Spin Polarization in d g → n p

1. Spin Polarization in dg → n p Chang Ho Hyun Daegu University Work with S. Ando (Daegu) Y.-H. Song (South Carolina) K. Kubodera (South Carolina) HNP2011, Pohang, Korea February 21-24

2. Outline • Motivation • Formalism • Spin polarization P’y • Results • Conclusion

3. Motivation • NN force well understood at Elab ≤ 300 MeV • Nevertheless many discrepancies in the polarization observables • Puzzles with spin • - Spin crisis: origin of the nucleon spin • - P’y in deuteron photo-disintegration • - Ay puzzle in neutron-deuteron scattering • Spin is still an open problem in few-body systems

4. Spin polarization P’y • - Experiments: 1961-1983 (John61, Jewell65, Nath72, Holt83) • - Jewell65 Jewell et al., PR 139, B71 (1965)

5. - SNPA (standard nuclear physics approach) R. Schiavilla, PRC 72, 034001 (2005) “New and more accurate measurements of the induced polarization in the 2H(γ,n)1H reaction are needed to establish unequivocally whether there is a discrepancy between theory and experiment.”

6. Need to investigate the problem with modern theories • - Effective field theories (EFTs) provide proper tools • Our choice: Pionless EFT with dibaryon fields

7. Formalism • NN force with chiral perturbation theory • Meson-exchange currents with systematic expansion rules • - Integrate out heavy degrees of freedom • Pionless theory • - Integrate out the pion • - NN contact interactions • - Low energy constants determined by low-energy data • Dibaryon fields • - Nucleon bubbles summed infinitely, the deuteron pole reproduced successfully • - Ensure fast convergence with only a few leading terms

8. Lagrangians • si, ti: dibaryon fields in 1S0 and 3S1 states, respectively • Bi : external photon fields

9. Low energy constants • ys, yt: dNN coupling constants. Fitted to effective range parameters • L1, L2 : ddV coupling constants. Fitted to np capture cross section at threshold and deuteron magnetic moment • Total np capture cross section

10. Spin polarization P’y • Definition z’ g y’ n p s+,-(q) : differential cross section with parallel and anti-parallel neutron spin Py’ becomes non-zero due to the interference of even and odd transition amplitudes

11. Amplitudes and cross sections • Feynman diagrams • Amplitudes

13. Cross section for unpolarized neutron • Projection operator for the neutron spin

14. Cross section for polarized neutron • Result for P’y

15. Results • Differential cross section of unpolarized neutron Eg = 19.8MeV De Pascale et al. PRC 32, 1830 (1985) Our work SNPA

16. P’y at Eg = 2.75 MeV SNPA Our work

17. P’y at q = 45

18. P’y at q = 90

19. P’y at q = 135

20. Conclusion • What is the correct answer? • Experiment side • - Large errors • - Data do not converge (q = 90) • - Revival of the measurement at modern facilities and instruments is absolutely necessary • Theory side • - Results for unpolarized cross section show good agreement • - SNPA results are discrepant from pionless EFT • - Systematic investigation at higher orders are inevitable works in the future

21. Possible scenarios • - New measurements or other theories consistent with SNPA • - New measurements or other theories consistent with pionless EFT • - New measurements still inconsistent with any theory • Spin is still an open and active subject in the nuclear physics • P’z ∝ cosq: Parity-violating effect. In progress with Ando and Shin