Decay asymmetry in non-mesonic weak decay of light hypernuclei

T.MarutaKEK,

JSPS fellow

Oct./13/2006 HYP2006@Mainz Univ.

KEK-PS E462/E508 collaborations

2

Asymmetry measurement of decay proton

Asymmetry : Volume of the asymmetric emission from NMWD

Difference of acceptance & efficiency is canceled out !

K >0

+ K+

/p

K

θ

P

K <0

+K+

/p

K

θ

P

N(θ) = N0(1 + Acosθ)

Asymmetry

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

R =N(θ-) N(θ+)

,

Asymmetryparameter

= N0(1 + Pcosθ)

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

polarization in n(+,K+)

K+ scattering angle(K)

1.05GeV/c +

E278

In large scattering angle, is much polarized.

accepted

3

If assuming initial S state

We can know the Interference between differentIsospin and Parity states.

Importance of NM measurement

StateParity Isospin

Amplitude

Initial Final Ifz = 0 If

z = -1

1S0

1S0 PC 13P0 PV 1

3S1

3S1 PC 0 ー3D1 PC 0 ー1P1 PV 0 ー3P1 PV 1

: d K : f

p

p

p

p

p

p

p

a

b

c

d

e

f

n

n

n

a

b

f

2 2 2 2 2 2

2( 3[ ] [ ] 2[ ] 6[ ] 3[ ])

| | | | 3(| | | | | | | | )p p p p p p p p p pNM

pp p p p p p

a e b c b d c f d f

a b c d e f

4

Motivation

Theoreticalprediction

-0.6 ～ -0.7

Previousexperiments

5He : 0.24±0.22

The aim of E462/E508 experiment

Ajimura et al.

Precise measurement of Asymmetry parameter high statistics

Reduction of Statistical error. p-n coincidence measurement

p→np measurements of 5He and 12C,11B in same setup 　

possible to compare them in low systematic error　

Present status

12C,11B : -0.9±0.3

OME Ex.+K+DQ Ex. etc.

5

Excitation spectra w/ coincident decay particles for 6Li

inclusive

w/ proton

w/ pion

4.0×104

contamination in p-gate Systematic error

PID spectrum6Li → 5He+p

++6Li → K++6Li

6

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

7

Procedure for NM calculation (5He)

A= P

A: Asymmetry of : Asymmetry Parameter of mesonic decay (=-0.642±0.013)PPolarization of : Attenuation factor (Monte-Carlo simulation)

・ Polarization of

・ Asymmetry Parameter of Proton

Ap = NMPp

p

Estimated from mesonic decay

We can obtain NM without theoretical help.p

M

8

Theory: - 0.6 ～ - 0.7

Asymmetry parameter of 5He

NM=0.08±0.08+0.08 pst

atis

tical

conta

mi

-0.00

5He

9

np coincidence analysis

Asymmetry Asymmetry

θ K<0 θ K>0 (Ap) Parameter

19/ 22 26/ 18 0.15±0.13 0.31± 0.286<|θ K|<15°

Horizontal

Scattering AngleNU/ ND

np

np

nnp

n

p

Coincidence

NMWD

np opening angle

cosθ

cosθ<-0.85He

10

Excitation spectra w/ coincident decay particles for 12C

B + p11

C + 11

C 12

p decay

B 11

0.0MeV

6.3MeV

2.5MeV

8.1MeV

decay

depolarizedeffect

inclusive

w/ proton

w/ pion

5.8×104 Level scheme of 12C

LH

N,decay

11

Polarization of (12C,11B)Itonaga et al.Prog. of Theo. Phys. Supp. 117(1994)14

2o 6～ o 6o 9～ o 9o 15～ o

12C 0.052 0.084 0.12211B 0.048 0.081 0.122

P Kaon Scattering angle

M1 transitionreduces

Polarization of

2o 15o

If assuming polarization isproportional to scattering angle.

12

Asymmetry parameter of 12C, 11B

NM=-0.14±0.28+0.18 pst

atis

tical

conta

mi

-0.00

E160 : - 0.9±0.3

13

Comparison with recent results

+2/+2/++K+/a1

　　　　　　　　　　　　　　 Itonaga et al.OPE

+K+DQ

+K

OME

p

Theoretical models such as +K reproduce n/p ratio, but predict large negative NM.

+K+

+K++DQ

+K++DQ model reproduces both n/p ratio and NM.

p-shell

Sasaki et al.PRC71 (2005) 035502

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

14

Summary

A series of experiments, E462 (5He) and E508 (12C, 11B) were carried out to measure NM precisely.

We obtained nearly zero NM’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.

-0.00 -0.00

p

15

Spare OHP’s

16

Excitation spectra w/ coincident decay particles for 6Li

inclusive

w/ proton

w/ pion

6Li (6Li→5He+p)

4.0×104

contamination in p-gateSystematic error

PID spectrumprevious experiment at BNL

6Li(K-,-)6Li

17

Proton Energy Dependence (5He)

Proton

Th

res

ho

ld

(Co

un

ts/5

MeV

)

0.07±0.09+0.09

0.15±0.10+0.09

0.05±0.14+0.08

Q/2Low energy region

Contamination of FSI 2N-induced

5He の結果における FSI 等の効果は小さいと考えられる -0.00

-0.00

-0.00

Neutron

Ep 30MeV -0.46 50MeV -0.52 70MeV -0.55

Garbarino’s calculation(with FSI effect)

PRL94 (2005) 082501

th

18

Production of polarized hypernuclei

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

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

K+ scattering angle(K)

1.05GeV/c +

In large scattering angle, is much polarized.

P

+

K+

/p

K

θ

E278

19

Polarization of

ー : E462

ー : E278

: Motoba et al. NPA577 (1994) 293c

Motoba et al.g.s.+4.5MeVex. state

20

One 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

21

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 I=1/2 rule)

Importance of asymmetry measurement

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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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Charged particle identification

TOF

dE/dx

E VS. TOF

E VS. dE/dx

T2

ener

gy

loss

(M

eVee

)T

OF

(1

/)

d

e

Total E (MeVee)

PID1

PID2

PID1

PID2

Total E

p

24

Comparison with E160P

ID f

unct

ion

Ene

rgy

spec

trum

p

E160 E508

w/protonw/proton w/pion

range&EtotdE/dx&Etot

TOF&Etot

25

Proton asymmetry of 12C and 11B

KEK E369 dataExcitation spectrum w/proton

Excitation spectrum of 12C is well known from previous experiment

eliminate the contamination such as QF event inside the gate

26

Asymmetry of 5He spectra

w/ proton w/ pion

27

Asymmetry parameter of mesonic decay

0 1 10 1 0

1 0 12 1

0 1 03 3S Y P Y Y

2 2

2Re *

| | | |

S P

S P

→ - + ps 1/2 0 1/2

l =0,1

wave function:

Angular distribution of decay proton1 -sinθ cosθ

2 2* | | | | 2Re * cosS P S P θ Asymmetry parameter

S : 88%、 P : 12% = 0.64

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One (Sasaki et al.) solution

Ex.+ K Ex. + Ex+D.Q.

n/

p r

atio

N

M p

weak coupling constant of meson

Sasaki et al.PRC71 (2005)035502

Both n/p and NM are consistent with experimental value.

N

NN

W S

,K,

p

Exp

eri

me

nta

lva

lue

29

PID1 ( dE/dX .vs. Etotal )

PID

2 (

TO

F .

vs.

Eto

tal )

PID window

(+, pX) reaction

or p

p gate gate

30

Identification of hypernuclear formation

21

p

M scattered

K+

+

T1 target

K

～ 340psec

0 1-1 2-2 3-3 (ns)0

2000400060008000

200 400 600 800 1000 12000(MeV/c2)

-200-400-600-800 0 200(mm)

Good /K separation Good /K/p separation

31

Neutron spectrum for 5He

Theoretical calc.

No peaking at half of Q-value (76MeV) even 5

ΛHesuggested larger contribution of NN→NNN or FSI than theoretical prediction.

Q / 2 = 76MeV

Theory :Garbarino et al.PRC69 (2004)054603

32

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

33

Coincidence Measurement (A=12)

cos

n + pn + p

n + nn + n

p + pp + p

EEn +n +EEpp

EEn +n +EEnn

EEp +p +EEpp

MeVMeVθθNNNN

Co

un

tsC

ou

nts

12ΛC

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

13