P10-2: Exclusive Study on the N Weak Interaction in A=4 -Hypernuclei (update from P10) S. Ajimura...
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Transcript of P10-2: Exclusive Study on the N Weak Interaction in A=4 -Hypernuclei (update from P10) S. Ajimura...
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.)
2
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
3
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
4
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
5
• 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
6
• 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
7
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
8
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
9
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
10
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
11
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
12
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
13
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
14
Time schedule– Ready in 2009– Collaboration with E05 and E15
K1.8 and K1.8 and SKS ReadySKS Ready
Decay arm Decay arm ReadyReady
15
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.
18
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
21
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
22
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
23
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
24
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
25
Asymmetry parameter of 12C, 11B
NM= - 0.14±0.28+0.18 pst
atis
tical
conta
mi
-0.00
E160 : - 0.9±0.3
26
Yield of 4He with (K-,-) reaction
– 33,000 4He/day: about 2 of (+,K+)
full beam (30GeV)full beam (30GeV)
SPES2SPES2
150
,,
KK
27
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
35
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
36
Nn-pair detection efficiency
– About 1.5 in back-to-back region
averageaverage averageaverage
back-to-backback-to-backregionregion
back-to-backback-to-backregionregion