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Non- Mesonic Weak Decay Λ Hypernuclei - Results at KEK-PS and open problems at J-PARCPowerPoint Presentation

Non- Mesonic Weak Decay Λ Hypernuclei - Results at KEK-PS and open problems at J-PARC

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Non- Mesonic Weak Decay Λ Hypernuclei - Results at KEK-PS and open problems at J-PARC

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Weak decay mode of Lhypernucleus

Γπ_(L→ ｐ + π－)

Γπ0(L → ｎ + π０ )

Mesonic

q～100MeV/c

Γm

1/tHY =Γtot

Γp(L +“ｐ”→ ｎ + ｐ)

Γn(L +“ｎ”→ ｎ + ｎ)

Γ2N (ΛNN →NNN)

Non-Mesonic(NMWD)

q～400MeV/c

Γnm

Workshop on Strangeness Nuclear Physics(SNP2013), Xiamen Dec. 13, 2013

RIKEN H. Outa

Non-Mesonic Weak Decay ΛHypernuclei- Results at KEK-PS and open problems at J-PARC

Study of the mechanism ofbaryon-baryon weak interaction

1. Results of NMWD experiments at KEK-PS

2. Λ Hypernuclear weak decay experiments at J-PARC

E18 and E22 experiments

E18: ΛNN → NNN for 12ΛC

E21: Γ（Λn→nn)/Γ(Λp→np) for 4ΛHe

1

If assuming

initial S state

OPE:

ΔS=2, ΔL=2

3S1 → 3D1

(L+S+T=odd; T=0)

Λn→nn

is suppressed

(Applying DI=1/2 rule)

We can know the interference between states with

differentIsospin and Parity .

２

Direct Quark

mechanism

Meson Exchange

mechanism

One Pion Exchange

(OPE)

N

N

Theo.

N

Λ

N

N

π,K,η,ρ,ω…

S

W

π

N

Λ

Gn / Gp

1.5

0.5

1

0

N

N

5LHe (E462)

Nnn / Nnp (5LHe)= 0.45±0.11±0.03

Kang et al. PRL 96 (2006) 062301

12LC (E508)

Γn / Γp (12LC)= 0.51±0.13±0.05

Λ

N

Kim et al. PLB641 (2006) 28

Gn/ Gp ratio

ΔS=2,ΔL=2 ； 3S1 → 3D1 (L+S+T=odd; T=0)

OPE: Λn→nn is suppressed

Previous exp. (at BNL)

0.93±0.55 (Szymanski et al.) for 5LHe

Exp.

３

LN→nN

LNN→nNN

FSI

re-scattering

n

n

n

n

n

n

n

n

p

p

p

p

p

p

p

p

p

p

p

p

p

p

n

n

n

n

n

n

n

distribute low energy

region up to Q/2

broad peak

around Q/2

continuous

distribution

counts

Energy

Energy spectra (image)

Q/2

Coincidence

Select ΛN→NN events

w/o FSI effect & ΛNN→NNN.

NMWD

L

NMWD

Coincidence

* cosθ<－0.8

* E(N1)+E(N2) cut

p

p

p

p

p

p

n

n

n

n

n

n

n

n

KEK-PS E462/E508

Direct measurement

of the Gn/ Gp ratio

NMWD : ΛN→NN

1) Angular correlation

( back-to-back, cosq<-0.8 )

2) Energy correlation

( Q～E(N1)+E(N2) ～152MeV )

Select light hypernuclei to minimize FSI effect, 5LHeand12LC

５

６

Charged PID

Charged particles from 5LHe

Constant background

very small

PID function

Decay particle identification @E462/E508

Neutral particles from 12LC

Sensitive to all the decay modes

1 / b spectrum

Good p p d separation

Good g n separation

7

Excitation spectra w/ coincident decay particles for 12LC

12LC

12LC

12LC

(in-flight K- ,π-)

Phys.Rev.C43 (1991) 849

(stopped K-,π-)

(π+,K+)

８

Lifetime & Decay Widths

?

Lifetime of very-heavy hypernuclei ?

COSY-13

KEK-E307

９

Coincidence Measurement(E462/E508)

nn-pairs

np-pairs

10

Ap=apPLe

Ap:Asymmetry of Pion

ap:Asymmetry Parameter of Pion

(=－0.642±0.013)

PL:Polarization of Lambda

e :Attenuation factor

We can directly measure aNM

p

Estimated from mesonic decay

・Polarization ofL

α

Λ

・Asymmetry Parameter of Proton

Ap=aNMPLe

p

11

Theory: －0.6～0.7

pcontami

statistical

aNM=0.08±0.08+0.08

-0.00

p

11

12

p+K,OME can reproduce

Gn/Gp ratio but predict

large negative aNM

p+2p/r+2p/s+w+K+rp/a1

Calculation

by Itonaga

p+K+DQ

Gn/Gpand aNM can be

reproduced by

p+K+s+DQ model

p+K+s

OME

p+K+s+DQ

Sasaki et al.

PRC71 (2005)035502

p+K

(1) Large b(1S0→3P0) and

f(3S0→3P1) amplitude

(2) Violation of ΔI=1/2

rule considered

OPE

NMWD of Λ-hypernuclei- achievement and open problems -

N

N

N

Λ

π,K,η,ρ,ω…

N

N

N

N

N

W

π

Λ

Λ

N

N

N

✔ Gn/ Gp～0.5

Meson Exchange

mechanism

Direct Quark

mechanism

× OPE

✔ Heavy meson exch./DQ

Spin/isospin dependence ??

✔ αNM～0

×Hard to be explained w/ large Gn / Gp

✔ Some cancellation mechanism

LNN→NNN

(2N-induced process)

Large contribution of 1S0 ΛN initial state??

✔ G２Ｎ/ Gnmwd～0.3 ?

E18

✔ Gnm (A～56）～1.2ΓΛ

15

n

n

p

n

W

p

π-

Λ

n

p

4

Λn→nn like

n

p

n

p

n

n

W

π0

Λ

1

n

p

n

Λp→np like

ΛNN→NNN consideration

Larger contribution

in n+n-pair ?

Seen as….

n

p

n

Theoretical prediction of 3-body process (Γ2N) of NMWD.

•First proposed by Alberico-Ericson for Nuc. Matter

(‘91) and Ramos-Oset extended to finite nuclei (‘94).

• Coupling of virtual π- to 2p-2h correlations.

- Λpπ- is dominant at the weak vertex and

- π- absorbed dominantly on the pn pair.

In the process 3 nucleons are emitted;

1p(LE) + 2n (HE)

• Γ2N/Γnm= 0.20 (12ΛC)

• Bauer and Garbarino; Recently, extended the

model to include K, , mesons and

other nucleon pairs.

Γ2N/Γnm→ 0.37 (12ΛC)

[E. Bauer, plb 698 (‘11)]

n

p

n

n

p

n

Extended to K, ρ,ω. .

16

Nn+Npand(Nnn+Nnp) back-to-back yield

✔ When we summed up Nnn+Nnp (back-to-back) and Nn+Np

the spectra becomes free from Γn/Γp ratio

✔ Both of Nnn+Nnp and Nn+Np yields are smaller than

those of INC calculation withonly ΛN→NN process (1N)

✔ΛNN → NNN decay is assumed to occur uniformly in

three-body phase space.

✔ Good agreement obtained when we assume

Γ2N/Γnmwd = 0.29±0.13 M. Kim et al., PRL103 (2009) 182502

N-N pair number

distribution

Nucleon number

distribution

17

11

Hypernuclear decay experiments

at J-PARC

OPEN PROBLEMS :

1) Large ΛNN→NNN contribution

2) 1S0 initial state contribution; ΔI=1/2rule

4ΛHe & 4ΛH : np-ratio

(E22 experiment at J-PARC)

ΛΛ→ΛN decay of double Λ hypernuclei

18

E18 Setup

T4

p+

Side Veto

T3

PDC

T2

K+

y

20cm

x

z

19

Comparison of acceptance

enn

enp

np

nn

E18

E18

n

qnp

E508

E508

cosqnp

cosqnn

eNN

epp

E18

pp

Double coincidence eff.

at EN=75MeV

→ Much increased!!

E18

pp

np

beam

p

E508

nn

cosqpp

cosqNN

20

Statistics of the Two-step Plan

Λp→np

Λn→nn

FSI

ΛNN→NNN

& FSI

@20% stat. ✔ Much improved statistics in non b-t-b region

✔ With the similar analysis as E508, can establish

the existence of ΛNN→NNN with ～4σ-level

@100% stat. ✔ Dalitz plot analysis for NNN-final state

✔ FSI-free(or less affected) analysis can be tried

✔ 10% error can be reached for both of Γn/Γp & Γ2N/ΓNM

✔ Also all the mesonic/non-mesonic widths will

be updated with much improved accuracy.

21

- NMWD of 4-, 5-body hypernuclei
- allowed initial LN states

p p n Λ

p p n n Λ

n n p Λ

0+

0+

0+

0+

Ln→nn:1S0, 3S11S01S0, 3S1

Lp→np:1S01S0, 3S11S0, 3S1

22

assuming initial S state

Our prospects

new constraint from 4LHe

np-ratio better than 15% error

Current status

Constraint from 5LHe data

other constraints are loose

J-PARC E21

23

Indication of Γ2N in 4ΛHe decay ??

26

PRC 76,035511 (2007)

EN1+EN2

Opening angle

p+n

p+n

n+n

n+n

?

p p n Λ

J-PARC E21 proposed by Osaka/RCNP

0+

0+

◆Γn/Γpratio

5ΛHe, 12ΛC ～0.5

◆ Asymmetry parameter

5ΛHe / 11ΛB and 12ΛC ～0

◆ Lifetime

Heavy hypernucleiΓtotal～1.2ΓΛΓnm(A→∞)

◆ ΛNN→NNN process

Γ2N/Γnmwd= 0.29±0.13

Suggesting possible large contribution of

spin-singlet initial state & ΛNN→NNN

[1] High statistics/wide acceptance run of 12ΛC at J-PARC (E18)

－ NN non-back-to-back & NNN coincidence ： Stage-2 approved

[2] Spin/isospin dependence of NMWD : np-ratio for 4ΛHe (E21)

－Stage-1 approved

[3] Lifetime measurement of very heavy hypernuclei

σ-meson exch. / ΔI=1/2 violation? ⇒ 4ΛH 4ΛHe & ΛΛ→ΛN@J-PARC

６

p

p

n

n

Rare NMWD of 4ΛHe

(Bｒ～1/1300)

4He(K－,d)

Inclusive

?

～105 events

～1% of

stopped K

% of stopped K－/(10MeV/c)

(1) Ｐｄ>500 MeV/c > 2×PFermi

Hard to explain the origin of

such high momentum deuterons

(2)Observation of back-to-back dd pair from

stopped 4ΛHe → d + d (Ｐｄ = 571.8MeV/c )

800

600

700

500

MeV/c

4He(K－,dd)

Back-to-Back

Coincidence

d

!!

30

cosθ < -0.9995

20

d

Counts / 0.0005

Counts / (10MeV/c)

10

0

ー0.96

ー0.92

ー0.9

ー1

ー0.94

ー0.98

500

600

700

cosθ(dd)

MeV/c

Oka’sTheorem for NMWD

Nucl.Phys. A754 (2005) 117-126

Our strategy to control FSI effect

✔On most of the observables we are not “free” from FSI effect.

But its effect is NOT the major part of error source in E508.

We have already achieved the control of strength of FSI

in 4% level.

✔ KEK-PS E508 setup is NOT optimized for non back-to-back

coincidence measurement where TN-NWWD & FSI becomes

dominant. Measurement of (1) 3N Dalitz plot and (2)Nnn/Nnp/Npp

measurement in whole cosθNN region and also for (3)singles

proton/nucleon spectra/NMWD will give good control data

for FSI effect

✔We are now seriously considering the method of analysis free

from (or less affected by) FSI.

By artificially changing FSI strength in SNU cascade simulation,

Nnn/Nnp ratio analysis coupled with precise Γn/Γpratio

measurement seems to be promising.

✔ To measure the TN-NMWD with different mass number is the

way to check FSI. Most of E18 member also joins the proposal

E21 for 4ΛHe, which will take sufficient data for TN-NMWD at A=4

15

Γπ/Γnm and Λ-Nucleus Potential

Mesonic decay rate

Γπ(YNG) > Γπ(ORG)

Non-mesonic decay rate

Γnm(ORG) >Γnm(YNG)

Calculation using two types of a-L potential,

ORG and YNG(Motoba et al. NPA577)

Central repulsion in a-L potential ?

Can be directly checked by the mesonic decay width!

12

Mesonic decay widths??

＊ Important to study the NMWD widths

Br(NMWD) = 1 － Br(π－) － Br(π0)

Γnm= 1/τ× Br(NMWD)

σ=119ps

ADC sum w/ Geantsim

20 MeVee

5LHe g.s.

quasi free

inclusive

N (inc)

= 42040

w/ gamma

bp0 = N (w/ g) / N (inc) × eff

= 0.212±0.008

N (w/ g)

= 848

- = 217 ± 6 ps
for 12ΛC

13

14

Gp- and Gp 0 for 5LHe

GL

GL

Gp0

Gp-

0.207±0.013 GL

Isle

SG

Isle

0.322±0.018 GL

SG

Measured with much

improved accuracy

Λ-nucleus potential

with inner repulsive core

can reproduce present

experimental results

Γ2N of 11ΛB (preliminary)

Nnn+Nnp (≤300MeV/c)

- Fitting with the known 12ΛC and 11ΛB structure information.
- Side band subtraction of Continuum contribution.

Γ2N/Γnm (%)

18

Neutron random background;

- E508 Neutral particle TOF spectrum

Beam rate at E508

～ 5×106 π+/spill

= 3Mπ+/sec

(thanks to the good

duty factor of 0.8～0.9)

n singles

~2% nn background @E508

… This number need to be controlled less than 8% in order to obtain n+n results in non b-t-b region. 2% → 4% by Ωnbut some room for higher beam rate

if D.F.=1

nn pair

Neutron background level

is beam-line dependent

… need to be checked with beam

23

Nn+Npand(Nnn+Nnp) back-to-back yield

✔ When we summed up Nnn+Nnp (back-to-back) and Nn+Np

the spectra becomes free from Γn/Γp ratio

✔ Both of Nnn+Nnp and Nn+Np yields are smaller than

those of INC calculation withonly ΛN→NN process (1N)

✔ ΛNN → NNN decay is assumed to occur uniformly in

three-body phase space.

✔ Good agreement obtained when we assume

Γ2N/Γnmwd = 0.29±0.13

N-N pair number

distribution

Nucleon number

distribution

19

7

N+N momentum sum analysis

INC

N1

•E508 Exp.

p12

PRL103(2009) 182502

N3

N2

*INC(IntraNuclear Cascade)

- -- - 1N

ーー 2N

● E508

- -- - 1N

ーー 2N

● E508

3. Comments on FSI

ー“Even though a triple coincidence is detected, there is no clear way to separate TN-NMWD from a NMWD (LN-NN) with final state interactions (FSI). One will have to rely on the inter-nuclear cascade (INC) calculations or equivalent procedures, which may limit the precision that can be reached to that of the previous KEK-PS experiment.

Meanwhile FINUDAshowed a new result based on mass number dependence where TN-NMWD was derived in a way related directly to the experimental spectra.” (PAC2011Jan. )

INC strength effect control data

Inelastic scattering(p,p’)

Charge exchange reaction(p,n)

Confirmed the validity of the INC parameters for the FSI over the nuceon energy region of 45-160MeV

and the mass region of A=12-90 within 4% accuracy for its strength

16

Ｓｔａｔｉｓｔｉｃａｌ error

～10%

>> Systematic error

～ 3%

at least at E508

17

1２

Our prospects

new constraint from 4LHe

np-ratio better than 15% error

Current status

Constraint from 5LHe data

other constraints are loose

J-PARC E22

FSI independent observables to extract G2N

In the left figure the E508 data is compared with those of INC whose FSI istoo weak. However in the ratio Nnn/Nnp, the weakness of FSI is

cancelled out so that the predicted branching ratio is about 0.2 but with a large error. We need to reduce this error bar in the main run.

FSI strength renormalization factor a determination

a = 1.09±0.04

3

K6/SKS setup

K+

decay

counter

π+

INC calculation

p

p

n

Flow Chart

Target nucleus

At first, we develop scattering and

reaction process to check its range

of validity by comparing experimental

data

* included important features

1)Pauli-effect and Fermi momentum

2)Woods-saxon type density

fix the strength of FSI on the nucleons. Then, applied in the calculation of decay particle propagation

Applying INC to a Λ hypernuclei decay process

1N- induced NMWD process:

Λ+N N + N + 176MeV

according to the momentum distribution of 1s –state in the harmonic oscillation potential

Primary energy dis. from 1N

Initial

Count/MeV

Count/MeV/c

EN1[MeV]

Count/MeV

PL[MeV/c]

PN[MeV/c]

(in the nucleus)

EN2[MeV]

1N-induced NMWD process INC results

proton

neutron

before

FSI

Counts/10MeV

Counts/10MeV

Λp np

after FSI

Ep[MeV]

En[MeV]

proton

neutron

Counts/10MeV

Counts/10MeV

Λn nn

Ep[MeV]

En[MeV]

After FSI suffering, recorded at outside of nucleus

2N-induced NMWD process

Λ+N+N N+N+N+176MeV

In a 2n NMWD process, three nucleons were produced at a point interaction site with the kinematics of uniform phase space sharing

LNN→nNN(Using Dalitz Plot)-uniform phase space sharing

proton

Counts/10MeV

neutron

EN[MeV]

EN[MeV]

energy dis. after FSI including, outside nucleus

Primary energy dis. in the nucleus before FSI

Comparison of INC results and Data

Inelastic scattering(p,p’)

Charge exchange reaction(p,n)

Confirmed the validity of the INC parameters for the FSI over the nuceon energy region of 45-160MeV

and the mass region of A=12-90

Signature of the 3-body process

-- Measured results are compared with those of INC(only 1N process)

• The quenching of the yields are universal for both proton and neutron and accordingly also for np,nn pair and momentum sum

Renormalized INC method

Г2N (ΛNN nNN)

INC reproductions including the 3-body process

INC

• E508 Exp.

G2N= 0.29±0.13Gnm