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Non-Mesonic Weak Decays of 5 Λ He in ( p + ,K + ) reaction

24 th . Oct. 2005 PANIC2005. Non-Mesonic Weak Decays of 5 Λ He in ( 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 , Univ. Tokyo f ,Tokyo Inst. Tech. g , KRISS h , RIKEN i

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Non-Mesonic Weak Decays of 5 Λ He in ( p + ,K + ) reaction

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  1. 24th. Oct. 2005 PANIC2005 Non-Mesonic Weak Decays of 5ΛHe in (p+,K+) reaction RIKEN H. Outa for KEK-PS E462 / E508 collaborations Osaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku Univ.e, Univ. 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 B.H. Kang et al.nucl-ex/0509015; PRL submitted Asymmetry parameter T. Maruta et al. nucl-ex/0509016; ＰＲL submitted Mesonic decay widths　⇒　omitted in this talk S. Kameokaet al. Nucl. Phys. A754 (2005) 173-177 S. Okadaet al. Nucl. Phys. A754 (2005) 178-183

  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) 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. 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. 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

  5. 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

  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-particle detection system Neutron Proton Neutral particle： ・TOF (T1T4) ・T3 VETO Solid angle: 26% = 9(T)+9(B)+8(S) % Charged particle： ・dE/dx (at T2) ・total energy deposit (T2+T3+T4) ・TOF (T2T3)

  8. 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

  9. Coincidence analysis 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

  10. np- & nn- angular distribution 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

  11. 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. nucl-ex/0509015 12LC (E508) Nnn / Nnp (12LC)= 0.50±0.14±0.03 Λ N Kim et al. PANIC05; session IV-6 Gn / Gp ratio Previous exp. (at BNL) 0.93±0.55 (Szymanski et al.) for 5LHe Exp.

  12. 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)

  13. L p/p θ K+ jK p+ (r + 1) N(θ+) A = r = (r - 1) , N(θ-) 1/2 N(θ+(+j))×N (θ-(-j)) r = N(θ+(-j))×N (θ-(+j)) Asymmetric proton emission from NMWD N(θ) = 1 + Acosθ = 1 + aP cosθ L P Asymmetry Parameter Asymmetry N Decay counter Acceptance |θ| > 0.7 1 -1 0 +1 cosθ SKSAcceptance jK = +15o～-15o Difference of acceptance & efficiency is canceled out !

  14. 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

  15. Asymmetry parameter of5ΛHe aNM=0.11±0.08±0.04 p Theory: - 0.6～- 0.7 T.Maruta et al. HYP03 Nucl.Phys.A754 (2005) 168-172 nucl-ex/050916 ; submitted to PRL Final !! statistical p contamination

  16. 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

  17. aNM=0.31±0.22 aNM=0.11±0.08±0.04 p p Summary • LN→NN was directly observed for the first time !! 5ΛHe : Γn / Γp ratio～ Nnn / Nnp = 0.45±0.11±0.03 nucl-ex/0509015 12ΛC : Γn / Γp ratio～ Nnn / Nnp = 0.50±0.14±0.03 Kim (Session IV-6) ◆ Asymmetry parameter measured with improved accuracy !! Single proton : n+p coincidence: nucl-ex/0509016 [1] Importance of shorter-range mechanism OPE 　⇒　Heavy meson & DQ exchange [2] Significant contribution of ΛN initial spin-singlet state contribution －　σ-meson exchange ?

  18. Spare OHP

  19. 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.

  20. 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

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

  22. 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

  23. 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 !!

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

  25. Gp- /GL of 5LHe E462 Other results Lifetime p- branching ratio p0 branching ratio p - decay width for 5LHe p0 decay width for 5LHe and 12LC Statistical errors were much improved !!

  26. 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

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

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

  29. Neutral decay particle ID Neutral particles from 12LC Neutron energy resolution (estimated from g width) → 7MeV(FWHM) at 75MeV • Good g n separation • Good S/N ratio (～30) ( previous exp. S/N ratio (5LHe) ～1 ) ～30 times (for 5LHe) higher than previous exp. • High statistics (～ 5000 neutrons) ～200 times (for 5LHe) ～30 times (for 12LC) higher than previous exp. 5MeV< En < 150MeV Constant background very small 1 / b spectra (TOF spectra)

  30. Charged decay particle ID Charged particles from 5LHe Gaussian fit (±2s cut) Deuterons were separated from the protons. (for the first time!) PID function • derived from • dE/dx (at T2) • Total energy deposit (sequentially fired counters (T2,T3,T4).) • TOF (between T2 and T3)

  31. 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

  32. 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.

  33. L-nucleus overlap for 5LHe Gp0/ GL = 0.201±0.011 (5LHe) Gp0/ GL locates in between ORG and YNG. 5ΛHe (ORG) ～ 40% 5ΛHe (YNG) ～ 20% 1/3 of Λ is inside α Gnm/ GL = 0.395±0.016 (5LHe) Γtotal (56ΛFe) ～Γnm(A→∞) ～ 1.2ΓΛ　（E307） Γnm (5ΛHe) = 0.4ΓΛ　（Present) 1/3 of Λ is inside α Both results are consistent, preferred larger overlap than YNG prediction.

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