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Decay asymmetry in non-mesonic weak decay of light L hypernuclei

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### Decay asymmetry in non-mesonic weak decay of lightLhypernuclei

Oct./13/2006 [email protected] Univ.

T.Maruta

KEK,

JSPS fellow

KEK-PS E462/E508 collaborations

1.05GeV/cp+

accepted

P

L

p/p

E278

θ

K+

p+

K+ scattering angle(jK)

jK

1/2

N(q+(+j))×N(q-(-j))

R =

Inlarge scattering angle,

Lis much polarized.

N(q+(-j))×N(q-(+j))

jK <0

jK

p+

K+

θ

p/p

P

L

Asymmetry measurement of decay protonAsymmetry : Volume of the asymmetric emission from NMWD

N(q) = N0(1 +Acosq)

jK >0

Asymmetry

=N0(1 +aPcosq)

L

Asymmetry

parameter

(R + 1)

N(q+)

A =

R =

,

(R - 1)

N(q-)

Difference of acceptance & efficiency is canceled out !

Importance ofaNM measurement

p

If assuming initial S state

p : d

K : f

We can know the Interference between different

Isospin and Parity states.

Motivation

Present status

Previous

experiments

Theoretical

prediction

5He : 0.24±0.22

L

-0.6～-0.7

12C,11B : -0.9±0.3

OME Ex.

p+K+DQ Ex. etc.

L

L

Ajimura et al.

The aim of E462/E508 experiment

Precise measurement of Asymmetry parameter

- high statistics
- Reduction of Statistical error.
- p-n coincidence measurement
- Lp→np
- measurements of 5He and 12C,11B in same setup
- possible to compare them in low systematic error

L

L

L

6Li → 5He+p

L

Excitation spectra w/ coincident decay particles for 6Li

L

p++6Li → K++6Li

L

PID spectrum

inclusive

4.0×104

w/ proton

w/ pion

pcontamination in p-gate

Systematic error

Null asymmetry test

(p, pX)reaction : Only Strong Interaction

p or p

Asymmetry = 0expected

Instrumental Asymmetry<0.3%

Ap: Asymmetry ofp

ap: Asymmetry Parameter of mesonic decay

(=-0.642±0.013)

PL: Polarization of L

h: Attenuation factor (Monte-Carlo simulation)

We can obtainaNMwithout theoretical help.

p

Procedure for aNM calculation (5He)p

L

Estimated from mesonic decay

・Polarization ofL

M

・Asymmetry Parameter of Proton

p

Ap = aNMhPL

LB +p

Excitation spectra w/ coincident decay particles for 12CL

Level scheme of 12C

L

5.8×104

inclusive

N,a decay

LH

p decay

11

C +L

w/ proton

LB

8.1MeV

11

6.3MeV

w/ pion

gdecay

2.5MeV

depolarized

effect

0.0MeV

LC

12

15o

Polarization of L (12C,11B)L

L

Itonaga et al.

Prog. of Theo. Phys. Supp. 117(1994)14

If assuming polarization is

proportional to scattering angle.

M1 transition

reduces

Polarization ofL

Theoretical models such as p+K reproduce Gn/Gp ratio, but predict large negative aNM.

p+K+DQ

p

- p+K+s+DQ model reproduces both Gn/Gp ratio and aNM.

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 DI=1/2

rule considered

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

Itonaga et al.

OPE

Comparison with recent resultsp-shell

Summary

- A series of experiments, E462 (5He) and E508 (12C, 11B) were carried out to measure aNM precisely.
- We obtained nearly zero aNM’s (0.08±0.08+0.08, -0.14±0.28+0.18) in both nuclei. We don’t observe the difference in both reaction mechanism.
- Recent theoretical calculation indicates the contribution of the initial state of 1S0. For more check, measurement of 4H is required at J-PARC.

L

L

L

p

-0.00

-0.00

L

6Li(K-,p-)6Li

L

Excitation spectra w/ coincident decay particles for 6Li

L

6Li (6Li→5He+p)

PID spectrum

L

L

L

inclusive

4.0×104

w/ proton

w/ pion

pcontamination in p-gate

Systematic error

(with FSI effect)

Ep 30MeV -0.46

50MeV -0.52

70MeV -0.55

0.05±0.14+0.08

0.15±0.10+0.09

0.07±0.09+0.09

Proton Energy Dependence (5He)L

Q/2

- Low energy region
- Contamination of
- FSI
- 2N-induced

Neutron

Proton

Threshold

(Counts/5MeV)

th

-0.00

PRL94 (2005) 082501

-0.00

-0.00

5Heの結果におけるFSI等の効果は小さいと考えられる

Production of polarized hypernuclei

1.05GeV/cp+beam

is injected.

E462/E508 experiments

Distribution ofLpolarization

in then(p+,K+)Lreaction.

1.05GeV/c p+

P

L

p/p

θ

K+

jK

p+

E278

K+ scattering angle(jK)

Inlarge scattering angle,Lis much polarized.

N

S

W

N

p,K,s

L

One solution- + K + s + DQ

Sasaki et al.

PRC71 (2005)035502

- b(1S0→3P0)とf(3S0→3P1) amplitudeに影響を与える
- DI = 3/2が大きく寄与する
- 今回Gn/Gp ratioとaNM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

Importance of asymmetry measurement

If assuming initial S state

(ApplyingDI=1/2 rule)

We can know the Interference between states with

different Isospin and Parity .

Decay counter Setup

π

p

n

n

K

p

(KEK-PS K6 & SKS)

Decay arm

Solid angle: 26%

9(T)+9(B)+8(S)%

polarization

axis

Charged particle：

・TOF (T2→T3)

・tracking（PDC）

Neutral particle：

・TOF (target→NT)

・T3 VETO

N: 20cm×100cm×5cm

T3: 10cm×100cm×2cm

T2: 4cm×16cm×0.6cm

Charged particle identification

E VS. dE/dx

PID1

T2 energy loss (MeVee)

E VS. TOF

TOF (1/b)

Total E

PID2

d

TOF

p

PID2

p

e

dE/dx

PID1

Total E (MeVee)

p

Comparison with E160E160

E508

dE/dx&Etot

TOF&Etot

range&Etot

PID function

w/proton

w/pion

w/proton

Energy spectrum

Proton asymmetry of 12C and 11B

L

L

Excitation spectrum w/proton

KEK E369 data

Excitation spectrum of 12C is well

known from previous experiment

eliminate the contamination

such as QF event inside the gate

Asymmetry parameter of mesonic decay

L → p- + p

1/2

s

0

1/2

l =0,1

wave function:

-sinq

cosq

1

Angular distribution of decay proton

Asymmetry parameter

S : 88%、P : 12%

a = 0.64

N

S

W

N

p,K,s

L

aNM

p

One (Sasaki et al.) solutionp Ex.+ K Ex. + s Ex+D.Q.

Sasaki et al.

PRC71 (2005)035502

Experimental

value

Gn/Gp ratio

weak coupling constant ofs meson

Both Gn/Gp and aNM are consistent with experimental value.

p

s ～340psec

p

6000

4000

K

2000

0

-3

-2

-1

1

2

3

0

(ns)

K+

p+

T1

target

Identification of hypernuclear formationGood p/K/p separation

Good p/K separation

0

200

400

600

800

1000

1200

(MeV/c2)

-800

-600

-400

-200

200

0

(mm)

Neutron spectrum for 5He

Theory :

Garbarino et al.

PRC69 (2004)

054603

Theoretical calc.

Q / 2 = 76MeV

No peakingat half of

Q-value (76MeV)even 5ΛHe

suggested larger contribution of

LNN→NNN or FSI than theoretical prediction.

np- & nn- angular distribution (5ΛHe)

Back-to-back

Back-to-back

Gn/Gp ~ Nnn/ Nnp= 0.45±0.11±0.03

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

Coincidence Measurement (A=12)

En +Ep

n + p

Counts

En +En

12ΛC

n + n

Ep +Ep

p + p

qNN

cos

MeV

Gn/Gp ~ Nnn/ Nnp= 0.51±0.13±0.05

Analysis detail on Kim’s Poster

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