Nonmesonic weak decay of 5 Λ He and 12 Λ C and the effect of FSI on its observables H. Bhang for KEK-PS SKS collaboration (Seoul National University) Dafne04 Conf. on Physics at Meson Factories LNF, Frascati June 07~ 11, 2004 I. Issues / focus of NMWD II. Recent Experiments III. Г n /Г p, FSI and 2N contribution.

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 Nonmesonic q~ 400 MeV/c Weak Decay Modes of Λ Hypernuclei Main focus of this talk High momentum interaction Γ 2N (ΛNN  NNN) (1N) (2N)

Non-Mesonic Weak Decay (NMWD) & Issues 1. B-B Weak Interaction ; Λ + N -> N + N (ΔS=1 B-B Weak Interaction ) 2. Long standing puzzle on : Γ n /Γ p ( ≡ np ratio) 3. 2N NMWD: Predicted to be a significant component of NMWD, though not experimentally identified yet. 4. Asymmetry ; The relative phase concern of PC and PV part of NMWD interaction. 5. Final State Interaction : It seems one of the important elements to understand NMWD.

Hyp. Nuc. Γ nm Γ n /Γ p BNL 5 Λ He0.41± ± Λ C1,14± ±1.12/0.81 KEK’95 12 Λ C0.89± ±0.91/1.59 Status of Γ n /Γ p puzzle Γ n /Γ p exp >> Γ n /Γ p th(OPE) ~ 1 ~ Γ n /Γ p Puzzle : 3. Recent Exp. Development: p n p,n simultaneous p,n coinc. ~ 1.0 ~0.5 ~0.5 ~ Recent Development of Γ n /Γ p theory : 0.3 ~ 0.7

N p /decay where 2N ; ΛNN  NNN. Proton Energy spectrum INC N p /nm ~0.4 Ramos et al. Λ+n  n+n Λ+p  n+p Sato et al., PRC submitted INC

Nn=0.69 Neutron Spectrum Efficiency Np = n yield is directly compared with that of p Simple number counting gives PRC 68 (03) Λ+n  n+n Λ+p  n+p

- Assume 1N process only ; r n + r p = 1. - The neutron (proton) number per NMWD N n =Y n /N nm Ω n ε n = (2r n +r p )f + r p g N p =Y p /N nm Ω p ε p = r p f + (2r n +r p )g, where f, g ; FSI effects. Correction for Cross over recoil effects - Obtained model independently assuming 1N process. - The INC β value is used only for second order correction. β=0.076  Γ n /Γ p = (0.45~0.51)+-0.15

Г n /Г p ratio E307/E369 : Г n /Г p ( 12 Λ C)| 1N = (0.45 ~0.51)± 0.14 (stat. only) - Almost model independent. - This ratio agrees well with the most recent theoretical values (~.5) In order to resolve the difficulties, Exclusive measurements 5 Λ He  E Λ C  E508 final state int. 2N induced NMWD. However, ambiguities due to

p n π K Setup E462/E508 (KEK-PS K6 beamline & SKS) Solid angle: 26% 9(T)+9(B)+8(S)%

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± MeV 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

n+p n+n Estimation of N nn, N np (N nn /N np =0.45±0.11±0.04) measured N(nn-pair)/  nn~ 1956 N(n+p)=90( -1.34) N(n+n)=30(-4.38) En,p>30MeV Cos(  )<-0.8 Angular correlation cos(  )  - contamination nn, np measured N(np-pair)/  np ~

Γ n /Γ p from N n /N p & N nn /N np Simple counting of singles yields of n,p and coincidence yeilds of nn, np pairs gives, N n /N p = 2Г n /Г p + 1, N nn /N np ~ Г n /Г p, neglecting FSI and 2N. Γ n /Γ p (N n /N p )  0.59 ( 5 Λ He), 0.5 ( 12 Λ C), Γ n /Γ p (N nn /N np )  0.45 ( 5 Λ He), 0.40 ( 12 Λ C).

For the singles of E462/E508 5 Λ He (E462)  Γ n /Γ p ( 5 Λ He) = 0.61 ± ± Λ C (E508)  Γ n /Γ p ( 12 Λ C) = 0.58 ± 0.06 ± Λ He (E462)  Γ n /Γ p ( 5 Λ He) = 0.61 ± ± Λ C (E508)  Γ n /Γ p ( 12 Λ C) = 0.58 ± 0.06 ± Correction for Cross over recoil effects Similiarly for the coincidence yields of 5 Λ He and 12 Λ C (E462/E508), where β’ is reduced β due to the back-to-back selection. 5 Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± 0.05.

5 Λ He : 0.61±0.081±0.082 (E462) 12 Λ C : 0.58±0.06±0.08 (E508) : ( 0.45~0.51)± 0.15 (E307/E369) Γ n /Γ p Status  n /  p Exp. OPE OME, DQ model 5 Λ He : 0.43 ±0.12 ±0.044 ± (E462) 12 Λ C : 0.34 ±0.13 ±0.05 (E508) Singles Coincidence

Summary 1.I have discussed the recent experimental results on NMWD of Λ hypernuclei and the effects of FSI on the Γ n /Γ p. 2.A series of experiments have been done at KEK-PS, the decay particle spectra of n and p (Nn, Np) and the exclusive coincidence pair yields (Nnn, Nnp) of NMWD 3. The ratios of singles yields of n/p and coincidence yeilds of nn/np gives Γ n /Γ p values which well supports the recent theoretical ratios. 4. The difference of Γ n /Γ p values from singles yields of n/p and coincidence yeilds of nn/np is interpretted as 2N contribution. This probably is the first experimental indication of 2N process. It is expected that the 2N contribution now can be extracted from the coincidence data.

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 KEK-PS E462/508 collaboration

Extra OHPs

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 !!

Neutron and Proton Energy spectra of 5 Λ He and 12 Λ C N n / N p (E>60MeV) ~ 2.17±0.15±0.16 Neutron Proton Neutro n Proton N n / N p (E>60MeV) ~ 2.00±0.09±0.14

For the singles of E462/E508 where α=2.00 ( 5 Λ He), 2.17 ( 12 Λ C) 5 Λ He (E462)  Γ n /Γ p ( 5 Λ He) = 0.61 ± ± Λ C (E508)  Γ n /Γ p ( 12 Λ C) = 0.58 ± 0.06 ± Λ He (E462)  Γ n /Γ p ( 5 Λ He) = 0.61 ± ± Λ C (E508)  Γ n /Γ p ( 12 Λ C) = 0.58 ± 0.06 ± β=0.041 Correction for Cross over recoil effects

Similiarly for the coincidence yields of 5 Λ He and 12 Λ C (E462/E508), 5 Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± where β’ is reduce β due to the back-to-back selection. Correction for Cross over recoil effects

Similiarly for the coincidence yields of 5 Λ He and 12 Λ C (E462/E508), 5 Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± Λ He (E462)  Γ nn /Γ np ( 5 Λ He) = 0.43 ± 0.12 ± Λ C (E508)  Γ nn /Γ np ( 12 Λ C) = 0.34 ± 0.13 ± where β’ is reduce β due to the back-to-back selection. Correction for Cross over recoil effects

5 Λ He : 0.61±0.081±0.082 (E462) 12 Λ C : 0.58±0.06±0.08 (E508) : ( 0.45~0.51)± 0.15 (E307/E369) Γ n /Γ p Status  n /  p Exp. OPE OME, DQ model 5 Λ He : 0.45±0.11±0.04 (E462) 12 Λ C : 0.40 ± 0.09 (E508) Singles Coincidence

CONTENTS I. Issues / focus of Nonmesonic Weak Decay (NMWD) II. Recent Experiments 1. Singles meas. ; N n and N p ; (KEK-PS E307 /E369) 2. Coincidence meas. : N nn and N np (E462/E508) III. Г n /Г p and Final State Interaction 1. Effects of FSI onГ n /Г p 2. 2N signature IV. Summary

Taking the ratios of singles yields of n/p and coincidence yeilds of nn/np give Γn/Γp different values whose differce is interpreted as 2N contribution, Γn/Γp (Nn/Np)  0.61(5He), 0.58(12C), Γn/Γp (Nnn/Nnp)  0.43(5He), 0.34(12C). 4. Showed that 2N component can be resolved from coincidence data. For the extraction of 2N contribution, one need FSI model calculation but only for the second order effect. 5. Pinning the The direct comparison of the spectra gave the Γn/Γp ratio close to 0.5. The coincidence exclusive measurements of NMWD produced the results on NMWD quite free from FSI and 2N contribution. The results show the Γn/Γp ratio ~ As long as Γn/Γp ratio concerned, the experimental and theoretical ratios now well agree to each other. However, αnm remains inconsistent between those of experiment and theory.

Therefore, Where  Γ n /Γ p = (0.45~0.51)±0.15 Obtained model independently assuming 1N process and f n =f p. The INC β value is used only for second order correction. HYP03 Conf INC calculation

Theory: ～ Preliminary  NM =0.07±0.08 p statistical  contami Asymmetry parameter of 5 Λ He (E462)

Preliminary  Blue : 12 C :  NM = ± 0.32  Red : 11 B :  NM = 0.11 ± 0.44 Theory: ～ p Asymmetry parameter of 12 Λ C, 11 Λ B (E508)  NM =-0.24±0.26 statistical  contami

N hyp Γn/Γp sas02 5 Λ He0.70 Ito02 5 Λ He0.39 Gar03 5 Λ He0.46 Ose01 12 Λ C0.53 Ito03 12 Λ C0.57 Gar03 12 Λ C0.34 N hyp Γn/Γp BNL 5 Λ He.93± Λ C1.33±1.12/0.81 KEK’95 12 Λ C1.87±0.91/1.59 KEK (Sat02) 12 Λ C 0.87±0.09±0.21 (1N) 0.60±0.11±0.23 (1N+2N) KEK (Kim03) 12 Λ C(0.45~0.51)±0.14 (1N) E462 (Inclusive) 5 Λ He(0.66~0.69)±0.12 (1N) E508 (Inclusive) 12 Λ C(0.49~0.55)±0.10 (1N) Theory Experiment HYP03 Conf. Current Status of Γ n /Γ p