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H. Bhang (Seoul National University) for KEK-PS E462/E508 collaboration HYP2006 conference

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. 10-14, 2006. I. Decay Modes of NMWD

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H. Bhang (Seoul National University) for KEK-PS E462/E508 collaboration HYP2006 conference

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  1. 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) forKEK-PS E462/E508 collaboration HYP2006 conference Mainz, Germany Oct. 10-14, 2006 I. Decay Modes of NMWD II. Recent Developments on Γn/Γp and III. Quenching of Nucleon Yield and the three-Body decay Process

  2. Weak Decay Modes of Λ Hypernuclei Mesonic q~ 100 MeV/c Γπ- ( Λ pπ- ) Γπo ( Λ nπo ) Γm Γnm Γtot(=1/τ) Γp ( Λp  np ) Γn ( Λn  nn ) (1N) Nonmesonic q~ 400 MeV/c (2N) Γ2N (ΛNN nNN) 3-Body Process - B-B weak interaction (ΔS=1) - Long standing Γn/Γp Puzzle; Γn/Γpexp>> Γn/Γpth(OPE) ~ 1 ~0.1 (3-Body)

  3. Recent Developments on Γn/Γp ratio OPE Gn / Gp 1.5 0.5 1 0 2. Experimental Developments; p n p,n singles spec np,nn pair no. ~ 1.0 ~0.5 ~0.5 ~ 0.5 1. Recent Development of Γn/Γptheory : 0.3 ~ 0.7 E462(5ΛHe)/E508(12ΛC) • Residual FSI effects • No 2N NMWD assumed!! Ambiguity Sources!!

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

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

  6. Coincidence Yields Pair energy sum (Esum) correlation 1. Sharp peak in np pair(5ΛHe) at Q value.  FSI negligible in He. 2. Broad spec in nn (5ΛHe). FSI? No. π- absorption or 2N? π- can not make it broad.  Seems 3B spectrum!! 3. Ynp(C); FSI is significant. 4. Ynn(C); Even further degraded.  Again points to 3B decay. np pair nn pair 2B Q Q Esum=En+Ep Esum=En+En 3B? Q Q Esum=En+Ep Esum=En+En Esum = En + Ep Esum = En1 + En2 (Esum)np=12(8), (Esum)nn=16(11) MeV ;E not enough to explain the broadening

  7. Coincidence Yields : NNN Angular Correlation Back-to-back(bb) (cosθ≤ -0.8) nbb nbb bb NNN YNN/(Ynm•εNN) • back-to-back(bb) dominant • Non-bb (nbb); In np; few events. In nn, more counts Gn/Gp ~ Nnn/ Nnp= 0.45±0.11±0.03 B.Kang et al., Pys. Rev. Lett. 96 (2006) 062301

  8. NN angular correlations Angular bb (cosθ≤ -.7) bb nbb • np ; bb dominant • nn ; nbb enhancement • Nbb~Nnbb • FSI corrected using pp • yields. • Nnn/Nnp;2N effect • kinematically reduced Γn/Γp= 0.51±0.13±0.05 M. Kim et al., PLB 641 (2006) 28

  9. Now Γn/Γp ratio is well determined removing the ambiguities of FSI and 2N. Then what has been the reason of the Γn/Γp puzzle ??

  10. Signatures of Three Body Processes 0.1 Compared to INC spectrum 0.5 2.0 12ΛC 2.0 (Nn+Np)/NMWD 0.5 0.1 EN (MeV) 1. Quenching of Singles Yields n  p • Quenching of Nn+Np can not be explained by 1N-nmwd only.!! • For 2N-nmwd, we adopted the kinematics of uniform phase space sharing of 3 nucleons

  11. 2. Quenching of Pair Yields

  12. 3. Enhancement of nn pair yields in nbb region 15 counts 8 counts This model tends to produce 2 HE neutron and one LE proton. Then protons are often cut off at the threshold. • Enhancement of Nnn in nbb. •  Assign it to Г2N. • 2. Estimation; • 1) Nnp(nbb)  all FSI eff. •  Same FSI on Nnn •  Г2N ~ The residual Nnn • after FSI sub. •  Г2N ~0.180.14 Γnm±±± • 2) Similarly, • but using INC for FSI •  Г2N ~0.300.19 Γnm

  13. Reproduction of Singles and Coincidence yields with INC np pair nn pair Гn/Гp=0.5 Г2N=0.4 Гnm Proton spectrum Neutron spectrum Гn/Гp=0.5 Г2N=0.4 Гnm

  14. Summary • The coincidence exclusive measurements of each NMWD channel, Λnnn and Λpnp, accurately determined Гn/Гp ~0.5 for 5He and 12ΛC. • The underlying reason for the long-stood Γn/Γp puzzle. •  TheQuenching of nucleon yields. • 3. The 3-body weak decay process, ie Γ2n, provides a good mechanism to explain the quenching. • 4. Both singles and coincidence yields indicate a fairly large Γ2N comparable to Γn, but with less than 2σ stat. significance. • 5. Now the accurate measurement ofГ2N becomes so important that we have to measure it before each determination ofГn andГp. J-PARC Proposal P18.

  15. KEK-PS E462/508 collaboration 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

  16. Extra Slides

  17. Simple Estimation of Г2N * Stat. error only.

  18. Rough Estimation of Γ2N * Stat. error only. • Consider the Nnpnbb all due to FSI. Then subtract the corresponding FSI amount from Nnnnbb. The remainder would be N2N. This give us a kind of lower limit of Γ2N which is about ~18% of Γnm. • Use INC calculation result to estimate the FSI component in Nnpnbb. Then it will give ~30% of Γnm.

  19. 2. Quenching of Pair Yields np pair nn pair

  20. Signatures of Three Body Processes Compared to INC spectrum 12ΛC (Nn+Np)/NMWD EN (MeV) 1. Quenching of Singles Yields n  p Quenching of Nn+Np can not be explained without Г2N.!! For 2N, we adopted the kinematics of uniform phase space sharing of 3 nucleons.

  21. σNN 2 x σNN

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