1. 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
Mesonic weak decay (MWD)
similar with free L decay
spin/isospin structure well known p L N N N
Non-Mesonic weak decay (NMWD)
new decay modes
spin/isospin structure: unknown
I=0 or 1 N N L
LN weak interaction N N N
3S1/1S0

3. Status of NMWD studies Old puzzle solved recently
np-ratio (GLnnn/GLp  pnGn/Gp) inconsistent
Experimental and theoretical improvements
(Exp.) Back-to-back coincidence for final two nucleons (E462/508) n p
LN→nN
LNN→nNN
FSI
re-scattering
in reality

4. Asymmetry written by amplitudes
A new puzzle arises
Decay asymmetry inconsistent HY p n )
Exp. ( NM p a Û )
Theory ( 7 . - NM p a
Asymmetry written by amplitudes
Large contribution ?
initial
final
amplitude
isospin
parity
1S0 a 1 no
3P0 b
yes
3S1
1S1 c
3D1 d
1P1 e
3P1 f
assuming initial S state

5. NMWD of 4-, 5-body hypernuclei
allowed initial LN states
n n p Λ
p p n Λ
p p n n Λ 0+ 0+ 0+ 0+
Ln→nn: 1S0, 3S1 1S0 1S0, 3S1
Lp→np: 1S0 1S0, 3S1 1S0, 3S1
initial
final
amplitude
isospin
parity
1S0 a 1 no
3P0 b
yes
3S1
1S1 c
3D1 d
1P1 e
3P1 f
assuming initial S state

6. Status of amplitude determination
Our prospects
new constraint from 4LHe
np-ratio better than 15% error
Current status
constraint from 5LHe data
other constraints are loose
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+p-
p-+A  nnX, npX

9. Decay arm system Large acceptance and high efficiency for NN
Good PID capability (n/p/p/g) n n p p
n/g TOF
p/p E/DE/range
n/p charge-veto

10. Yield estimation 19,000 4LHe/day  500,000 4LHe in 4 weeks
1,300 Lpnp and 75 Lnnn in 4 weeks
high beam intensity
large acceptance
large acceptance and high efficiency
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- ~ ⇔ Gn ~ 0.01
GEANT4 base simulation Li C
R~1/110
R~1/30
R: Reduction factor (En1+En2>50MeV)

13. Nonmesonic decay of A=4 hypernuclei
Allowed initial states for A=4, 5 hypernuclei
hypernucleus
Ln→nn
Lp → np
4LH
1S0, 3S1
1S0
4LHe
5LHe
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 (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

14. (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.

15. Beam Momentum bgct = 9.7 cm @ PK=1.53 GeV/c 1/5
At standard momentum for (p,K) reaction, 1050 MeV/c, bgct for kaon is 3.9cm.
bgct = 9.7 PK=1.53 GeV/c

16. High Intensity and High Resolution beamline (proposed by H
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

17. Resolution OK ? – angle/momentum of pion
Ex resolution
Dp/p= 1x10-5: DEx= – 0.014
1x10-4: – 0.142
1x10-3: – 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 4LH.