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Role of axial-vector meson exchange interaction in the hypernuclear nonmesonic weak decays

Role of axial-vector meson exchange interaction in the hypernuclear nonmesonic weak decays. K. Itonaga Univ. of Miyazaki T. Motoba Osaka E-C. Univ. T. Ueda Hiroshima Th. A. Rijken Radboud Univ. NP@JPARC Symposium Tokai, Jun. 1-2, 2007. §Basic problems:

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Role of axial-vector meson exchange interaction in the hypernuclear nonmesonic weak decays

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  1. Role of axial-vector meson exchange interaction in the hypernuclear nonmesonic weak decays K. Itonaga Univ. of Miyazaki T. MotobaOsaka E-C. Univ. T. Ueda Hiroshima Th. A. Rijken Radboud Univ. NP@JPARC Symposium Tokai, Jun. 1-2, 2007

  2. §Basic problems: □　 To understand the nonmesonic decay observables Γnm, Γn/Γp, α1 totally and consistently. □　 Meson theoretical model can explain the decay interaction or not? 　 　□If the meson exchange model works, which specific role can each meson play? □To find a link between the meson theoretical interaction and the quark physics.

  3. §Brief review • First step ; one-pion exchange int. • natural, but naïve • tensor force dominance • →Γp : enhanced • Γn/Γp : small • Second step; • (a) meson-octet (psendo-scalar, vector) • exchange int • heavy meson (ρ, ω, Ｋ, Ｋ＊) exch. int • ←since Non-mesonic weak • decay is high-mom. process

  4. (b) 2π/σ-exch. : tensor-free int. 2π/ρ-exch. : tensor : opposite-sign to that of 1π-exch. Γnm : can be explained Γn/Γp : ～0.3-0.4(improved) 3. 3rd step : K-exch. int. is recognized to be important. Vπ+ＶK : additive for (3S1 → 3P1) destructive for (1,3S →1,3S) and (3S1→ 3D1) :Γn/Γp : enhanced “exp” But, α1(αΛ) asymmetry parameter : cannot be explained in the meson-exchange model

  5. Present stage : • How to explain the asymmetry parameter? • α1(αΛ) : sign and magnitude? • Effective field theory. • Isoscalar int., σ-meson exchange + quark int. • Axial-vector meson (meson-pair) exchange? • a1-exchange is New. • [ note: a1-exchange is important in “strong” • V(ΛN-ΛN) & V(NN-NN) int. in ESC04 model ]

  6. §Angular distribution of an emittedproton from the polarized hypernuclei pure vector polarization PH density matrix Nonmesonic decay L+p → n+p k1 : neutron mom. k2 : proton mom. (kp=k2)

  7. Angular distribution of the proton

  8. §Why a1 meson exchange? □We need a potential which has properties: ・　short range ・　central force : negative = opposite sigh to V２π/σ ・　tensor force : positive = opposite sign to V2π/ρ □In strong NN-force, a1-exch. pot ・　central : opposite to Vσ（ＮＮ） ・　tensor : opposite sign to Vρ(NN) ⇒a1 meson (ρπ-meson pair) exch. favorable in weak case ??

  9. § A1 exchange weak decay potential N N N N N N s p r w a1 a1 a1 N N S r w w p p N N N L L L （A） （B） ρπ/a1exch. σπ/a1 exch.

  10. N N r a1 N p N L

  11. N N p w a1 S r N L ρπ/a1（B） exch.weak decay potential a1 meson m=1230. MeV Jπ＝1+ a1→ρ+π Γexp(a1→ρπ)＝250－600MeV （B）

  12. N N s a1 N w p N L σπ/a1 exch.weak decay potential a1 meson m=1230. MeV Jπ＝1+ a1 →σ+π seen ( Part. Data Booklet )

  13. N N p2´ p1´ p1´-p1+k ρ p1-k a1 π k p2 p1 N Λ （Ｎ） ２．Loop integral,parameters diverge at large k2 regularization factor introduced of ESC04 model

  14. Force characteristics transition channel Central negative, strong 1S0→1S0 opposite behavior 3S1→3S1 to V2p/s Tensor positive 3S1→3D1 opposite behavior to V2p/r Vector 1S0→3P0 similar behavior 3S1→1P1 to V2p/r

  15. Characteristic features of V(ΛN – NN) potentials

  16. Gnm[GL] , aL=a1 +0.08 -0.00 Exp. Gnm=0.424±0.024[1] Gｎ/Gp=0.45±0.11±0.09[1] aL=0.07±0.08 [3]

  17. Nonmesonic decay rates and asymmetry parameter p+2p/r+2p/s+w+K+rp/a1(A+B) +sp/a1 Gnm=0.358GL Gn/Gp=0.508 Asymmetry parameter

  18. p+2p/r+2p/s+w+K+rp/a1(A+B) +sp/a1 Gnm=0.364GL Gn/Gp=0.503 Asymmetry parameter

  19. Gnm [GL] +0.18 -0.00 Exp. Gnm=0.940±0.035[1] Gｎ/Gp=0.56±0.12±0.04[1] aL=-0.16±0.28[3]

  20. Nonmesonic decay rates and asymmetry parameter p+2p/r+2p/s+w+K+rp/a1(A+B) +sp/a1

  21. §Summary and outlook • The ρπ/a1and σπ/a1 –exch. interactions have vital roles in modifying the short-range part of the ΛＮ→ＮＮ transition potential. • In our π+2π/σ+2π/ρ+ω+Ｋ+ρπ/a1+σπ/a1exch. model, the potential has following features: • central force : negatively strong at • tensor force : positive at weak at • vector force : strong for (b), (e) & (f) channels.

  22. Nonmesonic decay rates Γnm, n/p ratio (Γｎ/Γp) and asymmetry parameter α1（αΛ) are evaluated for for the adopted coupling const. and parameters of ρπ/a1 and σπ/a1 – exch. int.

  23. The decay observables (exp. data) are rather • well explained in our model, though not enough, • especially for Γnm. • The coupling constants and the parameters • adopted in our model have some uncertainties • which should be further studied.

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