P10-2: Exclusive Study on the N Weak Interaction in A=4 -Hypernuclei

(update from P10)

S. Ajimura (Osaka Univ.)

Osaka-U, KEK, OsakaEC-U, RIKEN, Seoul-U, JAEA, Torino

Spokespersons: A. Sakaguch, S. Ajimura (Osaka Univ.)

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Subjects of this proposal

• Properties of N weak interaction– study on non-mesonic weak decay (NMWD) i

n hypernuclei N weak interaction• spin/isospin structure• parity information

– measurement of np-ratio (n/p) of 4He

n→nn, p→np

• Studies toward test of “I=1/2 rule”– “I=1/2 rule” valid or not in NMWD

– Study on A=4 hypernuclei (4He and 4

H)

– 1st step for the study

determination of determination of partial decay amplitudespartial decay amplitudes

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Weak decays in -Hypernuclei

N Mesonic weak decayMesonic weak decay (MWDMWD)

similar with free decayspin/isospin structure well known

NNN

N

Non-Mesonic weak decayNon-Mesonic weak decay (NMWDNMWD) new decay modes

spin/isospin structure: unknown

NNN

Nnnnnp,p

3S1/1S0

I=0 or 1

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Status of NMWD studies

• Old puzzle solved recently– np-ratio (nnn/p pnn/p) inconsistent

– Experimental and theoretical improvements

– (Exp.) Back-to-back coincidence for final two nucleons (E462/508)

)TheoryandExp.(5.0/ pn

)Theory(0/)Exp.(1/ pnpn

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 pin reality

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• A new puzzle arises– Decay asymmetry inconsistent

)Theory(7.0NMp)Exp.(0NM

pS

HY

p

n

Large contribution ?

Asymmetry written by amplitudes 1

31

3 SS 13

01 SS

assuming initial S state

initial final amplitude isospin parity1S0

1S0a 1 no

3P0b 1 yes

3S11S1

c 0 no3D1

d 0 no

1P1e 0 yes

3P1f 1 yes

)1(01 IS

)0(13 IS

)1(13 IS

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• NMWD of 4-, 5-body hypernuclei– allowed initial N states

assuming initial S state

initial final amplitude isospin parity1S0

1S0a 1 no

3P0b 1 yes

3S11S1

c 0 no3D1

d 0 no

1P1e 0 yes

3P1f 1 yes

)1(01 IS

)0(13 IS

)1(13 IS

p p n Λ

)(0 He4Λ

n n p Λ

)(0 H4Λ

p p n n Λ

He5Λ

0+ 0+ 0+ 0+

n→nn: 1S0, 3S11S0

1S0, 3S1

p→np: 1S01S0, 3S1

1S0, 3S1

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Status of amplitude determination

Current statusCurrent status constraint from 5

He data

other constraints are loose

Our prospectsOur prospects new constraint from 4

He

np-ratio better than 15% error

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Production of 4He

High intensity High intensity beam beam

~10M ~10M ++/spill/spill

(not limited by primary beam)(not limited by primary beam)

High resolutionHigh resolution

Efficient K+ detectionEfficient K+ detection

Use 4He(+,K+)4He reaction

19,000 4He/day

500,000 4He in 4 weeks

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Energy resolution– K1.8 bemline + SKS excellent resolution

• Liquid 4He 2 g/cm2 Ex ~ 2 MeV

• BE(4He) = 2.42 0.04 MeV

– Separation from QF production essential

p+

+A nnX, npX

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Decay arm system– Large acceptance and high efficiency for NN

– Good PID capability (n/p//)

%30

4.0

n

n

%80

25.0

p

p

n n

p p

n/ TOFp/ E/E/rangen/p charge-veto

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Yield estimation

– 19,000 4He/day 500,000 4

He in 4 weeks

– 1,300 pnp and 75 nnn in 4 weeks

high beam intensityhigh beam intensity

large acceptancelarge acceptance

large acceptance large acceptance and high efficiencyand high efficiency

in case of1% BR

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Background estimation

• Background sources– QF -production (p+, +AnnX)

• cut in Ex spectrum

– Mesonic weak decay of hypernuclei• 4

He3He+p+, +AnnX

• 0.3 n 0.01

• Reduction of background– veto: no track in CDC– less material at target

• LHe target 2 g/cm2

• range() 5 g/cm2

range2 g/cm2

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Background MC simulation

• Simulation of worst case– 1/5 of stop in material around target

1/5 - ~ 0.06 ⇔ n ~ 0.01

– GEANT4 base simulation

Li C

R: Reduction factor (En1+En2>50MeV)

R~1/110 R~1/30

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Time schedule– Ready in 2009– Collaboration with E05 and E15

K1.8 and K1.8 and SKS ReadySKS Ready

Decay arm Decay arm ReadyReady

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Summary of proposal

• We propose to measure the nonmesonic weak decay of 4

He.

– select initial spin state (1S0/3S1)

– first step to check the validity of I=1/2 rule

• 1300 np-decay and 75 nn-decay are expected in 4 weeks if B.R.(nn)=1%.

• Main background, absorption, will not affect the measurement

• Experiment will be ready in FY 2009.

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Backup Slides

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S=1 weak interaction

• Quark description

• Case of free decay

S = 1S = 1

II 00

N S = 1S = 1

II =1/2=1/2

ororII =3/2 =3/2

S= -1S= -1II=0=0

S=0S=0II=1/2=1/2

S=0S=0II=1=1

/=2/=0.5

Exp./=1.78 “I=1/2 rule”

s

u d

W

u

p/s = 0.38 p/s = 0.38

isospin and parity structure well known

PP

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– (Exp.) Overestimation of nnn decay• Assumption of clean NnN process

C12 He5

Old puzzleOld puzzle

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 pin reality

)Theory(0/

)Exp.(1/

pn

pn

20

NnN coincidence necessary

– Back-to-back emission of n+N– Sum of n+N kinetic energies

MeVcmmTT NNn 1702

Nn pp

Back-to-backBack-to-back Back-to-backBack-to-back

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New puzzle in asymmetry

NM=0.080.08+0.08 p-0.00

Large contribution ?

)Theory(7.0~

)Exp.(0~

NMp

NMp

Asymmetry written by amplitudes 1

31

3 SS 13

01 SS

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Partial decay amplitudes

• Block and Dalitz treatment– Initial S-wave (s-shell hypernuclei)– isospin=0 or 1 in final states– Represent spin/isospin and parity structure

)1(01 IS

)0(13 IS

)1(13 IS

spinspin isospinisospin parityparity

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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

Expected Spectrum

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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) 168cnucl-ex/050916

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Asymmetry parameter of 12C, 11B

NM= － 0.14±0.28+0.18 pst

atis

tical

conta

mi

　 -0.00

E160 : － 0.9±0.3

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Yield of 4He with (K-,-) reaction

– 33,000 4He/day: about 2 of (+,K+)

full beam (30GeV)full beam (30GeV)

SPES2SPES2

150

,,

KK

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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

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700,000

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High Intensity and High Resolution beamline

– handle 109 /spill– p/p ~ 10-4

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High Intensity and High Resolution beamline(new configuration)

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Others

33

Super Kaon Spectrometer (SKS)

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Production cross section– 4

He(g.s., 0+) production

– estimation with DWIA by T. Harada

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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 identification

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Nn-pair detection efficiency

– About 1.5 in back-to-back region

averageaverage averageaverage

back-to-backback-to-backregionregion

back-to-backback-to-backregionregion

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