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The Three-body Process in the Weak Interaction Observed in the Decay of Λ hypernuclei

The Three-body Process in the Weak Interaction Observed in the Decay of Λ hypernuclei. H. Bhang for KEK-PS SKS collaboration (Seoul National University) APCTP-KPS Workshop on Nuclei Far from Stability and Their Application Chonbuk National University Oct. 20-22, 2005.

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The Three-body Process in the Weak Interaction Observed in the Decay of Λ hypernuclei

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  1. The Three-body Process in the Weak Interaction Observed in the Decay of Λ hypernuclei H. Bhang forKEK-PS SKS collaboration (Seoul National University) APCTP-KPS Workshop on Nuclei Far from Stability and Their Application Chonbuk National University Oct. 20-22, 2005 I. Issues / focus of NMWD II. The status of Γn/Γp III. Signatures of 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) (2N) Γ2N (ΛNN NNN) Main focus Nonmesonic q~ 400 MeV/c • Focus: • Baryon-Baryon Weak Interaction • Long standing puzzle on Γn/Γp. • 2N NMWD: 3-Body, ΛNNnNN, Interaction Process

  3. Status of Γn/Γp puzzle OPE Gn / Gp 1.5 0.5 1 0 1. Γn/Γp Puzzle : Γn/Γpexp>> Γn/Γpth(OPE) ~ 1 ~0.1 2. Recent Development of Γn/Γptheory : 0.3 ~ 0.7 K.Sasaki, Nucl. Phys. A669 (2000) 371 D. Jido, Nucl. Phys. A694 (2001) 525

  4. 3. Recent Exp. Development at KEK-PS Nn(> 40 MeV) =0.69 Np(> 40 MeV) =0.40 K+ π+ p n p,n singles spec p,n pair no. meas. meas. ~ 1.0 ~0.5 ~0.5 ~ 0.5 E307etc. E369 E462/E508 Гn/Гp(12ΛC) = 0.51±0.14 Theory Independent!! Y. Sato et al., PRC 71 (2004) 025203 J. Kim et al., PRC 68 (2003) 065201

  5. Agrees well with the recent theoretical values, 0.3-0.7. • Then What has been the problem? If Γn/Γp=1/2, one would expect (Np,Nn)=(0.67, 1.33). [ 1 (0.5, 1.5) ] Instead we obtained (0.40, 0.69). Main reason of the difference ; Eth cutoff  However, the quenching was even more than what one would expect from the threshold cut.  So the quenching of proton yields were attributed to a strong neutron side strength giving us a large Γn/Γp ratio.

  6. Ambiguities in Singles Measurements E307/E369 : Гn/Гp (12ΛC) = 0.51± 0.14 (stat. only) • Derived from Nn/Np ratio • Almost model independent. • agrees well with the recent theoretical values (~.5) • In order to resolve the difficulties, • Exclusive measurements • • 5ΛHe  E462 • • 12ΛC  E508 However, ambiguities due to • effect of residual final state int. • 2N induced NMWD.

  7. Setup E462/E508 (KEK-PS K6 beamline & SKS) π Ep θ En K SKS π

  8. Excitation Energy Spectra 12LC g.s. 5LHeg.s. inclusive quasi free inclusive inclusive p-gate w/ p p-gate w/ n p-gate p-gate w/ n+p d-gate d-gate w/ n+n

  9. Singles spectrum in NMWD To avoid suffering from FSI effect & ΛNN→NNN, High energy threshold Nn / Np (60<E<110MeV) ~2.17±0.15±0.16  Γn/Γp =0.61±0.08±0.08. Nn / Np (E>60MeV) ~2.00±0.09±0.14  Γn/Γp= 0.58±0.06±0.08. Okada et al., PLB 597 (2004) 249

  10. Coincidence Observables p θ n • Nucleon Energy sum spectrum; • Ep+En, En+En • 2. Pair number per NMWD; • Nnp(cosθ), • Nnn(cosθ) Nnp  Ynp/(Ynm•εnp)

  11. Energy Sum Spectum of the Two Nucleons • Energy Sum spectrum • Sharp peak in Ynp(He) at Q value. •  FSI negligible in He. • Broad spec in Ynn(He). • FSI? No. • π- absorption or 2N? • π- can not make it broad. •  Seems 2N effect!! • 3. Ynp(C); FSI is significant. • 4. Ynn(C); Even further • degraded. •  Again points to 2N. Q Q Q Q Esum = En + Ep Esum = En1 + En2 - ΔI=1/2 rule - np pair absorp. (Esum)np=12(8), (Esum)nn=16(11) MeV

  12. NNNYNN/Ynmwd Angular Back-to-back(bb) (cosθ≤ -0.8) • bb dominant • nbb;  only a few events. • In nn, more counts Angular Correl. of np pair Angular Correl. of nn pair B.Kang et al., prl submitted (’05) Gn/Gp ~ Nnn/ Nnp= 0.45±0.11±0.03

  13. Angular Back-to-back (cosθ≤ -0.7) bb ; 2-body kinematics nbb ; ? Similar behavior • bb dominant in np • In nn, bb no more major one M. Kim et al,, Proc. of DAFNE04 (2004) 237 Gn/Gp ~ = 0.50±0.11±0.03

  14. Гn/Гp from singles and coincidence data From the singles of E462/E508 5ΛHe (E462)  Γn/Γp(5ΛHe) = 0.61 ± 0.081 ± 0.082. 12ΛC (E508) Γn/Γp(12ΛC) = 0.58 ± 0.06 ± 0.08. Here no Γ2N assumed !!  diffences from the coincidence pair yields, 5ΛHe (E462)  Γn/Γp(5ΛHe) = 0.43 ± 0.12 ± 0.044 12ΛC (E508) Γn/Γp(12ΛC) = 0.50 ± 0.13 ± 0.05. Γ2N component is kinematically removed !! Free from Г2N Ambiguity

  15. Г2N • Now we have Гn/Гp almost ambiguity free! • Then, can we determine each Γn, Γp itself ? • How about Г2N(ΛNNnNN)? Can we neglect it?

  16. Status of (ΛNNNNN) • Large contribution of the 3-body decay process, Λ+NNNNN (Γ2N), was predicted in the theoretical calculations. Γ2N was predicted about 1/5 of Γnm. • (PRC 256(’91) 134, PRC 50 (’94) 2314) • 2. So far no experimental identification has been made. • 3. Original motivation of the experiments, E462 and E508, was to identify Γ2N experimentally. We remember that the quenching of proton singles yield was the source of the long standing confusion on Γn/Γp. Later we found even severer quenching in the neutron yield. Why such severe Quenching?

  17. Signatures of Three Body Process in Weak Decay • Quenching of Singles yields ; • 2. Energy sum spectrum ; • 3. Quenching of Total pair yields ; • 4. Enhancement of nn pair yields in the non-back-to-back angular kinematic region • 5. The difference of Γn/Γp values derived from singles yields and coincidence pair numbers. So many places !! In every places !!

  18. Signatures of Three Body Processes Compared to INC spectrum 12ΛC (Nn+Np)/NMWD EN (MeV) 1. Quenching of Singles Yields Significant quenching of the Nn+Np could not be explained with 1N only INC.!! For 2N, we adopted the kinematics of uniform phase space sharing of 3 nucleons.

  19. INC(IntraNuclear Cascade) calculation Mass Dependence Kin. Energy Dependence (p,p’) M. Kim, JKPS 46 (’05) 805

  20. Total Pair Number is compared to that of INC

  21. Enhancement of nn pair yields in the nbb angular region 15 counts 8 counts • We know that FSI(He) not strong. Then what are those in Ynnnbb(He)? • R(np) enhancement in C over He.  FSI • R(nn) enhancement over R(np) both in He and C  2N? where R=Nbb/Nnbb This model tends to produce 2 HE neutron and one LE proton. Then protons are often cut off at the threshold.

  22. Rough Estimation of Γ2N • 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 18-19% of Γnm. • Use INC calculation result to estimate the FSI component in Nnpnbb. Then it will give ~25-30% of Γnm.

  23. Summary • A series of experiments have been done for the study of NMWD of Λ hypernucleiat KEK-PS, • 2. The coincidence exclusive measurement of NMWD were • done for the first time for 5He and 12ΛC and determined • the Гn/Гp to be ~0.5 almost free from the ambiguity • of FSI and 2N contribution. • 3. The Γn/Γp values, ~0.5, well support the recent • theoretical ratios. • 4. All the signatures indicates fairly large Γ2N comparable to Γn, but with only a 2σ confidence level. • 5. Now the accurate measurement ofГ2N becomes so important that the decay width of each NMWD mode can be determined only after it. • 6. A proposal for its accurate measurement is planned for JPARC.

  24. KEK-PS E462/508 collaboration KEK, RIKEN, Seoul Univ., GSI, Tohoku Univ., Osaka Univ., Univ. Tokyo Osaka Elec. Comm. Univ.G , 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

  25. Extra Slides

  26. Quenching of Total Pair Yields 5ΛHe 12ΛC np pair np pair nn pair nn pair nbb Total pair yields, NT: If Γ2N=0, Eth =0 and FSI=0, NT=1. If Γ2N=0, Eth =0 and FSI≠0, NT=1+α. If Γ2N ≠ 0 and Eth ≠ 0, NT=?. NT = 0.38 This also has the limitation as the singles.

  27. INC(IntraNuclear Cascade) calculation (p,p’) Mass Dependence • • A nucleus as a Fermi gas. • • ρ(x)  V(x) • • FSI is simulated as a cascsde free NN scattering along with Fermi blocking imposed. • • Density geometry parameters are determined fitting the reactions, (p,p’) and (p,n) data with which Mass and Energy dependence were checked • • These parameters are fixed for the decay INC calc. M. Kim, JKPS 46 (’05) 805

  28. Г2N • Both singles yields (E307, E369) and coincidence yields (E462, E508) gave Γn/Γp ~0.5 which now agrees well with the recently enhanced theoretical ratios distributed in the range of 0.3-0.7. • 2. The coincidence measurement of NMWD were done for • the first time and determined the Γn/Γp valuesexclusively • for the two body ΛNNN process. It is almost free • from the ambiguities of FSI and 2N contribution. • Now we have Гn/Гp almost ambiguity free! • Then, can we determine each Γn, Γp itself ? • How about Г2N(ΛNNnNN)? Can we neglect it?

  29. Γn/Γp Status 1.5 0.5 1 0 Coincidence 5ΛHe : 0.45 ±0.11 ±0.03±(E462) 12ΛC : 0.50 ±0.13 ±(0.05) (E508) OME, DQ model OPE Gn / Gp Exp. Singles 5ΛHe : 0.61±0.081±0.082 (E462) 12ΛC : 0.58±0.06±0.08(E508) : (0.45~0.51)±0.15 (E307/E369)

  30. Particle identification Neutral particle Charged particle p π 5MeV< energy < 150MeV d 1/β spectra Energy resolution σ～8MeV ( around 80MeV ) PID spectra

  31. Proton and neutron spectra Nn(> 40 MeV) =0.69 Np(> 40 MeV) =0.40 E369 decay counter setup () NnYn/Ynmwd Y. Sato et al., PRC 71 (2004) 025203 J. Kim et al., PRC 68 (2003) 065201 Гn/Гp(12ΛC) = 0.51±0.14 • Obtained directly from the experimental ratio, Nn/Np, • Almost theory independently while previous ones were derived comparing to that of INC.

  32. Proton Energy spectrum 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

  33. Angle/Energy sum Correlations Preliminary Results 12ΛC

  34. 12ΛC 5ΛHe Comparison of 5ΛHeand12ΛC np pair np pair Sharp back-to-back kinematic nature in 5ΛHeis moderated in that of 12ΛC due to FSI. nn pair nn pair Preliminary Results

  35. Estimation of pair number per NMWD Preliminary Results

  36. Γn/Γp from Nn/Np & Nnn/Nnp Simple counting of singles yields of n,p and coincidence yeilds of nn, np pairs gives, neglecting FSI and 2N Nn/Np = 2•Гn/Гp + 1, Nnn/Nnp = Гn/Гp, Γn/Γp~ (Nn/Np -1)/2= 0.59 (5ΛHe), 0.5 (12ΛC), Γn/Γp~ Nnn/Nnp= 0.45 (5ΛHe), 0.53 (12ΛC).

  37. β=0.11 Correction for Cross over recoil effects - Assume1N process only ; rn + rp = 1. - The neutron (proton) number per NMWD Nn=Yn/NnmΩnεn = (2rn+rp)f+ rpg Np=Yp/NnmΩpεp = rpf+(2rn+rp)g, where f, g ; FSI effects. Γn/Γp= 0.51 +-0.15 - Obtained FSI model independently, but assuming 1N process. - The INC β value is used only for second order correction.

  38. Decay counter system Solid angle: 26% 9(T)+9(B)+8(S)% p n N: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm π

  39. 5ΛHe 12ΛC np pair np pair nn pair nn pair nn pair nbb Comparison of Angular Correlation of He and C • We notice that • R(np) enhancement in C over He. •  FSI? • 2. R(nn) enhancement over R(np) both in He and C •  2N NMWD? • where R=Nbb/Nnbb Preliminary Results

  40. INC spectrum ; Fermi momentum and FSI model

  41. Energy Sum ; EN1+EN2 Energy Sum Correlations 5ΛHe En+Ep(MeV) En+En Back-to-back Back-to-back Back-to-back (cosθ≤ -0.8) Angular Correl. of np pair Angular Correl. of nn pair Uniform cosθnp cosθnn

  42. cosθcut dependence Nnn/Nnp

  43. Gn/Gp ~ Nnn/ Nnp= 0.45±0.11±0.03

  44. Neutral particle Charged particle p π 5MeV< energy < 150MeV d 1/β spectra Energy resolution σ～8MeV ( around 80MeV ) PID spectra Particle identification

  45. INC showing angular correlation due to Fermi Mom. Only. No FSI.

  46. INC showing angular correlation due to Fermi Mom. Only. No FSI.

  47. 3. Enhancement of nn pair yields in the non- back-to-back angular region R  Nnbb/Nbb, 2N  2N NMWD Suppose no 2N, then Nnbb due to FSI and we expect Rnp = Rnn, R = Rnn/Rnp = 1. But in reality, Rnn  Rnp, Rnn/Rnp ~ 2.  2N signature ! !

  48. Estimation of Γ2N /ΓNM

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