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Non-Mesonic Weak Decays of 5 Λ He and 12 Λ C Hypernuclei formed by ( p + ,K + ) Reaction

22. Aug. 2006 FB18 (P28). Non-Mesonic Weak Decays of 5 Λ He and 12 Λ C Hypernuclei formed by ( p + ,K + ) Reaction. RIKEN H. Outa. for KEK-PS E462 / E508 collaborations. Osaka Univ. a , KEK b , GSI c , Seoul Univ. d , Tohoku Univ. e ,

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Non-Mesonic Weak Decays of 5 Λ He and 12 Λ C Hypernuclei formed by ( p + ,K + ) Reaction

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  1. 22. Aug. 2006 FB18 (P28) Non-Mesonic Weak Decays of 5ΛHe and 12ΛC Hypernuclei formed by (p+,K+) Reaction 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. Fukudac,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 Γ(Λn→nn)/Γ(Λp→np)ratio A= 5B.H. Kang et al.PRL 96 (2006) 062301 A=12 M.J. Kim et al. nucl-ex/0601029 : PLB in press n/p spectra from A=5,12S. Okadaet al. PLB 597 (2004) 249-256 Asymmetry of proton from polarized hypernuclei T. Maruta et al. nucl-ex/0509016 Mesonic & non-mesonic decay widths　⇒　may be omitted in this talk S. Kameokaet al.Nucl. Phys. A754 (2005) 173c-177c S. Okadaet al.Nucl. Phys. A754 (2005) 178c-183c

  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 Weak decay of L hypernucleus L weak decay in free space tL = 263.2±2.0 ps L→ ｐ + π－: 63.9±0.5 % L → ｎ + π0 : 35.8±0.5 % → Well known. Study of the mechanism ofbaryon-baryon weak interaction

  3. [1]Γ(Λn→nn) /Γ(Λp→np) Ratio －　Long standing puzzle solved

  4. 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 strong tenser coupling (DL=2, DS=2) → dominant term 3S1→3D1 (amplitude “d”)

  5. 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 the 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

  6. 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 single nucleon spectrum distribute low energy region up to Q/2 broad peak around Q/2 continuous distribution counts Energy Energy spectra (image) Q/2

  7. The present experiment 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

  8. 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

  9. Decay counter Setup 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

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

  11. previous experiment at BNL inclusive w/ proton w/ π± w/ neutron w/ γ 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) 173-177 S. Okadaet al. Nucl. Phys. A754 (2005) 178-183

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

  13. 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= 0.50±0.13±0.05

  14. 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)= 0.50±0.13±0.05 Λ N Kim et al. nucl-ex/0601029 PLB in press Gn / Gp ratio Previous exp. (at BNL) 0.93±0.55 (Szymanski et al.) for 5LHe Exp.

  15. [2] Asymmetry of proton emission from polarized hypernuclei

  16. 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 is canceled out !

  17. 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)

  18. 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

  19. 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

  20. Asymmetry parameter of 5ΛHe Theory: - 0.6～- 0.7 L Nucl.Phys.A754 (2005) 168-172 nucl-ex/050916 ; submitted to PRL Instrumental Asymmetry <0.003 statistical p contami aNM=0.08±0.08+0.08 -0.00 p

  21. 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

  22. p+K,OME can reproduce Gn/Gp ratio but predict large negative aNM p+K+DQ Gn/Gp and aNM can be reproduced only 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 OPE

  23. [3] Precise measurement of π-mesonic / non-mesonic decay widths － Test of Λ-nucleus potential

  24. Lifetime measurements with(π+,K+) reaction & SKS Kameoka et al., HYP03 proc NPA754 (2005) 47

  25. Lifetime: 278+11 ps (E462) -10 Other Results-1: G p0 and Λ-α potential 5LHe : Gp0/ GL = 0.201±0.011 HYP03 proceesings Nucl. Phys. A754 (2005)

  26. Other results-2: A dependence of ΓNM HYP03 proceedings Nucl. Phys. A754

  27. 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. PLB, in press ◆ Asymmetry parameter measured with improved accuracy !! 5ΛHe : 11ΛB and 12ΛC : Maruta et al. nucl-ex/0509016; thesis ◆ Mesonic and Non-mesonic decay widths are precisely measured － Test of Λ-nucleus potential -0.00 -0.00 [1] Importance of shorter-range mechanism OPE 　⇒　Heavy meson & DQ exchange [2] Significant contribution from ΛN initial spin-singlet initial state －　σ-meson exchange ? / Violation of ΔI=1/2 rule ?

  28. Spare OHP

  29. n n p n W p π- Λ n p 4 Λn→nn like n p n p n n W π0 Λ 1 n p n Λp→np like ΛNN→NNN consideration Garbarino Seen as….

  30. 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

  31. 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 !!

  32. Very recent measurements with(π+,K+) reaction & SKS Kameoka et al., HYP03 proc nucl-ex/0402023 Preliminary 47

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

  34. 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

  35. 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

  36. Coincidence analysis (5ΛHe) Angular correlation Energy sum cos θ＜－0.8 n + p 90 event (1.34) 30 event n + n (4.38) Q-Value ~153MeV estimated contamination from π－ absorption

  37. Gn/Gp estimation from Coincidence Measurement

  38. Coincidence Measurement(E462/E508) Energy sum distribution Estimated contamination from Pair numbers/NMWD n + p n + n

  39. Uniform components subtraction

  40. 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%

  41. 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%

  42. Neutron Efficiency Correction

  43. L p/p θ K+ jK p+ 2o 15o Production of Polarized Hypernuclei 1.05GeV/c p+beam is injected. E462/E508 experiment Distribution of L polarization in the n(p+,K+)L reaction 1.05GeV/c p+ P K+ scattering angle(jK) In large scattering angle, L is much polarized.

  44. 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

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

  46. 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

  47. Neutron and Proton energy spectra of 5LHe and 12LC (submitted PLB. nucl-ex/0406020) To avoid suffering from FSI effect & ΛNN→NNN, High energy threshold Nn / Np (60<E<110MeV) ～ 2.17±0.15±0.16 135MeV 5LHe → n + a : Q～135MeV (rate : 0.049±0.01Gp-) apply upper energy limit of 110MeV ! Nn / Np (E>60MeV) ～2.00±0.09±0.14 apply a simple relation Gn / Gp = (Nn / Np - 1) / 2 Gn / Gp～ 0.5 Corrected proton energy loss inside the target !!

  48. Excitation spectra w/ coincident decay particles for 12LC inclusive w/ proton w/ pion w/ neutron w/ gamma

  49. 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

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

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