Spin Polarization in d g → n p

Chang Ho HyunDaegu University

Work withS. Ando (Daegu)

Y.-H. Song (South Carolina)K. Kubodera (South Carolina)

HNP2011, Pohang, KoreaFebruary 21-24

Outline

• Motivation

• Formalism

• Spin polarization P’y

• Results

• Conclusion

MotivationNN 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

Spin polarization P’y

- Experiments: 1961-1983 (John61, Jewell65, Nath72, Holt83)

- Jewell65 Jewell et al., PR 139, B71 (1965)

- SNPA (standard nuclear physics approach) R. Schiavilla, PRC 72, 034001 (2005)

“New and more accurate measurements of the induced polar-ization in the 2H(γ,n)1H reaction are needed to establish un-equivocally whether there is a discrepancy between theory and experiment.”

Need to investigate the problem with modern theories

- Effective field theories (EFTs) provide proper tools

Our choice: Pionless EFT with dibaryon fields

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 re-produced successfully - Ensure fast convergence with only a few leading terms

Lagrangians

- si, ti : dibaryon fields in 1S0 and 3S1 states, respectively- Bi : external photon fields

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

Definition

Spin polarization P’y

p

n

g z’y’

s+,-(q) : differential cross section with parallel and anti-parallel neu-tron spin

Py’ becomes non-zero due to the interference of even and odd transition amplitudes

Amplitudes and cross sections- Feynman diagrams

- Amplitudes

- Cross section for unpolarized neutron

- Projection operator for the neutron spin

- Cross section for polarized neutron

- Result for P’y

ResultsDifferential cross section of unpolarized neu-

tronEg = 19.8MeV De Pascale et al. PRC 32, 1830 (1985)

Our work SNPA

P’y at Eg = 2.75 MeV

Our work SNPA

P’y at q = 45

P’y at q = 90

P’y at q = 135

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 in-evitable works in the future

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 the-ory Spin is still an open and active subject in the nuclear

physics P’z ∝ cosq : Parity-violating effect. In progress with

Ando and Shin