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The Contribution of the Three-Body Process in the Nonmesonic Weak Decay of Λ Hypernuclei

The Contribution of the Three-Body Process in the Nonmesonic Weak Decay of Λ Hypernuclei. H. Bhang (Seoul National University) For KEK-PS E508 Collaboration FB09 Bonn, Germany Aug. 31-Sep. 05, 2009. I. The Weak Decay Modes of Λ Hypernuclei .

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The Contribution of the Three-Body Process in the Nonmesonic Weak Decay of Λ Hypernuclei

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  1. The Contribution of the Three-Body Process in the Nonmesonic Weak Decay of Λ Hypernuclei H. Bhang (Seoul National University) For KEK-PS E508 Collaboration FB09 Bonn, Germany Aug. 31-Sep. 05, 2009 I. The Weak Decay Modes of Λ Hypernuclei. II. Гn/Гp ratio puzzle and 3-body NMWD process. III. The Partial Decay widths of NMWD; Г2N,Гn, Гp

  2. I. The Decay Modes of Λ Hypernuclei Mesonic q~ 100 MeV/c Гπ- ( Λ pπ- ) Гπo ( Λ nπo ) Гm Гnm Гtot(=1/τ) Γn/Γp puzzle Гp ( Λp  np ) Гn ( Λn  nn ) (1N) Nonmesonic q~ 400 MeV/c (2N) Г2N (ΛNN nNN) 3-Body Process; Predicted theoretically (2N-NMWD)

  3. The Status of NMWD Study of Λ hypernuclei • I. Fundamental Motivation ; to study the elem. B-B Weak Interaction ; • Λ + N  N + N (ΔS=1 B-B Weak Interaction ) - Гn/Гp andAy have been mainly studied so far. • Outstanding Issues ; • - Decay widths; Гn, Гp Г2N (3-body process) • - Asymmetry; • - ΔI=1/2 rulein NMWD ;

  4. Гn/Гp puzzle and the previous searches N N 1. Γn/Γp Puzzle : Γn/ΓpexpΓn/Γpth(OPE) ~ 1 ~0.1 π N N π K ρ Λ N ωη .. Λ N HME DQ OPE Gn / Gp 1.5 0.5 1 0 All these derived from p spectra

  5. E508 & KEK-PS K6 beamline and SKS • 12C(π+,K+), Pπ+=1.05 GeV/c • SKS; 100msr & short flight path

  6. Coincidence Measurement (KEK-PS E462/E508) Ep θ En π - Measured all decay particles • Pair yields, Ynp and Ynn(θ) meas. • {Ynn(θ), Ynp(θ)}/Nnm{Nnn(θ), Nnp(θ)} • Can distinguish back-to-back(bb) and • non-bb kinematic events. • Require back-to-back (cosθ<-0.7) condition. •  can supress FSI and 3-b decay events.

  7. Test of Modified DEMONS Program for n Efficiency

  8. Coincidence Yields (NN correlations) • Detection efficiency corrected. • Normalized to unit NMWD. • back-to-back(bb) ; cosθ < -0.7 • FSI/3-B broaden the angular corr. • Well agreed with Th. Ratios. • Гn/Гp puzzle finally solved. • Why the exp. Гn/Гp ratios have been so high? Nnn/Nnp (bb) Гn/Гp Гn/Гp= 0.51±0.13±0.05 M. Kim et al., PLB (’06)

  9. Quenching of SinglesYield INC(1N) INC(1N) • Quenching in both p and n spectra from that of INC(1N). • What would be the mechanism for the nucleon Quenching? •  FSI & 3-Body process. •  Both reduce the energy of emitted nucleons. •  Quenching

  10. Momentum sum distribution. • Missing momentum dist. • - |p1+p2|  p12 Two groups? - low mom(~200 MeV/c); Essentially same as the bb kinematics events. - high mom(~500 MeV/c); Non-bb kinematics events and indicate a third party either via FSIor 3-B int. process.

  11. Theo. Prediction of 3-body process (Γ2N) of NMWD. N π Λ •Model for 2N-NMWD;ΛNN  nNN - Alberico-Ericson for Nuc. matter (‘91), and - Ramos-Oset extended to finite nuclei (‘94). - Recently Bauer and Garbarino; extensive calc. including complete set of mesons and NN pairs. • Γ2N = 0.25 – 0.27 ΓΛ (12ΛC)

  12. Previous INC calculation and Quenching Overproduction

  13. INC (IntraNuclear Cascade) calculation (p,p’) Mass Dependence • • A nucleus as a local density Fermi gas with Woods-Saxon density dist. • • FSI is simulated as a cascade free NN scattering along with Fermi blocking. • • Density geometry parameters are adopted from scattering data. • • These parameters are fixed for the decay INC calc. • • In principle, the INC does not have fitting parameters. M. Kim, JKPS 46 (’05) 805

  14. INC and a renormalization parameter • • In principle, INC model does not have fitting parameters. • However, to reproduce FSIin NMWD, we used INC as a fitting function for scattering data by renormalizing the σNNασNN. • Renormalization param., α=1.09+-0.04, defines the experimental FSI. • Then INC carry this exp. FSIto the emitted nucleons in NMWD 2=0.08(α-109.3)2+1.04

  15. Reproduction of the inelastic scatterings with α=1.09

  16. Three-Body Process (Γ2N)and the quenching of yields -3-Body NMWD; Uniform phase space distribution • Total sum of the yields under 300MeV/c is reproduce with the branching ratio 2N-NMWD of 0.29. • b2NΓ2N/Γnm =0.29±0.13

  17. Γ2N agrees with the current theo. Prediction. • Γn, Γp agree with those of the most extensive recent calculation. • Errors are big, 20-45%. •  Experiment(J-PARC E18)

  18. Summary • We have measured the branching ratio of 3-Body NMWD of 12ΛC, 0.29±0.13, for the first time. • Combining with the Γnm, we obtained the widths Γn and Γp including 3-body NMWD. • These results are very well agreed with the current 3-body model • predictions. • 4. However, the errors are big, 20-45%  For accurate meas. • J-PARC E18 ; Γ2N(3-body decay process), Γn ,Γp for 12ΛC

  19. KEK-PS E508 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

  20. Extra

  21. Partial decay widths of NMWD • Units ; ΓΛ • b2N is derived from the quenching of yields of low missing momentum which is of bb kinematics events. • Γ2N agrees with the current theo. Prediction. • Γn, Γp agree with those of the most extensive recent calculation. • Errors are big, 20-50%.  Experiment(J-PARC E18) ; Extensive study including the non-bb events, namely those of the high missing momentum.

  22. Korea-Japan Collaboration • 1990~1991; Prof. Yamazaki and Prof. Nakai visited Korea and introduced N. Physics activity in Japan and the new KEK-PS program. • 1992 ; started the KOSEF-JSPS international collaboration program and participated the E140a experiment of Prof. Hashimoto. • 1993 ; proposed the lifetime measurement E307 in collaboration with Prof. Hashimoto. It was the lifetime and proton measurement of Λ hypernuclei and found the saturation of the total decay width at around carbon. • E369 ; Neutron measurement; Breakthrough of the Γn/Γp puzzle. • E462/E508; First coincidence measurement of NMWD and solved the Γn/Γp puzzle problem. • 2006 ; JPARC P18; proposal for 3-body NMWD decay. • 2007; Got 1st stage approval and we are now preparing for it.

  23. The collaboration was very successful, I consider. • It was fun and enjoyable, so I would like to thank all the collaborators. • I am quite sure it to be continued and further blossomed in J-PARC collaboration experiments.

  24. Thank you very much.

  25. Decay Counter Setup (J-PARC E18) • Covers ~50% solid angle; 2(solid angle)x3(beam)

  26. Lifetime and Proton Measurement (E307) Comparison to INC results gave where 2N ; ΛNN  NNN. E307 decay counter setup Λ+nn+n Λ+pn+p Np/nm ~0.4 Np/decay

  27. Neutron Measurement (E369) • Neutron spectra measured. • Target; 12C, 51V and 89Y • n-γ seperation by TOF

  28. Neutron Spectrum (E369) Efficiency Nn=0.69 • Nn is comparedwith Np. • Nn (> 40 MeV) = 0.69 while • Np = 0.40 (E307) Np=0.40 HYP03 Conf.

  29. E307/E369 Experiment ; Γn / Γp (12ΛC) = (0.45 ~0.51)± 0.15(stat. only) Γn/Γp ratio • Obtained almost model independent way • Large sys. error due to Γnm is cancelled •First exp result to show a significantly smaller ratio than unity. However, some ambiguities still exist due to • final state int. • 2N induced NMWD. • Coincidence measurements • 5ΛHe  E462 • 12ΛC  E508 Asymmetry issue: Serious inconsistency among αnmfor C.

  30. Korea-Japan Collaboration • In retrospect, for me the collaboration was quite successful and

  31. Asymmetry parameter of 11ΛB,12ΛC

  32. Status of Γn/Γp and αpnm p+K+DQ Corr 2π OME p+K OPE • Models explain Γn/Γp, • but not apnm !! • Serious inconsistency • between theo. and • exp. αnm values. 37

  33. Most recent status of Γn/Γp and αpnm p+K+DQ +a1 +s Corr 2π OME +s p+K OPE  +σ meson; Sasaki et al., PRC 71 035502 (‘05)  +a1 meson; Itonaga et al. PRC 77 044605 (‘08)

  34. Summarising on the decay widths of NMWD • NN Correlation data show two groups of missing momentum, low and high mom groups. The HM group tells the existence of many-body process. • FSIcalculation strongly indicates that the reason behind the Гn/Гp puzzle was the quenching of the nucleon yields in NMWD. • The degree of the quenching is well explained simply by adopting the uniform phase space distribution of the 3-body process, but of the significant fraction of NMWD. • In order to extract the most fundamental decay observables, Гn,Гp accurately, one has to determine Γ2Nfirst.  J-PARC (E18). • The Γ2N is

  35. Statusof NMWD of Λ hypernuclei Urgent problems to be solved ; - ΔI=1/2 rule (I: 4ΛHe) - 3-body process of Weak Decay; Is there really such processes? How much contribution? Why 3-body effect is so strong that it is comparabe to 2-body effect? - Branching ratios of NMWD;It has been so long, but accurate branching ratios are not available yet.  Since the contribution of 3-body process seems significant, we have to measure first of all. J-PARC E22 J-PARC E18

  36. Asymmetry measurement of decay proton ー : E462 ー: E278 PΛ p/p θ : Motoba K+ jK p+ N(θ) = N0 ( 1 + Ay cosθ) = N0 ( 1 + αp PΛcosθ) (αp; Asymmetry parameter) Ay = N(0) – N(180) N(0) + N(180)

  37. Previoussituation of Asymmetry Parameter Asymmetry Parameter Previous experiments Theoretical prediction 5ΛHe : 0.24  0.22 -0.6 ~ -0.8 12ΛC,11ΛB : -1.3  0.4 OME Ex. p+K+DQ Ex. etc. Ajimura et al. • High Statistics • Exclusive identification of Λpnp channel Exclusive Coin. Exp. E462/E508

  38. Asymmetry parameter of 5ΛHe 5ΛHe Theory: - 0.6 ~ - 0.8

  39. Total yields in LM region is produced with • b2N= Γ2N/Γnm • = 0.261±0.086. • For 2N-NMWD, we adopted the kinematics of uniform phase space sharing of 3 nucleons.

  40. ΔI=1/2 rule and Nonmesonic Weak Decay of 4ΛH and 4ΛHe.

  41. Extraction of Г2N. Some enhancements!! For 2N-NMWD, we adopted the kinematics of uniform phase space sharing of 3 nucleons. In order to explain the quenching,  Г2N~0.4Гnm Г2N is extracted by fitting the Np+ Nn, Npn+Nnn (cosθ).

  42. Momentum sum distribution. • Missing momentum dist. • - |p1+p2|  p12 • - upper fig.; 12ΛC • - lower fig. ; 11ΛB • We observe two groups; • - low mom(~150 MeV/c); • 1N NMWD? • - high mom(~500 MeV/c); • What is this high mom group? Preliminary  Though the recoil momentum is high, the recoil energy should be small. • It seems that not only the 3-nucleon-induced, but also many-nucleon induced NMWD exists..

  43. E369 Experiment/Setup HYP03 Conf.

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