Non-Mesonic Weak Decays of 5 Λ He and 12 Λ C Hypernuclei formed by (  +,K + ) Reaction RIKEN H. Outa 22. Aug FB18 (P28) for KEK-PS E462 / E508 collaborations Osaka Univ. a, KEK b, GSI c, Seoul Univ. d, Tohoku Univ. e, Univ.of Tokyo f, Tokyo Inst. Tech. g, KRISS h, RIKEN i S. Ajimura a, K.Aoki b, A.Banu c, H. C. Bhang d, T. Fukuda c,O. Hashimoto e, J. I. Hwang d, S. Kameoka e, B. H. Kang d, E. H. Kim d, J. H. Kim d, M. J. Kim d, T. Maruta f, Y. Miura e, Y. Miyake a, T. Nagae b, M. Nakamura f, S. N. Nakamura e,H. Noumi b, S. Okada g, Y. Okayasu e, H. Outa b, H. Park h,P. K. Saha b, Y. Sato b, M. Sekimoto b, T. Takahashi e,H. Tamura e, K. Tanida i, A. Toyoda b, K. Tsukada e,T. Watanabe e, H. J. Yim d Γ(Λn→nn)/Γ(Λp→np) ratio A= 5 B.H. Kang et al. PRL 96 (2006) A=12 M.J. Kim et al. nucl-ex/ : PLB in press n/p spectra from A=5,12 S. Okada et al. PLB 597 (2004) Asymmetry of proton from polarized hypernuclei T. Maruta et al. nucl-ex/ Mesonic & non-mesonic decay widths ⇒ may be omitted in this talk S. Kameoka et al. Nucl. Phys. A754 (2005) 173c-177c S. Okada et al. Nucl. Phys. A754 (2005) 178c-183c

Weak decay of  hypernucleus Γ π _ Γ π _ (  → ｐ + π － ) Γ π 0 Γ π 0 (  → ｎ + π ０ ) Γ p Γ p (  +“ ｐ ”→ ｎ + ｐ ) Γ n Γ n (  +“ ｎ ”→ ｎ + ｎ ) Γ 2N (ΛNN →NNN) Mesonic q ～ 100MeV/c Non-Mesonic (NMWD) q ～ 400MeV/c 1/  HY =Γ tot ΓmΓm Γ nm Weak decay mode of  hypernucleus  weak decay in free space  → ｐ + π － : 63.9±0.5 %  → ｎ + π 0 : 35.8±0.5 %   = 263.2±2.0 ps → Well known. Study of the mechanism of baryon-baryon weak interaction

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

 n /  p Theoretical N N N Λ Direct Quark mechanism Meson Exchange mechanism Λ N NN π,K,η,ρ,ω,K * 0.93±0.55 (Szymanski et al.) Exp. (for 5  He)  n /  p ratio : The most important observable used to study the isospin structure of the NMWD.  n /  p ratio puzzle Γ p Γ p (Λ+“ ｐ ”→ ｎ + ｐ ) Γ n Γ n (Λ+“ ｎ ”→ ｎ + ｎ )  n /  p ～ 0.1 Λ N NN W S π One Pion Exchange (OPE) Simple theoretical model Tensor-dominant  requires the final Nn pair to have isospin 0. strong tenser coupling (  L=2,  S=2) → dominant term 3 S 1 → 3 D 1 (amplitude “d”)

n n n p p p p p n n n p rescattering Final state interaction (FSI) effect 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 ”  NN  nNN” process  NN→NNN (2N-induced process) Λ N N N W π N N (One of the theoretical model)

Expected single nucleon spectrum n p n p n  N→nN n n n p p p p p n n n p  NN→nNN FSI re-scattering n n n p p p p p n n n p counts Q/2 Energy spectra (image) Energy distribute low energy region up to Q/2 broad peak around Q/2 continuous distribution

n p n p  n n p n p n p n p n Coincidence NMWD The present experiment KEK-PS E462/E508 Angular correlation 1) Angular correlation ( back-to-back, cos  <-0.8 ) Energy correlation 2) Energy correlation ( Q ～ E(N1)+E(N2) ～ 152MeV ) Angular correlation 1) Angular correlation ( back-to-back, cos  <-0.8 ) Energy correlation 2) Energy correlation ( Q ～ E(N1)+E(N2) ～ 152MeV ) NMWD : ΛN→NN * cosθ< － 0.8 * E(N1)+E(N2) cut Direct measurement of the  n /  p ratio Select ΛN→NN events w/o FSI effect & ΛNN→NNN. Select light hypernuclei to minimize FSI effect, 5  He and 12  C

π+π+ K+K+ Target: 6 Li, 12 C Excitation-energy spectra for 6  Li and 12  C 6.2×10 4 events 4.6×10 4 events decay counter 5  He 6  Li (g.s.)  5  He + p The ground state of 6  Li is above the threshold of 5  He + p. 5  He

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

Charged particles from 5  He PID function Charged PID Neutral PID Constant background very small Neutron energy resolution →  7MeV(FWHM) at 75MeV 1 /  spectra Neutral particles from 12  C Good  n separation Good  p d separation Decay particle identification

The g.s. peak is clearly seen in all spectra with coincident decay particles. previous experiment at BNL inclusive w/ proton w/ π ± w/ neutron w/ γ Excitation spectra w/ coincident decay particles for 5  He S. Kameoka et al. Nucl. Phys. A754 (2005) S. Okada et al. Nucl. Phys. A754 (2005)

np- & nn- angular distribution ( 5 Λ He)  n  p  nn /  np = 0.45±0.11±0.03 systematic error is mainly come from efficiency for neutron (6%) + acceptance(3%) Back-to-back

Coincidence Measurement (A=12) cos n + p n + n p + p E n + E p E n + E n E p + E p MeV  NN Counts 12 Λ C  n  p  nn /  np = 0.50±0.13±0.05

 n /  p 0.93±0.55 (Szymanski et al.) for 5  He Exp. Λ N NN W S π One Pion Exchange (OPE) Theo. N N N Λ Direct Quark mechanism Meson Exchange mechanism Λ N NN π,K,η,ρ,ω…  n /  p ratio Previous exp. (at BNL) N nn / N np ( 5  He)= 0.45±0.11±  He (E462) Kang et al. PRL 96 (2006) Γ n / Γ p ( 12  C)= 0.50±0.13±  C (E508) Kim et al. nucl-ex/ PLB in press

[2] Asymmetry of proton emission from polarized hypernuclei

Asymmetry measurement of decay proton N(  ) = N 0 (1 + Acos  )  Asymmetry Asymmetry : Volume of the asymmetric emission from NMWD A = (R - 1) (R + 1) R = N(-)N(-) N(+)N(+), Asymmetry parameter = N 0 (1 +  Pcos  ) Difference of acceptance & efficiency is canceled out ! R = N(  + (-  ))×N(  - (+  )) N(  + (+  ))×N(  - (-  )) 1/2  K >0 ++ K+K+  /p KK    K <0 ++ K+K+ /p/p KK   P P

Initial stateFinal stateAmplitudeIsospinParity 1S01S0 1S01S0 a1No 3P03P0 b1Yes 3S13S1 3S13S1 c0No 3D13D1 d0 1P11P1 e0Yes 3P13P1 f1 If assuming initial S state We can know the interference between states with different Isospin and Parity. (Applying  =1/2 rule) Importance of α nm measurement

 NM for 5 Λ He NMWD A=PA=P A  :Asymmetry of Pion    Asymmetry Parameter of Pion (= － 0.642±0.013) P   Polarization of Lambda  :Attenuation factor ・ Polarization of  ・ Asymmetry Parameter of Proton A p =  NM P   p Estimated from mesonic decay We can calculate  NM without theoretical help ! p

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

Theory: ～ Asymmetry parameter of 5 Λ He   NM =0.08± p statistical  contami Nucl.Phys.A754 (2005) nucl-ex/ ; submitted to PRL Instrumental Asymmetry <0.003

Asymmetry parameter of 12 C, 11 B   NM =-0.14± p statistical  contami E160 : - 0.9±0.3

Comparison with recent calculations OPE  +K  +K+DQ OME  +K,OME can reproduce  n /  p ratio but predict large negative  NM  +K+   +K+  +DQ  n /  p and  NM can be reproduced only by  +K+  +DQ model Sasaki et al. PRC71 (2005) (1) Large b( 1 S 0 → 3 P 0 ) and f( 3 S 0 → 3 P 1 ) amplitude (2) Violation of ΔI=1/2 rule considered

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

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

Other Results-1:   0 and Λ-α potential   He   0 /   = 0.201±0.011 Lifetime : ps (E462) -10 HYP03 proceesings Nucl. Phys. A754 (2005)

Other results-2: A dependence of Γ NM HYP03 proceedings Nucl. Phys. A754

Summary   N→NN was directly observed for the first time !! 5 Λ He: Γ n / Γ p ratio ～ N nn / N np = 0.45±0.11±0.03 Kang et al. PRL 96 (2006) Λ C: Γ n / Γ p ratio ～ 0.50±0.13±0.05 Kim et al. PLB, in press ◆ Asymmetry parameter measured with improved accuracy !! 5 Λ He : 11 Λ B and 12 Λ C : Maruta et al. nucl-ex/ ; thesis ◆ Mesonic and Non-mesonic decay widths are precisely measured － Test of Λ-nucleus potential  NM =0.08± p [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 ?  NM = － 0.14± p -0.00

Spare OHP

ΛNN→NNN consideration Λ n n p W π-π- p n Λ n n n W π0π0 p p 4 1 Seen as…. n n p p n n Λn→nn like Λp→np like Garbarino

experimentaldata theoreticalcalc. Comparison with theoretical calc. for angular correlation Garbarino’s calc. assuming Gn/Gp = 0.46 (for 5  He ), 0.34 (for 12  C ) considered 2N-induced( ～ 20%), FSI 5  He (E462) 12  C (E508) n+p coincidence n+n coincidence n+p n+n cos  n+p coincidence n+n coincidence n+p n+n p+p Pair number /NMWD /NMWD Pair number /NMWD /NMWD n+p coincidence n+n coincidence p+p coincidence n+p coincidence n+n coincidence p+p coincidence Phys. Rev. Lett. 91 (2003)

E462   - /   of   He  - branching ratio  0 branching ratio Lifetime Other results  - decay width for 5  He Errors were much improved !!  0 decay width for 5  He and 12  C HYP2003 proceedings Nucl. Phys. A754 (2005)

Γπ: test of Λ-nucleus potential Λnucleus YN interaction attraction ～ repulsion Repusive core is the common feature of Y-nucleus potential

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

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

np coincidence analysis n p n p  n n p n p Coincidence NMWD np back-to-back event  NM =0.31±0.22 p

One and only(?) solution  + K +  + DQ Sasaki et al. PRC71 (2005)  N NN W S ,K,,K,  b( 1 S 0 → 3 P 0 ) と f( 3 S 0 → 3 P 1 ) amplitude に影響を与える   I = 3/2 が大きく寄与する  今回  n /  p ratio と  NM 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

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

 n /  p estimation from Coincidence Measurement

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

Before subtraction After subtraction Uniform components subtraction

FSI consideration using pp-pairs r n,p r n,p fraction ratio of the neutron and proton induced channels. f n,p f n,p is reduction factor due to FSI. g n,p g n,p is cross over influx of neutron(proton) from proton(neutron) due to FSI. p,q,q’ p,q,q’ are angular acceptance factor.,,, N np (bb) N nn (bb) N pp (bb) Before FSI correction 4%

Systematic error calculation 1)Intentionally add 2 pp events in -0.8<cos  <-0.7 2) 3)Uniform b.g. level for different angular regions:6.6%

Neutron Efficiency Correction

Production of Polarized Hypernuclei E462/E508 experiment 1.05GeV/c  + beam is injected. Distribution of  polarization in the n(  +,K + )  reaction 2o2o 15 o K + scattering angle(  K ) 1.05GeV/c  + In large scattering angle,  is much polarized. P ++ K+K+  /p KK  

6 Li Hypernuclear mass spectra 6 Li +  + →  6 Li + K +  6 Li →  5 He + p 5  He 6  Li 5 Li 0MeV 8.3MeV 18.3MeV ( P n -1,S  ) ( P n -1,P  ) ( S n -1,S  ) p decay  decay inclusive  coin p coin 5.2×10 4 events 3.2×10 3 events 1.6×10 3 events 

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

N n / N p Ratio Γ n Γ n (Λ+“ ｎ ”→ ｎ + ｎ ) Γ p Γ p (Λ+“ ｐ ”→ ｎ + ｐ ) If Γ n / Γ p = 1 → N n / N p = 3 If Γ n / Γ p = 0.5→ N n / N p = 2 If Γ n / Γ p = 0→ N n / N p = 1 Naive estimation (without considering FSI and ΛNN→NNN) To avoid suffering from FSI effect & ΛNN→NNN, High energy threshold Γ n :Γ p N n N p 1 : 1 … 3 : 1 1 : 2 … 2 : 1 0 : 1 … 1 : 1 N n / N p = 2×  n /  p + 1

Neutron and Proton energy spectra of 5  He and 12  C N n / N p (E>60MeV) ～ 2.00±0.09±0.14 N n / N p (60<E<110MeV) ～ 2.17±0.15±  He → n +  : Q ～ 135MeV (rate : 0.049±0.01   -) 135MeV apply upper energy limit of 110MeV ! apply a simple relation  n /  p = (N n / N p - 1) / 2   n /  p ～ 0.5 To avoid suffering from FSI effect & ΛNN→NNN, High energy threshold Corrected proton energy loss inside the target !! (submitted PLB. nucl-ex/ )

inclusive w/ proton w/ pion w/ neutron w/ gamma Excitation spectra w/ coincident decay particles for 12  C

Spin / isospin dependence p p n Λ n n p Λ p p n n Λ Non-mesonic weak decay of 4  He and 4  H 4 He (K -,  - ) 4  He or 4 He (  +,K + ) 4  He  n+n back-to-back 4 He (K -,  0 ) 4  H  p+n back-to-back (  0 spectrometer ) see S.Ajimura : J-PARC LOI 21 To J-PARC R NS … N :  n  nn,  p  np S : spin = 0 or 1  nm ( 4  H) = ( 3R n1 + R n0 + 2R p0 ) ×  4 / 6  nm ( 4  He) = ( 2R n0 + 3R p1 + R p0 ) ×  4 / 6  nm ( 5  He) = ( 3R n1 + R n0 + 3R p1 + R p0 ) ×  5 / 8  Need one-order higher statistics.  J-PARC

π+π+ K+K+ decay counter K6/SKS setup

Identification of hypernuclear formation K+K+  T1 target Z-vertex Mass Z

Neutral decay particle ID Good  n separation Good S/N ratio ( ～ 30) ( previous exp. S/N ratio ( 5  He) ～ 1 )  ～ 30 times (for 5  He) higher than previous exp. High statistics ( ～ 5000 neutrons)  ～ 200 times (for 5  He) ～ 30 times (for 12  C) higher than previous exp. 5MeV< E n < 150MeV Constant background very small Neutron energy resolution (estimated from  width) →  7MeV(FWHM) at 75MeV (TOF spectra) 1 /  spectra Neutral particles from 12  C

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

Non-mesonic weak decay strong tenser coupling (  L=2,  S=2) → dominant term 3 S 1 → 3 D 1 (amplitude “d”)  + N→ N + N  + p → n + p :  p = a 2 +b 2 +c 2 +d 2 +e 2 +f 2  + n → n + n :  n = a 2 +b 2 +f 2 OPE :  n /  p ～ 0.1 Λ N NN W S π One Pion Exchange (OPE) model  n /  p ratio : The most important observable to study the isospin structure of the NMWD. Exp. :  n /  p ～ 1  n /  p ratio puzzle Simple theoretical model with large error

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

 -nucleus overlap for 5  He   0 /   = 0.201±0.011 (   He)   nm /   = 0.395±0.016 (   He) Γtotal ( 56 Λ Fe) ～ Γnm(A→∞) ～ 1.2Γ Λ （ E307 ） Γnm ( 5 Λ He) = 0.4Γ Λ （ Present) 1/3 of Λ is inside α Both results are consistent, preferred larger overlap than YNG prediction. 5 Λ He (ORG) ～ 40% 5 Λ He (YNG) ～ 20% 1/3 of Λ is inside α     0 /   locates in between ORG and YNG.