Non-Mesonic Weak Decays of 5ΛHe and

12ΛC Hypernuclei formed by ( +,K+) Reaction

RIKEN H. OutaRIKEN H. Outa

22. Aug. 2006 FB18 (P28)

for KEK-PS E462 / E508 collaborationsOsaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku Univ.e,

Univ.of Tokyof, Tokyo Inst. Tech.g, KRISSh, RIKENi

S. Ajimuraa, K.Aokib, A.Banuc, H. C. Bhangd, 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. Outab, 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 A= 5 B.H. Kang et al. PRL 96 (2006) 062301 A=12 M.J. Kim et al. nucl-ex/0601029 : PLB in press n/p spectra from A=5,12 S. Okada et al. PLB 597 (2004) 249-256 Asymmetry of proton from polarized hypernuclei T. Maruta et al. nucl-ex/0509016 Mesonic & non-mesonic decay widths 　⇒　 may be omitted in this talk S. Kameoka et al. Nucl. Phys. A754 (2005) 173c-177c S. Okada et al. Nucl. Phys. A754 (2005) 178c-183c

Weak decay of Weak decay of hypernucleus hypernucleus

ΓΓππ__ 　　 (→ ｐ + π － )

ΓΓππ00 　　　　 ( → ｎ + π ０ )

ΓΓpp 　　 ( +“ ｐ”→ ｎ + ｐ )

ΓΓnn 　　 ( +“ ｎ”→ ｎ + ｎ )Γ2N (ΛNN →NNN)

Mesonic

q ～ 100MeV/c

Non-Mesonic(NMWD)

q ～ 400MeV/c

1/HY =Γtot

Γm

Γnm

Weak decay mode of Weak decay mode of hypernucleus hypernucleusWeak decay mode of Weak decay mode of hypernucleus hypernucleus

weak decay in free spaceweak decay in free space weak decay in free spaceweak decay in free space

→ ｐ + π － : 63.9±0.5 %

→ ｎ + π0 : 35.8±0.5 %

= 263.2±2.0 ps

→ Well known.

Study of the mechanism of baryon-baryon weak interaction

[1]Γ(Λn→nn) /Γ(Λp→np) Ratio－　 Long standing puzzle solved

10 0.5 1.5n / p

Th

eore

tica

l

N N

NΛ

Direct Quark mechanism

Meson Exchangemechanism

Λ N

NN

π,K,η,ρ,ω,K*

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

n n / / pp ratio ratio : The most important observable used to study the isospin structure of the NMWD.

n / p ratio puzzle

ΓΓpp 　　 (Λ+“ ｐ”→ ｎ + ｐ )

ΓΓnn 　　 (Λ+“ ｎ”→ ｎ + ｎ )n n / / pp ～～ 0.10.1

Λ N

NN

W S

π

One Pion Exchange(OPE)

Simple theoretical model

Tensor-dominant requires the final Nn pair to have isospin 0.

strong tenser coupling (L=2, S=2)→ dominant term 3S1→3D1 (amplitude “d”)

nn np

pp ppn

n

n p

rescattering

Final state interaction (FSI)

effect

Experimental difficulties in the nucleon measurementExperimental difficulties in the nucleon measurement

Difficulty in detecting neutrons. There is no experiment to observe both of the protons a

nd neutrons simultaneously with high statistics. Final state interaction (FSI) effect not well established Distinguish between the FSI and ”NNnNN” process

NN→NNN(2N-induced process)

ΛN

NN

Wπ

N

N

(One of the theoretical model)

Expected single nucleon spectrum

np

npn

N→nN N→nN

nn np

pp ppn

n

n p

NN→nNN NN→nNN FSI FSI re-scattering

nn np

pp ppn

n

n p

cou

nts

Q/2Energy spectra (image) Energy

distribute low energy region up to Q/2 broad peak

around Q/2 continuousdistribution

np

np

nnp

n

p

np

npn

Coincidence

Coincidence

NMWD

NMWD

The present experimentThe present experimentKEK-PS E462/E508

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

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

NMWD : N→NN

* cosθ< － 0.8 * E(N1)+E(N2) cut

n

p

p

n

coinpairnpN

coinpairnnN

FSIpn

.avpn

R1

nppN

FSI

2n

.avnn

R1

nnnN

Direct measurement of the n / p ratio

Select N→NN events w/o FSI effect & NN→NNN.

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

C

π+

K+

Target: 6Li,12C

Excitation-energy spectra for Excitation-energy spectra for 66Li and Li and 1212

CC

6.2×104

events

4.6×104

events

decay counter

6Li (g.s.) 55

He He + p

The ground state of 6Li is

above the threshold of 5He + p.

55HeHe

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

p

n

π

K

Decay counter Setup (KEK-PS K6 & SKS)

Decay arm

N: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm

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

n

p

polarizationaxis

Charged particles from 5He

PID function

Charged PIDCharged PIDNeutral PIDNeutral PID

Constant background very small

Neutron energy resolution→7MeV(FWHM) at 75MeV

1 / spectra

Neutral particles from 12C

Good n separation Good p d separation

Decay particle identificationDecay particle identification

The g.s. peak is clearly seen in all spectra with coincident decay particles.

previous experiment at BNL

inclusive

w/ proton

w/ π±

w/ neutron

w/ γ

Excitation spectra w/ coincident decay particles for Excitation spectra w/ coincident decay particles for 55HeHe

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

np- & nn- angular distribution (5ΛHe)

npnn / np = 0.45±0.11±0.03

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

Back-to-back Back-to-back

Coincidence Measurement (A=12)

cos

n + pn + p

n + nn + n

p + pp + p

EEn +n +EEpp

EEn +n +EEnn

EEp +p +EEpp

MeVMeVNNNN

Co

un

tsC

ou

nts

12ΛC

npnn / np = 0.50±0.13±0.05

10 0.5 1.5n / p

0.93±0.55 (Szymanski et al.) for 5He

Exp

.

Λ N

NN

W S

π

One Pion Exchange(OPE)

Th

eo.

N N

NΛ

Direct Quark mechanism

Meson Exchangemechanism

Λ N

NN

π,K,η,ρ,ω…

n n / / pp ratio ratio

Previous exp. (at BNL)

Nnn / Nnp (5He)= 0.45±0.11±0.0355

He (E462)He (E462)

Kang et al. PRL 96 (2006) 062301

Γn / Γp (12C)= 0.50±0.13±0.051212

C (E508)C (E508)

Kim et al. nucl-ex/0601029 PLB in press

[2] Asymmetry of proton emission from polarized hypernuclei

Asymmetry measurement of decay proton

N() = N0(1 + Acos)

Asymmetry

Asymmetry : Volume of the asymmetric emission from NMWD

A = (R - 1)(R + 1)

R =N(-) N(+)

,

Asymmetryparameter

= N0(1 + Pcos)

Difference of acceptance & efficiency is canceled out !

R =N(+(-))×N(-(+)) N(+(+))×N(-(-)) 1/2

K >0

+ K+

/p

K

K <0

+K+

/p

K

P

P

Initial state Final state Amplitude Isospin Parity

1S0

1S0 a 1 No3P0 b 1 Yes

3S1

3S1 c 0 No3D1 d 0 No1P1 e 0 Yes3P1 f 1 Yes

If assuming initial S state

)}(3{41

])2(3)2([23222222 fedcba

fdcdcbaeNMp

We can know the interference between states withdifferent Isospin and Parity .

222222

222 )(2/

fedcba

fbapn

(Applying =1/2 rule)

Importance of αnm measurement

NM for 5ΛHe NMWD

A=PA:Asymmetry of PionAsymmetry Parameter of Pion (= － 0.642±0.013)PPolarization of Lambda:Attenuation factor

・Polarization of

・ Asymmetry Parameter of ProtonAp=NMPp

Estimated from mesonic decay

We can calculate NM without theoretical help !p

Polarization of

ー : E462

ー : E278

: Motoba et al. NPA577 (1994) 293c

NOTE:Calculation by Motoba et al. considers excited state at E=4.5 MeV

Theory: - 0.6 ～ - 0.7

Asymmetry parameter of 5ΛHe

NM=0.08±0.08+0.08 pst

atis

tical

conta

mi

-0.00

Nucl.Phys.A754 (2005) 168-172nucl-ex/050916 ; submitted to PRLInstrumental Asymmetry <0.003

Asymmetry parameter of 12C, 11B

NM=-0.14±0.28+0.18 pst

atis

tical

conta

mi

-0.00

E160 : - 0.9±0.3

Comparison with recent calculations

OPE

+K

+K+DQ

OME

+K,OME can reproducen/p ratio but predict large negative NM

+K+

+K++DQ

n/p and NM can bereproduced only by+K++DQ model

Sasaki et al.PRC71 (2005)035502

(1) Large b(1S0→3P0) and f(3S0→3P1) amplitude(2) Violation of ΔI=1/2 rule considered

[3] Precise measurement of π-mesonic / non-mesonic decay widths － Test of Λ-nucleus potential

Kameoka et al., HYP03 procNPA754 (2005)

Lifetime measurements withLifetime measurements with(π(π++,K,K++) reaction & SKS) reaction & SKS

psHe 11278)( 5 psC 6212)(12 47

Other Results-1: Other Results-1: 0 0 and Λ-α potentialand Λ-α potential

He0 / = 0.201±0.011

Lifetime : 278+11 ps (E462)-10

HYP03 proceesingsNucl. Phys. A754 (2005)

Other results-2: A dependence of ΓOther results-2: A dependence of ΓNMNM

HYP03 proceedingsNucl. Phys. A754

SummarySummary N→NN was directly observed for the first time !! 5

ΛHe : Γn / Γp ratio ～ Nnn / Nnp = 0.45±0.11±0.03　　　　　　　　　　　　　　　　　　　　　　 Kang et al. PRL 96 (2006) 062301

12ΛC : Γn / Γp ratio ～ 0.50±0.13±0.05

　　　　　　　　　　　　　　　 　　　　　　 Kim et al. PLB, in press

◆ Asymmetry parameter measured with improved accuracy !!　　　 5

ΛHe :

11ΛB and 12

ΛC : Maruta et al. nucl-ex/0509016; thesis

◆ Mesonic and Non-mesonic decay widths are precisely measured － Test of Λ-nucleus potential

NM=0.08±0.08+0.08

p

[1] Importance of shorter-range mechanism 　 OPE 　⇒　 Heavy meson & DQ exchange [2] Significant contribution from ΛN initial spin-singlet initial state －　 σ-meson exchange ? / Violation of ΔI=1/2 rule ?

-0.00

NM= － 0.14±0.28+0.18

p -0.00

Spare OHP

ΛNN→NNN considerationΛNN→NNN consideration

Λn

np

Wπ-

p

n

Λn

nn

Wπ0

p

p

4

1

Seen as….

n

n

p

p

n

n

Λn→nn like

Λp→np like

Garbarino

exp

erim

enta

lex

per

imen

tal

dat

ad

ata

theo

reti

cal

theo

reti

cal

calc

.ca

lc.

Comparison with theoretical calc.for angular correlation

Garbarino’s calc.

assuming Gn/Gp = 0.46 (for 5He ), 0.34 (for 12

C )considered 2N-induced( ～ 20%), FSI

55He (E462)He (E462) 1212

C (E508)C (E508)

n+p coincidencen+n coincidence

n+pn+n

cos cos

n+p coincidencen+n coincidence

n+pn+np+pp+p

Pair numberPair number /NMWD/NMWD

Pair numberPair number /NMWD/NMWD

n+p coincidencen+n coincidencep+p coincidence

n+p coincidencen+n coincidencep+p coincidence

Phys. Rev. Lett. 91 (2003) 112501Phys. Rev. Lett. 91 (2003) 112501 21

E462

- / of He

- branching ratio0 branching ratio

Lifetime

Other resultsOther results

- decay width for 5He

Errors were much improved !!

0 decay width for 5He and 12

C

　 HYP2003 proceedingsNucl. Phys. A754 (2005)

Kameoka et al., HYP03 procnucl-ex/0402023

Very recent measurements withVery recent measurements with(π+,K+) reaction & SKS(π+,K+) reaction & SKS

psHe 10278)( 5 psC 6212)(12 47

Preliminary

Null asymmetry test

(,pX) reaction : Only Strong Interaction

Asymmetry = 0 expectedAsymmetry of

6Li target- 0.000± 0.002

0.003± 0.002

0.003± 0.002

- 0.001± 0.001

0.003± 0.001

0.000± 0.001

6<|θ |<9°

9<|θ |<15°

6<|θ |<9°

9<|θ |<15°

2<|θ |<6°

Scattering Angle2<|θ |<6°

Horizontal

Proton

Pion

Instrumental Asymmetry < 0.3%

p or

np coincidence analysis

Asymmetryθ <0 θ >0 Parameter

23/ 26 30/ 19 0.176± 0.122 0.306± 0.215

Asymmetry

6<|θ |<15°

Horizontal Scattering Angle

Nupper/ Nlower

np

np

nnp

n

p

Coincidence

NMWD

np back-to-back event

NM=0.31±0.22p

One and only(?) solution

+ K + + DQ

Sasaki et al.PRC71 (2005)035502 N

NN

W S

,K,

b(1S0→3P0) と f(3S0→3P1) amplitude に影響を与えるI = 3/2 が大きく寄与する

今回 n/p ratio と NM を高精度で測定したことにより、 こういう反応機構の必要性が認識された。

p

Angular correlationAngular correlation Energy sumEnergy sum

n +

pn

+ p

n +

nn

+ n

estimated contamination from π － absorption

Coincidence analysis (5ΛHe)

Q-Value ~153MeV

cos θ ＜－ 0.8

90 event

30 event

(1.34)

(4.38)

n/p estimation from Coincidence Measurement

Coincidence Measurement(E462/E508)

Pair Pair numbers/NMWDnumbers/NMWD

Estimated contamination from

Energy sum Energy sum distributiondistribution

7.0cos NNfor 1cos1 NNfor

n + pn + p n + nn + n

0034.00113.0

)018.0060.0()32

6(

a

0038.00156.0

)0204.00833.0()32

6(

b

Before subtraction After subtraction

)7.0(cos npnpN

)7.0(cos nnnnN

0138.01384.0

0144.00826.0 0149.00670.0

0142.01272.0

Uniform components subtractionUniform components subtraction

FSI consideration using pp-pairs

rrn,pn,p fraction ratio of the neutron and

proton induced channels.ffn,pn,p is reduction factor due to FSI.

ggn,n,pp is cross over influx of neutron(proton) from proton(neutron) due to FSI.p,q,q’p,q,q’ are angular acceptance factor.

fff np ggg np 02 gpn rrx /

0)(2 ''''2 ppnnnppp NNxNNx

,,,, ,,

Nnp(bb) 0.1272 0.0142

Nnn(bb) 0.0670 0.0149

Npp(bb) 0.0047 0.0017

MeVEthNN 30,7.0cos At

'2'

'

2'

2

qgrqgfrN

qgfrpffrN

qgfrpfrN

pnppppp

pnnpnpnp

nnpnnnn

))(( 'NNNN NbbN

Before FSI correctionBefore FSI correction

13.053.0)(

)(

bbN

bbN

np

nn4%4%

Systematic error calculationSystematic error calculation

05.013.051.0

p

n

1)1) Intentionally add 2Intentionally add 2 pp pp events events inin

-0.8<cos-0.8<cos<-0.7 <-0.7 %4:13.049.0

2)2)

3)3)Uniform b.g. level for different angular regions:6.6%Uniform b.g. level for different angular regions:6.6%

Neutron Efficiency Correction

Production of Polarized Hypernuclei

E462/E508 experiment 1.05GeV/c + beamis injected.

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

2o 15oK+ scattering angle(K)

1.05GeV/c +

In large scattering angle, is much polarized.

P

+

K+

/p

K

6Li Hypernuclear mass spectra

6Li + +→ 6Li + K+

6Li → 5He + p

5He

6Li

5Li0MeV

8.3MeV

18.3MeV

(Pn-1,S)

(Pn-1,P)

(Sn-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 InteractionAsymmetry = 0 expected

6Li target 12C target- 0.000± 0.002 0.000± 0.0020.003± 0.002 - 0.003± 0.0030.003± 0.002 0.001± 0.002- 0.001± 0.001 - 0.002± 0.002

0.003± 0.001 0.002± 0.0020.000± 0.001 - 0.003± 0.002

Proton

Pion

6<|θ |<9°

9<|θ |<15°

2<|θ |<6°

Scattering Angle2<|θ |<6°

Horizontal Asymmetry

6<|θ |<9°

9<|θ |<15°

Instrumental Asymmetry < 0.3%

NNn n / N/ Npp Ratio Ratio

ΓΓnn 　　 (Λ+“ ｎ”→ ｎ + ｎ )

ΓΓpp 　　 (Λ+“ ｐ”→ ｎ + ｐ )

If Γn / Γp = 1 → Nn / Np = 3

If Γn / Γp = 0.5 → Nn / Np = 2

If Γn / Γp = 0 → Nn / Np = 1

Naive estimationNaive estimation(without considering FSI and ΛNN→NNN)

To avoid suffering from FSI effect & ΛNN→NNN,

High energy threshold High energy threshold

Γn:Γp Nn Np

1 : 1 … 3 : 1 1 : 2 … 2 : 1 0 : 1 … 1 : 1

Nn / Np = 2×n / p + 1

Neutron and Proton energy spectra of Neutron and Proton energy spectra of 55He and He and 1212

CC

Nn / Np (E>60MeV)～ 2.00±0.09±0.14

Nn / Np (60<E<110MeV) ～ 2.17±0.15±0.16

5He → n + : Q ～

135MeV (rate : 0.049±0.01-)

135MeV

apply upper energy limit of 110MeV !

apply a simple relation n / p = (Nn / Np - 1) / 2

　 n / p ～ 0.5

To avoid suffering from FSI effect & ΛNN→NNN,

High energy threshold High energy threshold

Corrected proton energy loss inside the target !!

(submitted PLB. nucl-ex/0406020)

inclusive

w/ proton

w/ pion

w/ neutron

w/ gamma

Excitation spectra w/ coincident decay particles for Excitation spectra w/ coincident decay particles for 1212CC

Spin / isospin dependence

p p n Λ

He4Λ

n n p Λ

H4Λ

p p n n Λ

He5Λ

Non-mesonic weak decay of 4He and 4

H

4He (K-,-) 4He or

4He (+,K+) 4He

n+n back-to-back4He (K-,0) 4

H p+n back-to-back (0 spectrometer )

see S.Ajimura : J-PARC LOI 21

To J-PARCTo J-PARC

RNS … N : nnn, pnp S : spin = 0 or 1

nm(4H) = ( 3Rn1+ Rn0 + 2Rp0 ) ×4 / 6

nm(4He) = ( 2Rn0 + 3Rp1 + Rp0 ) ×4 / 6

nm(5He) = ( 3Rn1+ Rn0 + 3Rp1 + Rp0 ) ×5 / 8

Need one-order higher statistics. J-PARC

π+

K+

decay counter

K6/SKS setup

Identification of hypernuclear formation

21

p

M scattered

K+

T1 target

Z-vertexMass

Z

Neutral decay particle IDNeutral 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 5He)

～ 30 times (for 12C)

higher than previous exp.

5MeV< En < 150MeVConstant

background very small

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

(TOF spectra)1 / spectra

Neutral particles from 12C

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 IDCharged decay particle ID

Gaussian fit(±2 cut)

Non-mesonic weak decayNon-mesonic weak decay

strong tenser coupling (L=2, S=2)→ dominant term 3S1→3D1 (amplitude “d”)

+ N→ N + N + p → n + p: p = a2+b2+c2+d2+e2+f2

+ n → n + n : n = a2+b2+f2

OPE : n / p ～ 0.1

Λ N

NN

W S

π

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

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

Theoretical calc.

5ΛHe

w/ FSI

Q/2

-nucleus overlap for 5He

0 / = 0.201±0.011 (He)0 / = 0.201±0.011 (He)

nm/ = 0.395±0.016 (He)nm/ = 0.395±0.016 (He)

Γtotal (Γtotal (5656ΛΛFe) Fe) ～～ Γnm(A→∞) Γnm(A→∞) ～ ～ 1.2Γ1.2ΓΛΛ 　（　（ E307E307 ））

Γnm (Γnm (55ΛΛHe) = 0.4ΓHe) = 0.4ΓΛΛ 　（　（ Present)Present)

1/3 of Λ is inside α 1/3 of Λ is inside α

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

55ΛΛHe (ORG) He (ORG) ～ ～ 40%40%

55ΛΛHe (YNG) He (YNG) ～ ～ 20%20%

1/3 of Λ is inside α 1/3 of Λ is inside α

0 / locates in between ORG and YNG.