Λハイパー核の弱崩壊実験
S. Ajimura (RCNP)
- Nonmesonic weak decay of hypernuclei
- E22 experiment (4LHe)
- Future experiment (4LH)
2
Weak decays in L-Hypernuclei
N
p Mesonic weak decay (MWD)
similar with free L decay
L
spin/isospin structure well known
NNN
I=0 or 1 Non-Mesonic weak decay (NMWD)
N N
L
NNN
new decay modes
Lp  np, Ln  nn
spin/isospin structure: unknown
3S /1S
1
0
LN weak interaction
3
Status of NMWD studies
• Old puzzle solved recently
– np-ratio (GLnnn/GLp  pnGn/Gp) inconsistent
Gn / Gp  1 (Exp.)  Gn / Gp  0 (Theory )
– Experimental and theoretical improvements
Gn / Gp  0.5 (Exp. and Theory )
– (Exp.) Back-to-back coincidence for final two
nucleons (E462/508)
LN→nN
n
p
p n
FSI
n
in reality
re-scattering LNN→nNN
p
p n
p nnn
np
np p
p
p n
p nnn
np
np p
4
• A new puzzle arises

S
– Decay asymmetry inconsistent
a pNM  0 (Exp.)  a pNM  -0.7 (Theory)
HY
n
Asymmetry written
by amplitudes
S0  S1
1
3
3
S13S1
Large contribution ?
initial
1S
0
3S
1
final
1S
0
3P
0
1S
1
3D
1
1P
1
3P
1
amplitude isospin
a
1
b
1
c
0
d
0
e
0
f
1
parity
no
yes
no
no
yes
yes
assuming initial S state
S0 ( I  1)
1
S1 ( I  0)
3
S1 ( I  1)
3
p
5
• NMWD of 4-, 5-body hypernuclei
– allowed initial LN states
4
Λ

H (0 )
n n pΛ
0+ 0+
Ln→nn: 1S0, 3S1
Lp→np: 1S0
initial
1S
0
3S
1
final
1S
0
3P
0
1S
1
3D
1
1P
1
3P
1
4
Λ

He (0 )
p p nΛ
0+ 0+
1S
0
1S , 3S
0
1
amplitude isospin
a
1
b
1
c
0
d
0
e
0
f
1
parity
no
yes
no
no
yes
yes
assuming initial S state
5
Λ
He
p p n nΛ
1S , 3S
0
1
1S , 3S
0
1
S0 ( I  1)
1
S1 ( I  0)
3
S1 ( I  1)
3
6
Status of amplitude determination
Current status
constraint from 5LHe data
other constraints are loose
Our prospects
new constraint from 4LHe
np-ratio better than 15% error
Sakaguch-plot: assuming DI=1/2
7
Production of 4LHe
High resolution
Efficient K+ detection
Use 4He(p+,K+)4LHe reaction
19,000 4LHe/day
 500,000 4LHe in 4 weeks
High intensity p beam
~10M p+/spill
(not limited by primary beam)
8
Energy resolution
– K1.8 bemline + SKS  excellent resolution
• Liquid 4He 2 g/cm2  DEx ~ 2 MeV
• BE(4LHe) = 2.42  0.04 MeV
– Separation from QF L production essential
L  p+pp-+A  nnX, npX
9
Decay arm system
– Large acceptance and high efficiency for NN
n
n
p
p
– Good PID capability (n/p/p/g)
n/g TOF
p/p E/DE/range
n/p charge-veto
n   0.4
 n   30%
 p   0.25
  p   80%
10
Yield estimation
high beam intensity
large acceptance
large acceptance
and high efficiency
– 19,000 4LHe/day  500,000 4LHe in 4 weeks
– 1,300 Lpnp and 75 Lnnn in 4 weeks
in case of
1% BR
11
Background estimation
• Background sources
– QF L-production (Lp+p-, p-+AnnX)
• cut in Ex spectrum
– Mesonic weak decay of hypernuclei
• 4LHe3He+p+p-, p-+AnnX
• Gp-  0.3 G  Gn  0.01 G
• Reduction of background
– veto: no p track in CDC
– less material at target
• LHe target  2 g/cm2
• range(p-)  5 g/cm2
range
2 g/cm2
12
Background MC simulation
• Simulation of worst case
– 1/5 of p- stop in material around target
1/5 Gp- ~ 0.06 ⇔ Gn ~ 0.01
– GEANT4 base simulation
Li
C
R~1/110
R~1/30
R: Reduction factor (En1+En2>50MeV)
Nonmesonic decay of A=4 hypernuclei
Allowed initial states for A=4, 5 hypernuclei
hypernucleus
4 H
L
4 He
L
5 He
L

Ln→nn
1S , 3S
0
1
1S
0
1S , 3S
0
1
Lp → np
1S
0
1S , 3S
0
1
1S , 3S
0
1
Gp(4LH), Gn(4LHe)
⇒ we can measure 1S0 amplitudes directly.

If DI=1/2 rule holds, Gn(4LHe)/Gp(4LH)=2.
⇒ we can check the validity of the DI=1/2 rule in B-B weak
interaction.
Existing experimental results
Gn(4LHe) /GL=0.01+0.04/-0.01 (KEK), 0.04±0.02(BNL) NP A639(1998)261c
Gp(4LHe) /GL= 0.16±0.02(KEK), 0.16±0.02(BNL)
NP A639(1998)251c
(p-,K0) reaction
The final state of K0 is two
“charged” pions.
It is easy to measure the energy of
pions and their opening angle,
compared with gamma-ray in p0 case.
background
Multi-pion production in the target or other material
around target should be most serious background.
Such background can be rejected
by measuring K0 decay point.
Beam Momentum
1/5
At standard momentum for (p,K) reaction, 1050 MeV/c, bgct for kaon is
3.9cm.
bgct = 9.7 cm @ PK=1.53 GeV/c
16
High Intensity and High Resolution beamline
(proposed by H. Noumi: 2nd NPFC L08)
– handle 109 p/spill
x 100 of K1.8
– Dp/p ~ 10-4
Resolution OK ? – angle/momentum of pion
Ex resolution
Dp/p= 1x10-5:
1x10-4:
1x10-3:
DEx= 0.013 – 0.014
0.133 – 0.142
1.332 – 1.417
18
Summary
• We propose to measure the nonmesonic
weak decay of 4-body L hypernuclei (4LHe/H).
– select initial spin state (1S0/3S1)
– check the validity of DI=1/2 rule in baryon weak
interaction
• We are now preparing E22 exp. (4LHe) at
K1.8 beamline.
• High intensity and high resolution beamline
by H. Noumi, is suitable for measurement of
4 H.
L

N G i N tt lll f l h New Geiger-Nuttalllaw of alpha decay and new law

10 - SHIP Collaboration

Allegato 1 Classificazione delle zone delle acque interne del