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Weak decay of 5 Λ He and 12 Λ C : experimental results

10. Oct. 2006 HYP2006(Mainz). 1. Weak decay mode of L hypernucleus. Γ π _ ( L → ｐ + π － ) Γ π 0 ( L → ｎ + π ０ ). Mesonic q ～ 100MeV/c. Γ m. 1/ t HY =Γ tot. Γ p ( L +“ ｐ”→ ｎ + ｐ ) Γ n ( L +“ ｎ”→ ｎ + ｎ ) Γ 2N (ΛNN →NNN). Non-Mesonic ( NMWD ) q ～ 400MeV/c. Γ nm.

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Weak decay of 5 Λ He and 12 Λ C : experimental results

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  1. 10. Oct. 2006 HYP2006(Mainz) 1 Weak decay mode of L hypernucleus Γπ_(L→ ｐ + π－) Γπ0(L → ｎ + π０ ) Mesonic q～100MeV/c Γm 1/tHY =Γtot Γp(L +“ｐ”→ ｎ + ｐ) Γn(L +“ｎ”→ ｎ + ｎ) Γ2N (ΛNN →NNN) Non-Mesonic(NMWD) q～400MeV/c Γnm Weak decay of 5ΛHe and 12ΛC : experimental results RIKEN H. Outa for KEK-PS E462 / E508 collaborations Osaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku Univ.e, Univ.of Tokyof, Tokyo Inst. Tech.g, KRISSh, RIKENi S. Ajimuraa, K.Aokib, A.Banuc, H. C. Bhangd, T. Fukudab,O. Hashimotoe, J. I. Hwangd, S. Kameokae, B. H. Kangd, E. H. Kimd, J. H. Kimd, M. J. Kimd, T. Marutaf, Y. Miurae, Y. Miyakea, T. Nagaeb, M. Nakamuraf, S. N. Nakamurae,H. Noumib, S. Okadag, Y. Okayasue, H. Outab, H. Parkh,P. K. Sahab, Y. Satob, M. Sekimotob, T. Takahashie,H. Tamurae, K. Tanidai, A. Toyodab, K. Tsukadae,T. Watanabee, H. J. Yimd Study of the mechanism ofbaryon-baryon weak interaction

  2. 2 Weak decay mode of L hypernucleus Γπ_(L→ ｐ + π－) Γπ0(L → ｎ + π０ ) Mesonic q～100MeV/c Γm 1/tHY =Γtot Γp(L +“ｐ”→ ｎ + ｐ) Γn(L +“ｎ”→ ｎ + ｎ) Γ2N (ΛNN →NNN) Non-Mesonic(NMWD) q～400MeV/c Γnm Contents of the present talk Γ(Λn→nn)/Γ(Λp→np)ratio n/p spectra from A=5,12S. Okadaet al. PLB 597 (2004) 249-256 A= 5B.H. Kang et al.PRL 96 (2006) 062301 A=12 M.J. Kim et al. PLB 641 (2006) 28 [ M. Kim, poster session ] Asymmetry parameter T. Maruta et al.nucl-ex/0509016 [ T.Maruta, parallel session A2] Mesonic & non-mesonic decay widths[ S. Okada, poster session ] S. Kameokaet al.Nucl. Phys. A754 (2005) 173c-177c S. Okadaet al.Nucl. Phys. A754 (2005) 178c-183c Two nucleon-induced NMWD (ΛNN→NNN)[H.Bhang, parallel session A2]

  3. 3 Simple theoretical model Gn / Gp～0.1 One Pion Exchange (OPE) Meson Exchange mechanism Direct Quark mechanism N N N N S N W Λ π N Tensor-dominant  requires the final Nn pair to have isospin 0. Λ π,K,η,ρ,ω,K* Gn / Gp 1.5 0.5 1 0 N N Exp. (for 5LHe) 0.93±0.55 (Szymanski et al.) Λ N : The most important observable used to study the isospin structure of the NMWD. Gn / Gp ratio Γp(Λ+“ｐ”→ ｎ + ｐ) Γn(Λ+“ｎ”→ ｎ + ｎ) Theoretical Gn / Gp ratio puzzle

  4. 4 n n n n Final state interaction (FSI) effect LNN→NNN (2N-induced process) rescattering p N N p p p p p N n W π Λ (One of the theoretical model) n N N Experimental difficulties in single nucleon measurement • Difficulty in detecting neutrons.  There is no experiment to observe both of the protons and neutrons simultaneously with high statistics. • Final state interaction (FSI) effect  not well established • Distinguish between the FSI and ”LNNnNN” process

  5. The present experiment 5 Coincidence Select ΛN→NN events w/o FSI effect & ΛNN→NNN. NMWD L NMWD Coincidence * cosθ<－0.8 * E(N1)+E(N2) cut p p p p p p n n n n n n n n KEK-PS E462/E508 Direct measurement of the Gn / Gp ratio NMWD : ΛN→NN 1) Angular correlation ( back-to-back, cosq<-0.8 ) 2) Energy correlation ( Q～E(N1)+E(N2) ～152MeV ) Select light hypernuclei to minimize FSI effect, 5LHeand12LC

  6. 6 The ground state of 6LLi is above the threshold of 5LHe + p. 6LLi (g.s.)5LHe + p 4.6×104events decay counter K+ 6.2×104events π+ Target: 6Li,12C Excitation-energy spectra for 6LLi and 12LC 5LHe

  7. Decay counter Setup 7 π p n n K p (KEK-PS K6 & SKS) Decay arm Solid angle: 26% 9(T)+9(B)+8(S)% polarization axis Charged particle： ・TOF (T2→T3) ・tracking（PDC） Neutral particle： ・TOF (target→NT) ・T3 VETO N: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm

  8. ８ Excitation spectra w/ coincident decay particles for 12LC previous experiment at BNL inclusive w/ proton w/ pi± w/neutron w/ gamma Excitation spectra w/ coincident decay particles for 5LHe The g.s. peak is clearly seen in all spectra with coincident decay particles. S. Kameokaet al. Nucl. Phys. A754 (2005) 173c S. Okadaet al. Nucl. Phys. A754 (2005) 178c

  9. ９ LN→nN LNN→nNN FSI re-scattering n n n n n n n n p p p p p p p p p p p p p p n n n n n n n Expected Spectrum distribute low energy region up to Q/2 broad peak around Q/2 continuous distribution counts Energy Energy spectra (image) Q/2

  10. 10 Calculation by Garbarino et al. Single proton/neutron spectra from 5ΛHe and 12ΛC Nn～2Np S.Okada et al., PLB 597 (2004) 249

  11. 11 np- & nn- angular distribution (5ΛHe) Back-to-back Back-to-back Gn/Gp ~ Nnn/ Nnp= 0.45±0.11±0.03 systematic error is mainly come from efficiency for neutron (6%) + acceptance(3%)

  12. 12 Coincidence Measurement (A=12) En +Ep n + p Counts En +En 12ΛC n + n Ep +Ep p + p qNN cos MeV Gn/Gp ~ Nnn/ Nnp= 0.51±0.13±0.05 Analysis detail on Kim’s Poster

  13. 13 Direct Quark mechanism Meson Exchange mechanism One Pion Exchange (OPE) N N Theo. N Λ N N π,K,η,ρ,ω… S W π N Λ Gn / Gp 1.5 0.5 1 0 N N 5LHe (E462) Nnn / Nnp (5LHe)= 0.45±0.11±0.03 Kang et al. PRL 96 (2006) 062301 12LC (E508) Γn / Γp (12LC)= 0.51±0.13±0.05 Λ N Kim et al. PLB641 (2006) 28 Gn / Gp ratio Previous exp. (at BNL) 0.93±0.55 (Szymanski et al.) for 5LHe Exp.

  14. 14 1/2 N(q+(+j))×N(q-(-j)) R = N(q+(-j))×N(q-(+j)) Asymmetry measurement of decay proton Asymmetry : Volume of the asymmetric emission from NMWD P N(q) = N0(1 +Acosq) jK >0 L p/p Asymmetry θ K+ =N0(1 +aPcosq) p+ L jK Asymmetry parameter jK <0 (R + 1) N(q+) A = R = , (R - 1) N(q-) jK p+ K+ θ p/p P L Difference of acceptance & efficiency → canceled out !!

  15. 15 Importance of αnm measurement If assuming initial S state We can know the interference between states with different Isospin and Parity . (Applying DI=1/2 rule)

  16. 16 Ap=apPLe Ap:Asymmetry of Pion ap:Asymmetry Parameter of Pion (=－0.642±0.013) PL:Polarization of Lambda e :Attenuation factor We can calculate aNMwithout theoretical help ! p aNM for 5ΛHe NMWD Estimated from mesonic decay ・Polarization ofL ・Asymmetry Parameter of Proton Ap=aNMPLe p

  17. Asymmetry parameter of 5ΛHe 17 Theory: - 0.6～- 0.7 Nucl.Phys.A754 (2005) 168c nucl-ex/050916 statistical p contami aNM=0.08±0.08+0.08 -0.00 p

  18. 18 statistical p contami aNM=－0.14±0.28+0.18 p E160 : －0.9±0.3 Asymmetry parameter of 12C, 11B L L -0.00

  19. 19 p p Comparison with previous experiment E508 Precious Experiment dE/dx&Etot TOF&Etot range&Etot PID function w/proton w/pion w/proton Energy spectrum

  20. 20 p+K,OME can reproduce Gn/Gp ratio but predict large negative aNM p+2p/r+2p/s+w+K+rp/a1 Calculation by Itonaga p+K+DQ Gn/Gp and aNM can be reproduced by p+K+s+DQ model p+K+s OME p+K+s+DQ Sasaki et al. PRC71 (2005)035502 p+K (1) Large b(1S0→3P0) and f(3S0→3P1) amplitude (2) Violation of ΔI=1/2 rule considered Comparison with recent calculations T. Maruta ; parallel A2 OPE

  21. 21 Decay Widths 　→ Okada’s poster

  22. 22 aNM=0.08±0.08+0.08 aNM=－0.14±0.28+0.18 p p Summary • LN→NN was directly observed for the first time !! 5ΛHe : Γn / Γp ratio～ Nnn / Nnp = 0.45±0.11±0.03 Kang et al.PRL 96 (2006) 062301 12ΛC : Γn / Γp ratio =0.50±0.13±0.05 Kim et al.PLB641 (2006) 28 ◆ Asymmetry parameter measured with improved accuracy !! 5ΛHe : 11ΛB and 12ΛC : Maruta et al. nucl-ex/0509016; thesis ◆ Total & partial decay rates are measured very accurately -0.00 -0.00 [1] Importance of shorter-range mechanism OPE 　⇒　Heavy meson & DQ exchange [2] Suggesting significant contribution from ΛN initial spin-singlet initial state σ-meson exch. / ΔI=1/2 violation? ⇒ 4ΛH & ΛΛ→ΛN@J-PARC

  23. Spare OHPs

  24. 8 Charged PID Charged particles from 5LHe Constant background very small PID function Good pp d separation Decay particle identification Neutral PID Neutral particles from 12LC Neutron energy resoltion → 7MeV(FWHM) at 75MeV 1 / b spectra Good gn separation

  25. n+p coincidence n+p coincidence n+p coincidence n+p coincidence n+n coincidence n+n coincidence n+n coincidence n+n coincidence p+p coincidence p+p coincidence n+p n+p n+n n+n p+p Comparison with theoretical calc. for angular correlation 5LHe (E462) 12LC (E508) experimental data Pair number /NMWD theoretical calc. Pair number /NMWD cosq cosq Garbarino’s calc. assuming Gn/Gp = 0.46 (for 5LHe ), 0.34 (for 12LC ) considered 2N-induced(～20%), FSI 21 Phys. Rev. Lett. 91 (2003) 112501

  26. Neutron Efficiency Correction

  27. Nn / Np Ratio Γn:Γp NnNp 1 : 1 … 3 : 1 1 : 2 … 2 : 1 0 : 1 … 1 : 1 Γn(Λ+“ｎ”→ ｎ+ ｎ) Γp(Λ+“ｐ”→ ｎ+ ｐ) If Γn / Γp = 1 → Nn / Np = 3 If Γn / Γp = 0.5 → Nn / Np = 2 If Γn / Γp = 0 → Nn / Np = 1 Naive estimation (without considering FSI and ΛNN→NNN) Nn / Np = 2×Gn / Gp + 1 To avoid suffering from FSI effect & ΛNN→NNN, High energy threshold

  28. Mass number dependence of neutron energy spectra ( A=5,12,89 ) 5ΛHe No peaking at Q / 2 (76MeV) even 5ΛHe suggested larger contribution of ΛNN→NNN or FSI than theoretical prediction. ( previous experiment ) Theoretical calc. w/ FSI Q/2 Q / 2 = 76 MeV As the mass number become lager, the number of neutron become lager in the low energy part, and smaller in the high energy part.

  29. Spare OHPsfor asymmetry

  30. E160 12C event L 11B event L Statistical comparison with E160 E508 P-coin spectrum ×11 246event 2779event ×5 393event 2122event ×8 Total 639event 4901event

  31. Comparison with E160 PID function Energy spectrum

  32. Polarization (P), asymmetry parameter(a) W(q) = 1 + A1P1(cosq) A1 = ka1PL Asymmetry parameter S.Ajimura et al., PLB282, 293 (1992)

  33. KEK-PS E278 experiment Experimental target: 6Li Observable: Mesonic decay branching ratios Polarization p / p separation by dE/dx and Etot E278 Decay counter

  34. Polarization of 5LHe 6Li(p+, K+) spectrum S. Ajimura et al., PRL80, 3471 (1998) Consistent with experimental data

  35. Proton asymmetry parameter of 5LHe 6Li(p+, K+) spectrum S. Ajimura et al., Submitted to PRL Inconsistent with meson exchange model

  36. ー : E462 ー: E278 : Motoba et al. NPA577 (1994) 293c Polarization of L NOTE: Calculation by Motoba et al. considers excited state at E=4.5 MeV

  37. Gp- /GL of 5LHe E462 HYP2003 proceedings Nucl. Phys. A754 (2005) Other results Lifetime p- branching ratio p0 branching ratio p - decay width for 5LHe p0 decay width for 5LHe and 12LC Errors were much improved !!

  38. Null asymmetry test (p,pX)reaction : Only Strong Interaction p or p Asymmetry = 0expected Instrumental Asymmetry<0.3%

  39. aNM=0.31±0.22 p L Coincidence p p p p n n n n n np coincidence analysis np back-to-back event NMWD

  40. N N S W N p,K,s L One and only(?) solution • + K + s + DQ Sasaki et al. PRC71 (2005)035502 • b(1S0→3P0)とf(3S0→3P1) amplitudeに影響を与える • DI = 3/2が大きく寄与する • 今回Gn/Gp ratioとaNMを高精度で測定したことにより、 こういう反応機構の必要性が認識された。 p

  41. Mijung のOHPから

  42. Uniform components subtraction

  43. FSI consideration using pp-pairs rn,pfraction ratio of theneutron and proton induced channels. fn,pis reduction factor due to FSI. gn,pis cross over influx of neutron(proton) from proton(neutron) due to FSI. p,q,q’are angular acceptance factor. , , , Before FSI correction 4%

  44. Systematic error calculation • Intentionally add 2 pp events in • -0.8<cosq<-0.7 2) 3)Uniform b.g. level for different angular regions:6.6%

  45. 6Li Hypernuclear mass spectra L inclusive 5.2×104 events 6Li + p+→ L6Li+ K+ L6Li→ L5He+ p (Sn-1,SL) p coin 3.2×103 events 18.3MeV (Pn-1,PL) p decay L decay 8.3MeV p coin 1.6×103 events (Pn-1,SL) 5Li 0MeV 5LHe 6LLi

  46. Instrumental Asymmetry (p,pC)reaction : Only Strong Interaction Asymmetry = 0expected Instrumental Asymmetry<0.3%

  47. p p n Λ p p n n Λ n n p Λ To J-PARC Non-mesonic weak decay of 4LHe and 4LH see S.Ajimura : J-PARC LOI 21 Spin / isospin dependence Gnm(4LH) = ( 3Rn1+ Rn0 + 2Rp0 ) ×r4 / 6 Gnm(4LHe) = ( 2Rn0 + 3Rp1 + Rp0 ) ×r4 / 6 Gnm(5LHe) = ( 3Rn1+ Rn0 + 3Rp1 + Rp0 ) ×r5 / 8 RNS … N : Lnnn,Lpnp S : spin = 0 or 1 4He (K-,p-) 4LHe or 4He (p+,K+) 4LHe  n+n back-to-back 4He (K-,p0) 4LH  p+n back-to-back (p0 spectrometer )  Need one-order higher statistics. J-PARC

  48. K6/SKS setup K+ decay counter π+

  49. Z-vertex Identification of hypernuclear formation K+ p+ T1 Z target Mass

  50. Non-mesonic weak decay N N S W π N Λ • + N→ N + N • + p → n + p : Gp = a2+b2+c2+d2+e2+f2 L + n → n + n : Gn = a2+b2+f2 Gn / Gp ratio : The most important observable to study the isospin structure of the NMWD. Simple theoretical model strong tenser coupling (DL=2, DS=2) → dominant term 3S1→3D1 (amplitude “d”) One Pion Exchange (OPE) model OPE : Gn / Gp～0.1 Exp. : Gn / Gp～ 1 Gn / Gp ratio puzzle with large error

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