The Quenching of Nucleon Yields in the Nonmesonic Weak Decay of Λ Hypernuclei and the Three-body Weak Interaction Process. H. Bhang (Seoul National University) for KEK-PS E462/E508 collaboration HYP2006 conference Mainz, Germany Oct , 2006 I. Decay Modes of NMWD II. Recent Developments on Γ n /Γ p and III. Quenching of Nucleon Yield and the three-Body decay Process

Nonmesonic q~ 400 MeV/c Weak Decay Modes of Λ Hypernuclei Γ tot (=1/τ) Γ m Γ nm Γ π- ( Λ  pπ - ) Γ πo ( Λ  nπ o ) Γ p ( Λp  np ) Γ n ( Λn  nn ) Mesonic q~ 100 MeV/c Γ 2N (ΛNN  nNN) (1N) (2N) 3-Body Process (3-Body) - B-B weak interaction (ΔS=1) - Long standing Γ n /Γ p Puzzle; Γ n /Γ p exp >> Γ n /Γ p th(OPE) ~ 1 ~0.1

2. Experimental Developments; p n p,n singles spec np,nn pair no. ~ 1.0 ~0.5 ~0.5 ~ 0.5 E462 ( 5 Λ He) /E508 ( 12 Λ C) Recent Developments on Γ n /Γ p ratio - Residual FSI effects - No 2N NMWD assumed!!  Ambiguity Sources!!  n /  p OPE 1. Recent Development of Γ n /Γ p theory : 0.3 ~ 0.7

K Coincidence Measurement (KEK-PS E462/E508) π SKS EpEp EnEn π θ To exclude FSI effect and 3-body decay in Г n /Г p and to identify 2N channel,  Exclusive meas. of each decay channel. Related presentations; 1. H. Outa (Plenary talk) 2. M. Kim ( poster session ) 3. Maruta (this session) K+K+

N n / N p (E>60MeV) ~ 2.00±0.09±0.14  Γ n /Γ p =0.58±0.06±0.08. N n / N p (60<E<110MeV) ~ 2.17±0.15±0.16  Γ n /Γ p =0.61±0.08±0.08. Singles spectrum in NMWD Okada et al., PLB 597 (2004) 249 (0.59) (0.50)

1. Sharp peak in np pair( 5 Λ He) at Q value.  FSI negligible in He. 2.  Broad spec in nn ( 5 Λ He). FSI? No. π - absorption or 2N? π - can not make it broad.  Seems 3B spectrum!! 3. Y np (C); FSI is significant. 4. Y nn (C); Even further degraded.  Again points to 3B decay. E sum = E n + E p E sum = E n1 + E n2 QQ Q Q Coincidence Yields  (E sum ) np =12(8),  (E sum ) nn =16(11) MeV ;  E not enough to explain the broadening 2B Pair energy sum (Esum) correlation E sum =E n +E p E sum =E n +E n 3B? np pair nn pair

Back-to-back(bb) (cosθ≤ -0.8) nbb bb  n  p  nn /  np = 0.45±0.11±0.03 B.Kang et al., Pys. Rev. Lett. 96 (2006) back-to-back(bb) dominant Non-bb (nbb); In np; few events. In nn, more counts N NN  Y NN /(Y nmε NN ) Coincidence Yields : N NN Angular Correlation

Γ n /Γ p = 0.51±0.13±0.05 M. Kim et al., PLB 641 (2006) 28 - np ; bb dominant - nn ; nbb enhancement N bb ~N nbb - FSI corrected using pp yields. - N nn /N np ;2N effect kinematically reduced NN angular correlations Angular bb (cosθ≤ -.7) bb nbb cosθY np N np Y nn N nn Y pp N pp bb ± ± ±.002 nbb12.060± ±.0200

Now Γ n /Γ p ratio is well determined removing the ambiguities of FSI and 2N. Then what has been the reason of the Γ n /Γ p puzzle ??

1. Quenching of Singles Yields Signatures of Three Body Processes Compared to INC spectrum (N n +N p )/NMWD E N (MeV) 12 Λ C -Quenching of N n +N p can not be explained by 1N-nmwd only.!! - For 2N-nmwd, we adopted the kinematics of uniform phase space sharing of 3 nucleons  n p

2. Quenching of Pair Yields N np (bb)N np (nbb) N nn (bb)N nn (nbb) E ± ± ±0.014 INC (1N only) INC (  2N =0.4  NM )

15 counts 8 counts 3. Enhancement of nn pair yields in nbb region This model tends to produce 2 HE neutron and one LE proton. Then protons are often cut off at the threshold. 1.Enhancement of N nn in nbb.  Assign it to Г 2N. 2. Estimation; 1) N np (nbb)  all FSI eff.  Same FSI on N nn  Г 2N ~ The residual N nn after FSI sub.  Г 2N ~0.18  0.14 Γ nm ±±± 2) Similarly, but using INC for FSI  Г 2N ~0.30  0.19 Γ nm cosθY np N np Y nn N nn bb ± ±.014 nbb12.060± ±.020 N nbb /N bb 0.43± ±.30

np pair nn pair Reproduction of Singles and Coincidence yields with INC Г n /Г p =0.5 Г 2N =0.4 Г nm Proton spectrum Г n /Г p =0.5 Г 2N =0.4 Г nm Neutron spectrum

Summary 1.The coincidence exclusive measurements of each NMWD channel, Λn  nn and Λp  np, accurately determined Г n /Г p ~0.5 for 5 He and 12 Λ C. 2.The underlying reason for the long-stood Γ n /Γ p puzzle.  The Quenching of nucleon yields. 3. The 3-body weak decay process, ie Γ 2n, provides a good mechanism to explain the quenching. 4. Both singles and coincidence yields indicate a fairly large Γ 2N comparable to Γ n, but with less than 2σ stat. significance. 5. Now the accurate measurement ofГ 2N becomes so important that we have to measure it before each determination ofГ n andГ p.  J-PARC Proposal P18.

KEK, RIKEN, Seoul N.Univ., GSI, Tohoku Univ., Osaka Univ., Univ. Tokyo, Osaka Elec. Comm. Univ., Tokyo Inst. Tech. S. Ajimura, K. Aoki, A. Banu, H. Bhang, T. Fukuda, O. Hashimoto, J. I. Hwang, S. Kameoka, B. H. Kang, E. H. Kim, J. H. Kim, M. J. Kim, T. Maruta, Y. Miura, Y. Miyake, T. Nagae, M. Nakamura, S. N. Nakamura, H. Noumi, S. Okada, Y. Okatasu, H. Outa, H. Park, P. K. Saha, Y. Sato, M. Sekimoto, T. Takahashi, H. Tamura, K. Tanida, A. Toyoda, K.Tsukada, T. Watanabe, H. J. Yim KEK-PS E462/508 collaboration

Extra Slides

No kinematic seperationWith kinematic seperation Methods Singles Quenching N NN Quenching N 2N ;in N nn nbb N pn 2N =0 N 2N  N NN exp -N NN INC Γ 2N /Γ NM ±0.14*0.30±0.19* Simple Estimation of Г 2N * Stat. error only.

No kinematic seperationWith kinematic seperation Methods Singles Quenching N NN Quenching N 2N ;in N nn nbb N pn 2N =0 N 2N  N NN exp -N NN INC Γ 2N /Γ NM ±0.14*0.30±0.19* Rough Estimation of Γ 2N 1.Consider the N np nbb all due to FSI. Then subtract the corresponding FSI amount from N nn nbb. The remainder would be N 2N. This give us a kind of lower limit of Γ 2N which is about ~18% of Γ nm. 2. Use INC calculation result to estimate the FSI component in N np nbb. Then it will give ~30% of Γ nm. * Stat. error only.

N np (bb)N np (nbb) N nn (bb)N nn (nbb) E ± ± ±0.014 INC (1N only) INC (  2N =0.4  NM ) Quenching of Pair Yields np pair nn pair

1. Quenching of Singles Yields Signatures of Three Body Processes Compared to INC spectrum (N n +N p )/NMWD E N (MeV) 12 Λ C Quenching of N n +N p can not be explained without Г 2N.!! For 2N, we adopted the kinematics of uniform phase space sharing of 3 nucleons.  n p

σ NN  2 x σ NN