Weak decay of 5ΛHe and 12

ΛC : experimental results

RIKEN H. OutaRIKEN H. Outa

10. Oct. 2006 HYP2006(Mainz)

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. Fukudab,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

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 hypernucleus

Study of the mechanism of baryon-baryon weak interaction

1

Contents of the present talkContents of the present talkΓ(Λn→nn)/Γ(Λp→np) ratio 　　 n/p spectra from A=5,12 S. Okada et al. PLB 597 (2004) 249-256 A= 5 B.H. Kang et al. PRL 96 (2006) 062301 A=12 M.J. Kim et al. PLB 641 (2006) 28 [ M. Kim, poster session ] Asymmetry parameter T. Maruta et al. nucl-ex/0509016 [ T.Maruta, parallel session A2] Mesonic & non-mesonic decay widths 　 [ S. Okada, poster session ] S. Kameoka et al. Nucl. Phys. A754 (2005) 173c-177c S. Okada et al. Nucl. Phys. A754 (2005) 178c-183c Two nucleon-induced NMWD (ΛNN→NNN) [H.Bhang, parallel session A2]

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 hypernucleus

2

10 0.5 1.5n / p

The

oret

ical

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

NNW S

π

One Pion Exchange(OPE)

Simple theoretical model

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

3

nn np

pp ppn

n

n p

rescattering

Final state interaction (FSI)

effect

Experimental difficulties in single Experimental difficulties in single nucleon measurementnucleon 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 ”NNnNN” process

NN→NNN(2N-induced process)

ΛN

NNW π

N

N

(One of the theoretical model)

4

np

np

nnp

np

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 )

NMWD : N→NN

* cosθ< － 0.8 * E(N1)+E(N2) cut

n

p

p

n

coinpairnpNcoinpairnnN

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

5

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

6

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

7

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

inclusive

w/ proton

w/ pi±

w/neutron

w/ gamma

previous experiment at BNL

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

S. Kameoka et al. Nucl. Phys. A754 (2005) 173cS. Okada et al. Nucl. Phys. A754 (2005) 178c

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

８

npnp

n

N→nN

nn np

pp ppn

nn p

NN→nNN FSI re-scattering

nn np

pp ppn

nn p

coun

ts

Q/2Energy spectra (image) Energy

distribute low energy region up to Q/2 broad peak

around Q/2 continuousdistribution

Expected Spectrum９

Single proton/neutron spectra from Single proton/neutron spectra from 55ΛΛHe and He and 1212

ΛΛCC

S.Okada et al., PLB 597 (2004) 249

Nn ～ 2Np

Calculation byGarbarino et al.

10

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

11

Coincidence Measurement (A=12)

cos

n + pn + p

n + nn + n

p + pp + p

EEn +n +EEpp

EEn +n +EEnn

EEp +p +EEpp

MeVMeVNNNN

Cou

nts

Cou

nts

12ΛC

npnn / np = 0.51±0.13±0.05Analysis detail on Kim’s Poster

12

10 0.5 1.5n / p

0.93±0.55 (Szymanski et al.) for 5He

Exp

.

Λ N

NNW S

π

One Pion Exchange(OPE)

The

o. 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 (12

C)= 0.51±0.13±0.051212C (E508)C (E508)

Kim et al. PLB641 (2006) 28

13

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 → canceled out !!

R =N(+(-))×N(-(+)) N(+(+))×N(-(-))

1/2

K >0

+ K+

/p

K

K <0

+K+

/pK

P

P

14

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([23

222222 fedcbafdcdcbaeNM

p

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

222222

222 )(2/fedcba

fbapn

(Applying =1/2 rule)

Importance of αnm measurement 15

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

16

Theory: - 0.6 ～ - 0.7

Asymmetry parameter of 5ΛHe

NM=0.08±0.08+0.08 psta

tistic

al

contam

i

-0.00

Nucl.Phys.A754 (2005) 168cnucl-ex/050916

17

Asymmetry parameter of 12C, 11B

NM= － 0.14±0.28+0.18 psta

tistic

al

contam

i

　 -0.00

E160 : － 0.9±0.3

18

Comparison with previous experimentP

ID fu

nctio

nE

nerg

y sp

ectru

m

p

Precious Experiment E508

w/protonw/proton w/pion

range&EtotdE/dx&Etot

TOF&Etot

19

)}(3{41])2(3)2([23

222222 fedcbafdcdcbaeNM

p

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 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

T. Maruta ; parallel A2a Calculation by Itonaga

20

Decay Widths 　→ Okada’s poster21

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 12

ΛC : Γn / Γp ratio = 0.50±0.13±0.05　　　　　　　　　　　　　　　 Kim et al. PLB641 (2006) 28

◆ Asymmetry parameter measured with improved accuracy !!　　　 5

ΛHe :

11ΛB and 12

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

◆ Total & partial decay rates are measured very accurately

NM=0.08±0.08+0.08p

[1] Importance of shorter-range mechanism 　 OPE 　⇒　 Heavy meson & DQ exchange [2] Suggesting significant contribution from ΛN initial spin-singlet initial state σ-meson exch. / ΔI=1/2 violation? ⇒ 4

ΛH & ΛΛ→ΛN@J-PARC

-0.00

NM= － 0.14±0.28+0.18p -0.00

22

Spare OHPs

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

Neutral PIDNeutral PID

Constant background very small

1 / spectra

Neutral particles from 12C

Good n separation

Charged particles from 5He

PID function

Charged PIDCharged PID

Good p d separation

Decay particle identificationDecay particle identification8

expe

rimen

tal

expe

rimen

tal

data

data

theo

retic

al

theo

retic

al

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 coincidencen+p coincidencen+n coincidencep+p coincidence

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

Neutron Efficiency Correction

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

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

Spare OHPsfor asymmetry

E160 E508Statistical comparison with E160

12C event 246 event

11B event 393 event

2779 event

2122 event

×11

×5

Total 639 event 4901 event×8

P-coinspectrum

Comparison with E160PID function Energy spectrum

• Polarization (P), asymmetry parameter(a)

1

14uA

uB

dA

dB

dB

dA

uB

uA

A1A1

)(N)(N)(N)(N)(N)(N)(N)(N

4.03.1a p1

Asymmetryparameter

S.Ajimura et al., PLB282, 293 (1992)

W() = 1 + A1P1(cos) A1 = ka1P

• KEK-PS E278 experiment

Experimental target: 6LiObservable: Mesonic decay branching ratios Polarization

E278 Decay counter

/ p separation by dE/dx and Etot

pHeLi 56

Polarization of 5He

6Li(+, K+) spectrum

PA

A1A1

)(N)(N)(N)(N

S. Ajimura et al., PRL80, 3471 (1998)

Consistent with experimental data

Proton asymmetry parameter of 5He

pp

p

p

pp

pp

PA

A1A1

)(N)(N)(N)(N

6Li(+, K+) spectrum

S. Ajimura et al., Submitted to PRL

Inconsistent with meson exchange model

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

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)

Null asymmetry test(,pX) reaction : Only Strong Interaction

Asymmetry = 0 expectedAsymmetry of

6Li target- 0.000± 0.0020.003± 0.0020.003± 0.002- 0.001± 0.0010.003± 0.0010.000± 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±0.215Asymmetry

6<|θ |<15°

Horizontal Scattering Angle

Nupper/ Nlower

np

np

nnp

np

Coincidence

NMWD

np back-to-back event

NM=0.31±0.22p

One and only(?) solution

+ K + + DQSasaki et al.PRC71 (2005)035502 N

NNW 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

Mijung のOHPから

0034.00113.0

)018.0060.0()326(

a

0038.00156.0

)0204.00833.0()326(

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)()(

bbNbbN

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%

6Li Hypernuclear mass spectra6Li + +→ 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±0.0020.003± 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%

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

pM scattered

K+

T1 target

Z-vertexMass

Z

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

NNW 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 ～ 1n / p ratio puzzle

Simple theoretical model

with large error

-nucleus overlap for 5He

0 / = 0.201±0.011 (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.

ちょっと前の講演のから ..

Mesonic Weak Decay Decay mechanism is known fairly well “How to use it”

Mass number

4 ～ 5

～ 10

50 ～ 100

q ～ 400MeV/cΛN→NN

•Spin/isospin dep.•ΔI=1/2 rule test 　

)( Anm

Mass number dependence

Λ→Nπ•Λ-nucleus potential•Spin/parity assignment 　Pion distortion effect in nuclei

q ～ 100MeV/c

Non-Mesonic Weak Decay (NMWD) Decay mechanism is unknown “What is it ?”

Fermi momentum～ 270MeV/c >>

4

History of hypernuclear History of hypernuclear weak decay experiments… weak decay experiments…

Year

～ 1960

1985

1995

2000 ～ 2002

2004 ～

ΓΓππ__ 　　 (→ ｐ + π － )

ΓΓππ00 　　　　 ( → ｎ + π ０ )

ΓΓpp 　　 ( +“ ｐ”→ ｎ + ｐ )ΓΓnn 　　 ( +“ ｎ”→ ｎ + ｎ )

1/HY =Γtot

Γm

ΓnmNew era started!→ FINUDA@DAFNE / J-PARC

Merit/Demerit

＊ Clean decay identification→ ground state spin assignment＊ Low energy threshold for p＊ Hypernuclear formation is not identified＊ Blind for neutral particles＊ No timing information

＊ Hypernuclear formation tagged　→　 branching ratio !＊ Direct lifetime meas. w/TOF counter

＊ Still hard to see neutral particles＊ High energy threshold for p (Ep>30 ～ 40MeV)

Emulsion andBubble chamber

Method

Counter experimentstart @BNL/KEK w/ (K,π)

SKS experimentsw/ (π,K) reaction

n+p/n+ncoincidence

＊ Heavy Λ hypernuclear production＊ Neutral particle detection＊ Asymmetry of p from NMWD ＊ Improved statistics → n+p/n+n double coincidence

7

Hyperon-nucleus potential Hyperon-nucleus potential

ΛnucleusYN interaction attraction ～ repulsion

Repusive core is the common feature of Y-nucleus potential

18

Spin/isospin dependence

8/RR3 R R3 = He

6/RR3 R2 = He6/R2 R R3 = H

50p1p0nn15

nm

40p1pn04

nm

40p0nn14

nm

Test of =1/2 ruleI0/1=Spin S

nnn npp N RNS

??)He(2)H(rule 1/2I ? 2

44p0n0

np

RR

p p n Λ

He4Λ

n n p Λ

H4Λ

p p n n Λ

He5Λ

NMWD ofNMWD of 4 4ΛΛHe and He and 44

ΛΛHH

×

35

Theoretical approachTheoretical approach

N N

NΛ

Direct Quark (DQ) mechanism

q ～ 400MeV/c 　 (← large!) → short-distance interaction

One Meson Exchange (OME) mechanism

Λ N

NN

π,K,η,ρ,ω,K*

→ large n / p ( ～ 0.4-0.7)

• Kaon exchange model (OME)

dominant term 3S1→1P1 (amplitude “f”)

( range ～ 0.5 fm)

36

Hypernuclear mass spectra on C, Si and Fe target

natC(+,K+) natSi(+,K+) natFe(+,K+)

45

KEK-E307

COSY-13dr

ρΓ 2

Λ0

2N

nm

?

Lifetime of very-heavy hypernuclei ? (J-PARC)

(π,K) K1.1BR

49

By-products of E307 exp.By-products of E307 exp.

1. Mesonic decay widths of midium-heavy Λ hypernuclei

2. Proron energy spectra → 　 Γn/Γp ratio “puzzle”

50

Enhancement of π-mesonic Enhancement of π-mesonic decay widthsdecay widths

Momentum transfer q ～ 100MeV/c << 270MeV/c→ 　“ Forbidden” process in nuclear matter

1. Fermi motion of Λ2. Smaller 　 local Fermi momentum at the surface regi

on3. Pion distortion effect 1) Pion feels attraction in nuclear medium due to the P-wave part of the optical potential 2) Inside the nucleus, pion can carry smaller energy for fixed momentum q

3) Due to the energy conservation, final nucleon has more chance to come out above the Fermi surface

22)( mqq

＜

51

Effects of nuclear shell structure and -nucleus potential

T. Motoba et al. Prog. Theo. Phys. Suppl. No.117, 477 (1994)

H. Noumi et al. PRC52, 2936 (1995)J. J. Szymanski et al. PRC43, 849 (1991)A.Sakaguchi et al. PRC43, 73 (1991)T. Motoba et al., PTP Suppl. No.117, 477 (1994)

Potential dependence of decay rate mesonic decay rates for p-shell hypernuclei

52

Hypernuclear mass spectra on C, Si and Fe target

natC(+,K+) natSi(+,K+) natFe(+,K+)

53

Results of - Mesonic decay width

CL) (90%012.0)(

015.0008.0032.0)(

002.0008.0036.0)(

036.0027.0170.0)(

004.0011.0099.0)(

27

28

11

12

FeB

AlB

SiB

BB

CB

Branching ratio

CL) (90%015.0)(

019.0010.0041.0)(

002.0011.0046.0)(

045.0036.0212.0)(

005.0014.0113.0)(

27

28

11

12

Fe

Al

Si

B

C

- Mesonic decay width

Motoba

Motoba

Y.Sato et al.Recently re-submitted to PRC

54

Gross behavior of hypernuclear -mesonic decay rate

E.Oset et al. Prog. Theo. Phys. Suppl. No.117,461 (1994) T. Motoba et al. Nucl. Phys. A547, 115c (1992)

55

Measurement of π0 Measurement of π0 mesonic decay mesonic decay

33

one gamma ray from “0 2” process was detected.

For observing 0 particle,

Constant backgroundlevel is very low.

Gated ground state of hypernuclei (5He, 12

C)

1 / 1 / ( TOF ) spectrum ( TOF ) spectrum

Neutral particle identificationNeutral particle identification

Layer multiplicity 2

ADC sum 20MeVee

Good separation (between and neutron) 34

Large plastic scintillator arrays were used as detector.

Start timing counter

Charged VETO30

cm

detection system

00 identification identification

Background (low energy): from nuclear decay process

0

EM shower

(~70MeV)

K++

0 emit energetic gamma. (~70MeV) set threshold of ADC sum. The gammacascade in many layers. select high multiplicity event.

To reject the nuclear decay

In these cut conditions, It is hard toestimate gamma detection efficiency. So we simulated with same conditions using GEANT code.

5He

35

Mul 2

Mul 3

Mul 4

Mul 5

Mul 6

Layer multiplicity

ADC sum distribution ADC sum distribution

Mul 1

efficiency estimation using GEANT simulationefficiency estimation using GEANT simulation

* Blue histogram : GEANT simulation * Plot (with error bar) : Experimental data

Nuclear is shown only Mul 1.To remove it completely,we apply Mul 2 andADCsum 20MeVee .

Well agree with Geant simulation.

assuming 0 momentum in GEANT simulation as 5He : 104.9 MeV (mono)

12C : Motoba’s calculation

PTP117(1994)

from 0

20MeV

nuclear

36

00 branching ratio of branching ratio of 55HeHe

Free b / b0 = 1.78±0.03

5Heb / b0 = 1.70±0.08

MeV

Mass spectra for Mass spectra for 66Li(Li(++,K,K++) ) 5He g.s.

quasi free

w/ gamma

Mul 220 MeVee

inclusiveefficiency= 9.59%N (inc)

= 42040

N (w/ )= 848.03

b0 = N (w/ ) / N (inc) × eff = 0.212±0.008

ADC sum w/ Geant simADC sum w/ Geant sim

5He

+ p + + n + 0

Same Q-value as that of free

37

Results of Results of 00

He0 / = 0.201±0.011

C0 / = 0.165±0.008Lifetime : 278+11 ps (E462)

-10 Lifetime : 212+7 ps (E508)

-6

Much improved accuracyOkada et al., HYP03 nucl-ex/0402022

38

Fuse-Kumagai’s recent calculationJPS meeting (2004 Spring)

39

SG: excluded→ 　 Existence of repulsive core!!

40

Henm/ = 0.406±0.020

Cnm / = 0.953±0.032

tot = 0.947±0.038 ΓΛ

bπbπ00 = = 0.212±0.008 bπbπ-- = = 0.359±0.009 bπbπ-- = = 0.099±0.011

bπbπ00 = = 0.133±0.005

tot = 1.242±0.042 ΓΛ

0.41±0.140.41±0.14 0.83±0.090.83±0.09

Non-mesonic deca rate Non-mesonic deca rate 55He and He and 1212

ΛΛCC

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

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

Γnm calculation must use the Λ w.f.Γnm calculation must use the Λ w.f.which can well reproduce Γπwhich can well reproduce Γπ

41

Mass number dependence of ΓMass number dependence of ΓNMNM

42