First ExclusiveFirst Exclusive Measurement of the Measurement of the Non-Mesonic Weak Deacay of Non-Mesonic Weak Deacay of 1212

CC

Seoul national UniversityM.J.Kim

KEK-PS E462/E508 Collaboration

2004.06.08 dane2004

Contents of talkContents of talk

n+n/n+p coincidence measurementn+n/n+p coincidence measurement nn//pp~ 0.40~ 0.40±0.09±0.09(statistical error)(statistical error)

Preliminary!!Preliminary!!

E462/E508E462/E508 Collaboration CollaborationKEK, RIKEN, Seoul Univ., GSI,

Tohoku Univ., Osaka Univ., Univ. TokyoOsaka Elec. Comm. Univ. , Tokyo Inst. Tech.

S. Ajimura, K. Aoki, A. Banu, H. Bhang, S.Shin,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,

T. Nagae, 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

ΓΓpp 　　 (Λ+ ｐ→ ｎ + ｐ )ΓΓnn 　　 (Λ+ ｎ → ｎ + ｎ )

Non Mesonic(NMWD) q ~ 400MeV/c

1/τHY =Γtot

Γm

Γnm

Γπ_ 　 (Λ→ ｐ + π － )

Γπ0 　 (Λ→ ｎ + π ０ )

Mesonic q ~ 100MeV/c

Weak decay mode of hypernucleus

ΓΓ2NN 　　 (ΛNN → NNN ) ?

Direct determination of the ratio !!!!Direct determination of the ratio !!!!nn//pp

Purpose

Model nn / / pp

Ramos-Benhold et al. (1994)

OPE OME

Dubach et al. (1996)

OPE OME

Parreno et al.(1996)

OPE

Ramos et al. (1997)

OPE

n / p

Szymanski et al. (BNL 1991)

Noumi et al. (KEK 1995)

Theory

Experiment

n/pexp >> n/p

th

n/p ratio puzzle!

Status of n/p of 12C(until recently)

0.190.19

0.270.27

0.200.20

0.830.83

0.120.12

0.100.10

1.33±1.21/0.81

1.87±0.59 ±0.32/1.00

Problems in Experiment Problems in Experiment 1)high threshold 1)high threshold 2) Large error bar2) Large error bar

Problems in determination Problems in determination method of method of nn//pp ratio ratio from datafrom data

1)model dependent1)model dependent2) Effects of FSI and 2N proces2) Effects of FSI and 2N processs

* FSI: Final State Interaction* FSI: Final State Interaction

12C g.s.

NMWDN → NN

N

N

Direct Measurement of the n /p ratio

Top Detector

Bottom Detector

Angular correlationAngular correlation ( back-to-back, cos<-0.8 ) Energy correlationEnergy correlation ( energy sum ~ Q value~150MeV )

Angular correlationAngular correlation ( back-to-back, cos<-0.8 ) Energy correlationEnergy correlation ( energy sum ~ Q value~150MeV )

Select ΛN→NN events without FSI and 2N process effect

YYnpnp=N=Nnpnpx x npnpx x nnx x ppx (1-Rx (1-RFSIFSI))npnp

YYnnnn=N=Nnnnnx x nnnnx x nnx x nnx (1-Rx (1-RFSIFSI))nnnn

By charge symmetry (1-RFSI)np=(1-RFSI)nn

~ =

YYnpnp=N=Nnpnpx x npnpx x nnx x ppx (1-Rx (1-RFSIFSI))npnp

YYnnnn=N=Nnnnnx x nnnnx x nnx x nnx (1-Rx (1-RFSIFSI))nnnn

By charge symmetry (1-RFSI)np=(1-RFSI)nn

~ = nnnnp

pnpnn

εΩYεΩY

p

n

Γ

Γnp

nn

N

N

NucleonNucleon

p

n

+

K+

NT: 20cm×100cm×5cmT3: 10cm×100cm×2cmT2: 4cm×16cm×0.6cm

NT: 20cm×100cm×5cmT3: 10cm×100cm×2cmT2: 4cm×16cm×0.6cm

Solid angleNeutral: 26.5%Charged : 10%

Decay armDecay arm

Active Target

SETUPSETUP

beambeam

Charged particle ： ・ TOF (T2→T3) ・ tracking （ PDC ）Neutral particle ： ・ TOF (target→NT) ・ T3 VETO

(KEK-PS K6 & SKS)

NTNT

Excitation energy Excitation energy spectra for spectra for 1212C(C(++,K,K++) )

Excitation Energy(MeV)Excitation Energy(MeV)

12C g.s. quasi free

11B

inclusive

w/ proton

w/ neutron

w/ n+p

w/ n+n

1111C + C +

BB + p+ p1111

0.0 MeV0.0 MeV

2.5 MeV2.5 MeV

6.3 MeV6.3 MeV

8.1 MeV8.1 MeV

C

1111 B

1212

Level scheme Level scheme of Cof C

Proton decayProton decay

1212

9.219.21MeVMeV

10.7610.76MeVMeV

Particle identificationCharged particleNeutral particle

proton

d

PID1 : total energy vs dE/dxPID2 : total energy vs TOF (PID1+PID2) /

PID1 : total energy vs dE/dxPID2 : total energy vs TOF (PID1+PID2) /

Neutron

1/Constantback ground→ very small

Constantback ground→ very small

gated 12C ground state

5MeV< energy < 150MeV

Energy resolution

~8MeV at ~75MeV

2

Coincidence MeasurementAngular CorrelationAngular Correlation Energy sumEnergy sum

n + pn + p

n + nn + n

cos θ MeVMeVEstimated Contamination from - absorption

Number:109

cos θ ＜－ 0.8

EENN>30MeV>30MeV

Number:33

cos θ ＜－ 0.8

EENN>30MeV>30MeV

Q value~150

EEn +n +EEpp

EEn +n +EEnn

Estimation of Nnp,Nnn (Nnn/Nnp = 0.40±0.09)

Number:109

Number:33

MeasuredMeasured YYnn,nn,YYnpnp Simulated coincide efficiency

npnp(Ep,n=65MeV)(Ep,n=65MeV)

nnnn(En,(En,nn=65=65MeV)MeV)

Angular Correlation(cos )

θ

Estimated Nnn,Nnp

NNnp=np=YYnpnp//np np

~~56775677±572±572

NNnp=np=YYnpnp//np np

~~56775677±572±572

NNnn=nn=YYnnnn//

nn~nn~22672267±437±437

NNnn=nn=YYnnnn//

nn~nn~22672267±437±437

Event by event correctionEvent by event correctionEvent by event correctionEvent by event correction

cos θ ＜－ 0.8EENN>30MeV>30MeV

EENN>30MeV>30MeVcos θ ＜－ 0.8

Coincidence Measurement(E462/E508)

Estimated Contamination from - absorptioncos θ

Co

un

tsC

ou

nts

Angular distributionAngular distribution Energy sum distributionEnergy sum distribution

Q value Q value

~ 153MeV~ 153MeV

He

C

s-shells-shell

p-shellp-shell

55

1212

n + pn + p

n + pn + p

n + nn + n

n + nn + n

C He

Szym

anski

Szym

anski et a

let a

l..

Noumi

Noumi e

t al

et al..

Kim

Kim et a

let a

l.(1N)

.(1N)

Sato

Sato et a

let a

l.(1N)

.(1N)

SatoSato

et al

et al.(1

N+2N)

.(1N+2N)

nn//pp

Results

Various ExperimentsVarious Experiments

E508 E508

OMEOMEOME,DQOME,DQ

OPEOPEOPEOPE

E462 E462

E307E307 E307E307 E369E369

Szym

anski

Szym

anski etet a

lal..

55

1212

5He (E462):0.45 ± 0.11 ± 0.04 (Final !!)

12C (E508):0.40 ± 0.09(statis.) (Preliminary !!)

Theory(OME or DQ)Theory(OME or DQ)

Theory(OPE)Theory(OPE)

Summary• The np- and nn-pairs from NMWD of 12

C were observed successfully.

The energy sum of back-to-back np- and nn- pairs are approximately equal to the Q-values(~150MeV) of the NMWD.

N nN process were clearly identified !!

• We got n/p ratio of 12C at cos <-0.8 and EN>30MeV .

n/p ~ Nnn/Nnp ~ 0.40 ±0.09 (statis. ) (Preliminary!! )

(expected systematic error ~ ±0.04)

Consistent with recent theoretical calculations !!!!

→

θ

Spare OHPS Spare OHPS

1.87±0.59±0.32/1.00

1.33±1.21/0.81

Experimental results Experimental results

Syzmanski et al.

(BNL)

Noumi et al. (KEK)

Sato et al.(KEKE307)

Sato et al.(KEKE307)

Kim et al.(KEKE369)

nn//pp

0.87±0.09±0.21(1N)

0.60±0.11/0.09±0.23/0.21(1N +2N)

0.51±0.15(1N)

Neutron and proton spectra from 12C

N

n,p

/NM

WD

/10

MeV

(MeV)(MeV)

Neutron

Proton

Nucleon energyNucleon energy