# Presentation: 693632

## Non-Mesonic Weak Decays of 5ΛHe in ( +,K+) reaction

**RIKEN  H. Outa**

***24******th******. Oct. 2005  PANIC2005***

**for KEK-PS E462 / E508 collaborations**

- Osaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku Univ.e,
- Univ. Tokyof,Tokyo Inst. Tech.g, KRISSh, RIKENi
-  S. Ajimuraa, K.Aokib, A.Banuc, _H. C. Bhang_d, T. Fukudac,O. Hashimotoe, J. I. Hwangd, S. Kameokae,
- B. H. Kangd, E. H. Kimd, J. H. Kimd, M. J. Kimd, T. Marutaf, Y. Miurae,Y. Miyakea, T. Nagaeb, M. Nakamuraf,
-  S. N. Nakamurae,H. Noumib, S. Okadag, Y. Okayasue, _H. Outa_b, H. Parkh,P. K. Sahab, Y. Satob,
- M. Sekimotob, T. Takahashie,H. Tamurae, K. Tanidai, A. Toyodab, K. Tsukadae,T. Watanabee, H. J. Yimd

**Γ(Λn→nn)/Γ(Λp→np)**** ****ratio**** **
**    **B.H. Kang *et al.*  **nucl-ex/0509015**;  PRL submitted
** ****Asymmetry parameter**
**    **T. Maruta *et al.***   nucl-ex/0509016**;  PRL submitted
** ****Mesonic decay widths**** ⇒ ****omitted in this talk**
**    **S. Kameoka** ***et al.*** Nucl. Phys. A754 (2005) 173-177**
**    **S. Okada** ***et al.***      Nucl. Phys. A754 (2005) 178-183 **

## Weak decay of  hypernucleus

***Weak decay of ************ hypernucleus***

****** ****weak decay in free space**

******→ ****p ****+ π****-****	****: ****63.9±0.5 %**
****** → ****n ****+ π****0****	****: ****35.8±0.5 %**

**********= 263.2±2.0 ps**

**→**** ****Well known.**

**Study of the mechanism of**** ****baryon-baryon weak interaction****.**

## Theoretical

**Theoretical**

*********n ******/ ************p****** ratio***

**: The most important observable used to study the ****isospin structure of the NMWD****.**

*********n ******/ ************p****** ratio puzzle***

**Γ****p**** ****(Λ+“****p”→ n ****+ ****p****)**
**Γ****n**** ****(Λ+“****n”→ n ****+ ****n****)**

## Experimental difficulties in the nucleon measurement

***Experimental difficulties in the nucleon measurement***

** ****Difficulty in detecting neutrons****. **
**    ******** ****There is no experiment to observe both of the protons and neutrons simultaneously with high statistics. **
** ****Final state interaction**** (FSI) effect**
**     ******** ****not well established**
**Distinguish between the FSI and ****”********NN********nNN” process**

## The present experiment

***The present experiment***

**KEK-PS E462/E508**

** ****1) ****Angular correlation**** **
** ****( back-to-back, ****cos********<****-0.8 ****)**
** ****2) ****Energy correlation**
** ****( Q****~****E(N1)+E(N2) ****~****152MeV  ****)**

**NMWD :  ****Λ****N→NN**

**Direct measurement **
**of the ********n ****/ ********p**** ratio**

**Select light hypernuclei to minimize FSI effect, ****5********He**** ****and**** ****12********C**** **

## Excitation-energy spectra for 6Li and 12C

***Excitation-energy spectra for ******6************Li and ******12************C***

**5********He**

## Decay-particle detection system

***Decay-particle detection system***

**Solid angle: 26% **
**= 9(T)+9(B)+8(S) %**

**Neutral particle****:**
**  ****・****TOF (T1********T4)**
**  ****・****T3 VETO**

**Charged particle****:**
** ****・****dE/dx (at T2)**
** ****・****total energy deposit**
**   ****(T2+T3+T4)**** **
** ****・****TOF (T2********T3)**

## Neutral PID

***Neutral PID***

**1 / ******** spectra**

**Neutral particles from  ****12********C**

**Good ******** *****n***** separation**

**Good ******** *****p d***** separation**

***Decay particle identification***

## Excitation spectra w/ coincident decay particles for 5He

***Excitation spectra w/ coincident decay particles for ******5************He***

- S. Kameoka** ***et al.*** Nucl. Phys. A754 (2005) 173-177**
- S. Okada** ***et al.***      Nucl. Phys. A754 (2005) 178-183 **

## Angular correlation

**Angular correlation**

**Energy sum**

**n + p**

**n + n**

**estimated contamination from π****- ****absorption **

***Coincidence analysis ***

**Q-Value**** ~****15****3****MeV**

***cos θ******<-******0.8***

***90 event***

***30 event***

***(1.34)***

***(4.38)***

##  np- & nn- angular distribution

*** ******np- & nn- angular distribution***

******n********p********n****n**** ****/ ********np**** ****= 0.4****5****±0.1****1****±0.0****3**

***systematic error is mainly come from efficiency for neutron (6%) + acceptance(3%)***

**Back-to-back**** **

**Back-to-back**** **

## Exp.

**0.93±0.55 (Szymanski et al.) for ****5********He**

**Exp.**

*********n ******/ ************p****** ratio***

**Previous exp. (at BNL)**

## Importance of  αnm measurement

| Initial state | Final state | Amplitude | Isospin | Parity |
| --- | --- | --- | --- | --- |
| 1S0 | 1S0 | a | 1 | No |
|  | 3P0 | b | 1 | Yes |
| 3S1 | 3S1 | c | 0 | No |
|  | 3D1 | d | 0 | No |
|  | 1P1 | e | 0 | Yes |
|  | 3P1 | f | 1 | Yes |

- If assuming initial S state

**We can know the interference between states with**
**different**** Isospin**** and ****Parity**** .**

- (Applying ******=1/2** rule)

***Importance of  α******nm****** measurement***

## N(θ) = 1 + Acosθ = 1 + P cosθ

- SKS Acceptance
******K ****= +15****o****~****-15****o**

- Decay counter
- Acceptance
**|****θ****| > 0.7**

- -1

- +1

**cosθ**

- 0

- 1

****

**Asymmetry**

**Asymmetry**
**Parameter**

- P

**Asymmetric proton emission from NMWD**

**N**

- Difference of acceptance & efficiency is canceled out !

## NM  for 5ΛHe NMWD

- ・**Polarization of**** ******

- ・Asymmetry Parameter of Proton

- A**p****=********NM**P********

**p**

**Estimated from mesonic decay**

## Asymmetry parameter of 5ΛHe

***Final !!***

**Asymmetry parameter of**** ****5****Λ****He**

**statistical**

****** ****contamination**

**T.Maruta et al. HYP03**
**Nucl.Phys.A754 (2005) 168-172**
**nucl-ex/050916 ; submitted to PRL**

## np coincidence analysis

***np coincidence analysis***

**NMWD**

**np back-to-back event**

## Summary

** ********N→NN was directly observed for the first time !!**
**    ****5****Λ****He****	****: Γ****n ****/ Γ****p**** ratio**** ****~ ****N****nn ****/ N****np**** = ****0.45±0.11±0.03**
**                       ****nucl-ex/0509015**
**    ****12****Λ****C****	****: Γ****n ****/ Γ****p**** ratio**** ****~ ****N****nn ****/ N****np**** = ****0.50±0.14±0.03**
**                      ****Kim (Session IV-6)**
**◆  ****Asymmetry parameter measured with improved accuracy !!**
**   ****Single proton :  **
**        ****n+p coincidence: **
**                                                                              **** ****nucl-ex/0509016**

**[1] Importance of shorter-range mechanism **
**     **** ****OPE **** ⇒ ****Heavy meson & DQ exchange **
**[2] Significant contribution of ΛN initial spin-singlet **
**     ****state contribution**
**       ****- ****σ-meson exchange ? **

## Spare OHP

## Production of Polarized Hypernuclei

- E462/E508 experiment

- 1.05GeV/c ******+**** ****beam**
**is injected. **

**Distribution of ******** polarization **
**in the n(********+****,K****+****)******** reaction**

**K****+**** scattering angle(********K****)**

**1.05GeV/c ********+**

**In large scattering angle, ******** is much polarized.**

- P

## 6Li Hypernuclear mass spectra

**6****Li**** + ********+****→ ********6****Li**** ****+ ****K****+**

******6****Li**** ****→ ********5****He**** ****+**** p**

**5********He**

**6********Li**

**5****Li**

**0MeV**

**8.3MeV**

**18.3MeV**

**(****P****n****-1****,S********)**

**(****P****n****-1****,P********)**

**(****S****n****-1****,S********)**

**p decay**

******decay**

**inclusive**

****** ****coin**

**p coin**

- 5.2×104 events

- 3.2×103 events

- 1.6×103 events

******

## Instrumental Asymmetry

- (****,pC) reaction : Only Strong Interaction

**Asymmetry = 0** expected

**Instrumental Asymmetry**** ****<**** ****0.3%**

## Nn / Np Ratio

**Γ****n**** ****(Λ+“****n”→ ****n**** ****+ ****n****)**
**Γ****p**** ****(Λ+“****p”→ ****n**** ****+ ****p****)**

**  ****If Γ****n ****/ Γ****p ****= 1 ****	****→ N****n**** / N****p**** = 3 **
**  ****If Γ****n ****/ Γ****p ****=  0.5****	****→ N****n**** / N****p**** = 2**
**  ****If Γ****n**** / Γ****p ****=  0****	****→ N****n**** / N****p**** = 1 **

**Naive estimation**

**(without considering ****FSI**** and ****ΛNN→NNN****)**

**To avoid suffering from FSI effect & ΛNN→NNN,**

**High energy threshold **

**Γ****n****:Γ****p         ****N****n****   ****N****p**
**  ****1 : 1  ...   ****3****  :  ****1**
**  ****1 : 2  ...   ****2****  :  ****1**
**  ****0 : 1  ...   ****1****  :  ****1**

**N****n**** / N****p  ****=  2×********n**** / ********p ****+ 1**

## Neutron and Proton energy spectra of  5He and  12C

***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±0.16**

**5********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 !!***

- (submitted PLB. nucl-ex/0406020)

## Excitation spectra w/ coincident decay particles for 12C

**inclusive**

**w/ proton**

**w/ pion**

**w/ neutron**

**w/ gamma**

***Excitation spectra w/ coincident decay particles for ******12************C***

## Other results

- - branching ratio

- 0 branching ratio

- Lifetime

**Other results**

******-**** decay width for ****5********He**

**Statistical errors were much improved !!**

******0**** decay width for ****5********He and ****12********C**

## To J-PARC

**Non-mesonic weak decay of ****4********He and ****4********H**

**4****He (K****-****,********-****) ****4********He   or **
**4****He (********+****,K****+****) ****4********He**
**  ******** ****n+n back-to-back**

**4****He (K****-****,********0****) ****4********H**
**  ******** ****p+n back-to-back**
**     ****(********0**** spectrometer )**

***see S.Ajimura : J-PARC LOI 21 ***

**To J-PARC**

**R****NS ****... N : ********n********nn, ********p********np**
**            ****S : spin =  0 or 1              **

******nm****(****4********H)   = ( 3R****n1****+  R****n0                  ****+ 2R****p0 ****) ×********4 ****/ 6**
******nm****(****4********He) = (            2R****n0 ****+ 3R****p1**** +  R****p0 ****) ×********4 ****/ 6**
******nm****(****5********He) = ( 3R****n1****+  R****n0  ****+ 3R****p1 ****+   R****p0 ****) ×********5 ****/ 8**

*********  ******Need one-order higher statistics.******  ************  J-PARC***

## K6/SKS setup

**π****+**

**K****+**

**decay **
**counter**

***K6/SKS setup***

## Identification of hypernuclear formation 

**K****+**

********

- T1

- target

**Mass**

**Z**

## Neutral decay particle ID

***Neutral decay particle ID***

** ****Good *********n***** separation**
** ****Good S/N ratio (****~****30)**
** ****( previous exp.**
**   ****S/N ratio (****5********He) ****~****1 )**
******~****30 times ****(for ****5********He)**
**   ****higher than previous**
**   ****exp.**
**  ****High statistics**
**         ****(****~ ****5000 neutrons)**
******~****200 times**** (for ****5********He)**
**    ****~****30 times**** (for ****12********C)**
**    ****higher than previous**
**    ****exp. **

**5MeV< E****n**** < 150MeV**

**Constant background **
** ****very small**

**Neutron energy resolution**
**(estimated  from ******** width) **
**→****7MeV(FWHM)  ****at 75MeV**

**(TOF spectra)**

**1 / ******** spectra**

**Neutral particles from  ****12********C**

## Charged decay particle ID

**Charged particles from  ****5********He**

**derived from**
*** ******dE/dx*****  (at T2)**
** ****Total energy deposit****  (sequentially fired counters (T2,T3,T4).)**
** ****TOF****  (between T2 and T3)**

**PID function**

**Deuterons**** were separated from the protons.**
**(for the first time!)**

***Charged decay particle ID***

**Gaussian fit**
**(±2******** cut)**

## Non-mesonic weak decay

**strong tenser coupling (********L=2, ********S=2)**
**→  ****dominant term ****3****S****1****→****3****D****1  ****(amplitude “****d****”)**

** ****+ N→ N + N**

**+ p → n + p**
**: ********p ****= a****2****+b****2****+c****2****+d****2****+e****2****+f****2**

******+ n → n + n **
**: ********n**** = a****2****+b****2****+f****2**

**OPE : ********n ****/ ********p**** ****~****0.1**

**One Pion Exchange (OPE) model**

** ********n ****/ ********p**** ratio**

**: The most important observable to study the isospin structure of the NMWD.**

**Exp. : ********n ****/ ********p**** ****~ ****1**

*********n ******/ ************p****** ratio puzzle***

***Simple theoretical model***

***with large error***

## Mass number dependence of neutron energy spectra ( A=5,12,89 )

***Mass number dependence ******of neutron energy spectra ( A=5,12,89 )***

**( previous experiment )**

**As the mass number become lager,  the number of neutron become ****lager in the low energy part,**** and ****smaller in the high energy part****.**

**Q / 2 = 76 MeV**

**No peaking**** **
**at Q / 2 (76MeV) ****even ****5****Λ****He**

**  ****suggested larger contribution of ΛNN→NNN or FSI than theoretical prediction.**

**Theoretical calc.**

**5****Λ****He**

**w/ FSI**

**Q/2**

## -nucleus overlap for 5He

**********0**** ****/ ************ = ****0.201±0.011 ****(************He****)**

- 

******nm****/ ************ = ****0.395±0.016 ****(************He****)**

**Γtotal (****56****Λ****Fe) ****~****Γnm(A→∞) ****~ ****1.2Γ****Λ**** (****E307****)**

**Γnm (****5****Λ****He)   = 0.4Γ****Λ**** (****Present)**

**1/3 of Λ is inside α **

**Both results are consistent,**
**preferred larger overlap than YNG prediction.**

**5****Λ****He (ORG) ****~ ****40%**
**5****Λ****He (YNG) ****~ ****20%**

**1/3 of Λ is inside α **

- 

- 

**********0**** ****/ ************ locates in between ORG and YNG.**