ) [ β = ρ n ρ p ρ ; x = ρ ρ ] 0 3ρ 0 9

10 The Nuclear Equation of State: Infinite System neutron matter e(ρ,δ=1) e ( MeV ) 10 0 Saturation (0.16 fm 3, 16.0 MeV) S(ρ)~ -10 e(ρ,δ=0) symmetric matter ρ ( fm 3 ) E A (ρ,β) = E (ρ,β = 0)+β2 A ( J + L x+ 1 ) 2 K sym x 2 +O(x 3 ) S(ρ 0 ) = J d dρ S(ρ) = L = P 0 ρ0 3ρ 0 [ ρ 2 0 β = ρ n ρ p ρ d 2 dρ 2S(ρ) = K sym ρ0 9ρ 2 0 ; x = ρ ρ ] 0 3ρ 0 10

11 Isovector properties in nuclei In the past (and also in the present), neutron properties in stable medium and heavy nuclei have been mainly measured by using strongly interacting probes. Limited knowledge of isovector properties At present, the use of rare ion beams has opened the possibility of measuring properties of exotic nuclei more info parity violating elastic electron scattering (PVES), a model independent technique, has allowed to estimate the weak form factor at low q of a stable heavy nucleus like 208 Pb Promising perspectives for the near future 11

12 STATISTIC UNCERTAINTIES IN EDFs 12

13 Covariance analysis: χ 2 test Observables O used to calibrate the parameters p m ( O χ 2 theo. ı O ref. ) 2 ı (p) = 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 13

14 binding 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: energies, charge radii, di«raction 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. 14

15 Covariance analysis: SLy5-min (NON-Rel) and DD-ME-min1 (Rel) J. Phys. G: Nucl. Part. Phys (2015). The neutron skin is strongly correlated with J and L in both models but NOT with α D. (I will discuss on that latter) 15

16 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... 16

17 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: α D 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 17

18 DIPOLE POLARIZABILITY 18

19 Polarizability, Strength distribution and its moments 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 ν F 0 2 /E where m 1 is the inverse energy weighted moment of the strength function, defined as, S(E) = ν F 0 2 δ(e E ν ) ν 19

20 Let us compare some experiment with theory within the measured energy range... 20

21 º Example 1: Cumulative sum of α D and EWSR Cumulative sum of α D (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 E x (MeV) E x (MeV) 0 The correction in the RPA predictions for the α RPA D due to the fact that not all possible correlations have been included in the model (α corr D ) that lead to a fair reproduction of the experimental resonance width can be estimated to be αrpa D αcorr D α RPA D Γ2 4E 2 This correction might not be negligible x 21

22 Example 2: Cumulative sum of α D and EWSR 5 68 Ni D. M. Rossi et al., PRL 111, (2013) 1.25 Cumulative sum of α D (fm 3 ) SAMi-J27 SAMi SAMi-J29 SAMi-J30 SAMi-J31 SAMi (CPVC) Cumulative sum of EWSR / TRK E x (MeV) E x (MeV) 0 22

23 Theory versus experiment RPA strength concentrated at a single peak: EWSR and α D saturate earlier in E PVC strength follow reasonable trends with E, thought no calculations exist for the r np and refitting of the interaction would be needed at the PVC level. RPA has been proben to be successful in describing experimental E x and sum rules Experimental data on the α D analysed via RPA need to include the full dipole response with low and high-energy parts: Data within a given energy range, better if it is extrapolated: low energy part is more important than high energy part Data in the high energy range, should be taken carefully due to quasi-deuteron excitations contaminations (small but sizeble correction to α D ) 23

24 Isovector Giant Dipole Resonance: Dipole polarizability: a macroscopic approach electric polarizability measures tendency of the nuclear charge distribution to be distorted (α electric dipole moment external electric field ) 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 A r2 1/2 ( ) J 48J 4 Q A 1/3 within the same model, connection with the neutron skin thickness: α D A r e r np + 2 Z 5 70J rsurface np 12J 2 r 2 1/2 (I I C ) 24

25 Isovector Giant Dipole Resonance in 208 Pb: Dipole polarizability: microscopic results HF RPA α D (fm 3 ) r=0.62 FSU NL3 (a) DD-ME Skyrme SV SAMi TF r np (fm) 10 2 α D 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). Experimental dipole polarizability α D = 20.1±0.6 fm 3 ; A. Tamii et al., PRL 107, (RCNP) [No quasi-deuteron α D = 19.6±0.6 fm 3 ]. α D J is linearly correlated with r np and no α D alone within EDFs 25

26 Isovector Giant Dipole Resonance in 68 Ni: Dipole polarizability: microscopic results HF α D (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) α D J (MeV fm 3 ) (b) 68 Ni RPA r = ,15 0,2 0,25 0,3 r np (fm) X. Roca-Maza et al. PRC 92 Experimental dipole polarizability α D = 3.40±0.23 fm 3 D. M. Rossi et al., PRL 111, (GSI). α D = 3.88±0.31 fm 3 full response D. M. Rossi, T. Aumann, and K. Boretzky. 26

27 208 Pb vs 68 Ni: Dipole polarizability: microscopic results HF α D ( 208 Pb) (fm 3 ) KDE0-J FSUΛ NL3Λ TAMU-FSU DD-ME SAMi-J Skyrmes RPA RCNP 18 GSI r = 0.96 (a) 3,4 3,6 3,8 4 4,2 4,4 4,6 4,8 α D ( 68 Ni) (fm 3 ) Just as an indication: α D (A = 208)/α D (A = 68) (208/68) 5/3 ; Cercled models predict r np ( 208 Pb) = fm and r np ( 68 Ni) = fm;j = MeV;L = MeV. 27

28 PARITY VIOLATING ASYMMETRY 28

29 While Dirac Some basics... Electrons interact by exchanging a γ or a Z 0 boson. protons couple basically to γ, neutrons do it to Z 0. Electron motion governed by the Dirac equation: [ α p+βm e +V(r)]ψ = Eψ where V(r) = V C (r)+γ 5 V W (r) equation for helicity states (m e 0) [ α p+(v C (r)±v W (r))]ψ ± = Eψ ± Ultra-relativistic electrons, depending on their helicity, will interact with the nucleus seeing a slightly di«erent potential αz ± G F. Refs: Phys. Rev. C (1998); Phys. Rev. C 63, (2001); Phys. Rev. C 78, (2008); Phys. Rev. C 82, (2010); Phys. Rev. Lett (2011) 29

30 Ultra-relativistic ect Some basics... The interference between the DCS of electrons with + and helicity states, A pv = dσ +/dω dσ /dω dσ + /dω+dσ /dω electrons moving under the e«of V ± where Coulomb distortions are important solution of the Dirac equation via the Distorted Wave Born Approximation (DWBA). Input for the calculation ofv ± are theρ n andρ p (main uncertainty in ρ n ) and nucleon form factors for the e-m and the weak neutral current. 30

31 Qualitative considerations... Within the Plane Wave Born Approximation, A pv = G Fq 2 [ 4πα 4 sin 2 θ W + F n(q) F p (q) ] 2 F p (q)... which depends on F n (q) F p (q). For q 0, it is approximately, q2 6 ( r 2 n r 2 p ) = q2 6 = q2 6 [ ] r np ( r 2 n 1/2 + r 2 p 1/2 ) ( ) 2 r 2 p 1/2 r np + r 2 np variation of A pv at a fixed q dominated by the variation of r np. F p (q) well fixed by experiment 31

32 Qualitative considerations... Within the Plane Wave Born Approximation, A pv = G PWBA Fq 2 [ 4πα 4 sin 2 θ W + F n(q) F p (q) ] DWBA 3 GeV 4e-05 DWBA 850 MeV DWBA MeV F p (q)... which depends on F n (q) F p (q). For q 0, it is approximately, q2 6 Parity Violating Asymmetry 5e-05 3e-05 2e-05 ( r 2 n r 2 p ) = q2 [ ] 1e-05 r np ( r 2 6 n 1/2 + r 2 p 1/2 ) = q2 ( ) 0 0 0,5 1 1,5 2 r 6 p 2 1/2 r 2,5 np + r 3 2 np q (1/fm) variation of A pv at a fixed q dominated by the variation of r np. F p (q) well fixed by experiment 32

33 Qualitative considerations... Within the Plane Wave Born Approximation, A pv = G Fq 2 [ 4πα 4 sin 2 θ W + F n(q) F p (q) ] 2 F p (q)... which depends on F n (q) F p (q). For q 0, it is approximately, q2 6 ( r 2 n r 2 p ) = q2 6 = q2 6 [ ] r np ( r 2 n 1/2 + r 2 p 1/2 ) ( ) 2 r 2 p 1/2 r np + r 2 np variation of A pv at a fixed q dominated by the variation of r np. F p (q) well fixed by experiment 33

34 So, let us check DWBA results... 34

35 208 Pb: direct correlations δa pv 1%; δ r np 0.02 fm; δl 10 MeV (systematic) X. Roca-Maza, et al., PRL (2011) 10 7 A pv 7,4 7,2 7,0 6,8 MSk7 HFB-8 v090 D1S D1N SGII SkP HFB-17 Tarbert Hoffmann Sk-T6 Klos SLy4 SkM* SkX SLy5 BCP MSkA MSL0 DD-ME2 SIV SkMP DD-ME1 Sk-Rs RHF-PKA1 SkSM* DD-PC1 RHF-PKO3 FSUGold Zenihiro Ska Sk-T4 NL3.s25 PC-PK1 G2 SkI5 PC-F1 PK1 G1 SV SkI2 r np (fm) ,1 0,15 0,2 0,25 0,3 r np (fm) HFB-8 MSk7 v090 Linear Fit, r = Mean Field From strong probes Sk-Gs PK1.s24 NL-SH NL-SV2 TM1, NL-RA1 NL3 SkP D1S Linear Fit, r = Mean Field SkX Sk-T6 HFB-17 SGII D1N SkM* DD-ME1 DD-ME2 SLy5 SLy4 FSUGold SkMP SkSM* SIV MSL0 MSkA BCP Ska DD-PC1 G2 Sk-T4 NL3.s25 PK1.s24 Sk-Rs SkI2 RHF-PKA1 SV Sk-Gs L (MeV) NL3* NL2 NL1 RHF-PKO3 NL3* NL3 PK1 NL-SV2 TM1 SkI5 G1 NL-RA1 PC-F1 NL-SH PC-PK1 EDF correlations allows to determine r np and L without direct assumpt. on ρ, JLab and Mainz forthcoming experiments Di«erent experiments on proton elastic scattering, antirpotonic atoms and pion-photoproduction agrees with the correlation 35 NL2 NL1

36 CONCLUSIONS 36

37 Conclusions: A precise and model-independent determination of r np in 208 Pb via PVES experiments probes the symmetry energy. We demonstrate a close linear correlation between A pv and r np within the same framework in which the r np is correlated with L (expected to be better as heavier the nucleus). Other experiments fairly agree with the correlation between A pv and r np in 208 Pb. EDFs (RPA) show a linear correlation between α D J and r np A pv and α D are complementary observables that may set tight constraints on the density dependence of the symmetry energy around saturation density, if precisely and» or systematically measured. 37

38 B. K. Agrawal 1 P. F. Bortignon, M. Brenna, G. Colò 2,3 W. Nazarewicz 4,5,6 N. Paar, D. Vretenar 7 J. Piekarewicz 8 Collaborators: P.-G. Reinhard 9 Michal Warda 10 Mario Centelles, Xavier Viñas 11 Misha Gorchtein 12 Chuck Horowitz 13 1 Saha Institute of Nuclear Physics, Kolkata , India 2 Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, I Milano, Italy 3 INFN, Sezione di Milano, via Celoria 16, I Milano, Italy 4 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 5 Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6 Institute of Theoretical Physics, University of Warsaw, ulitsa HoÅija 69, PL Warsaw, Poland 7 Physics Department, Faculty of Science, University of Zagreb, Zagreb, Croatia 8 Department of Physics, Florida State University, Tallahassee, Florida 32306, USA 9 Institut für Theoretische Physik II, UniversitÃd t Erlangen-Nürnberg, Staudtstrasse 7, D Erlangen, Germany 10 Katedra Fizyki Teoretycznej, Uniwersytet Marii Curie-Sklodowskiej, ul. Radziszewskiego 10, PL Lublin, Poland 11 Departament d Estructura i Constituents de la Matèria and Institut de Ciències del Cosmos, Facultat de Física, Universitat de Barcelona, Diagonal 647, E Barcelona, Spain 12 PRISMA Cluster of Excellence, Institut für Kernphysik, Johannes Gutenberg-Universität, Mainz, Germany 13 Department of Physics and Nuclear Theory Center, Indiana University, Bloomington, IN 47405, USA 38