NonMesonic Weak Decays of 5He in p ,K reaction

1
Non-Mesonic Weak Decays of 5?He in (p ,K)
reaction
24th. Oct. 2005 PANIC2005
RIKEN H. Outa
for KEK-PS E462 / E508 collaborations
Osaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku
Univ.e, Univ. Tokyof,Tokyo Inst. Tech.g, KRISSh,
RIKENi S. Ajimuraa, K.Aokib, A.Banuc, H. C.
Bhangd, T. Fukudac,O. Hashimotoe, J. I. Hwangd,
S. Kameokae, B. H. Kangd, E. H. Kimd, J. H.
Kimd, M. J. Kimd, T. Marutaf, Y. Miurae,Y.
Miyakea, T. Nagaeb, M. Nakamuraf, S. N.
Nakamurae,H. Noumib, S. Okadag, Y. Okayasue, H.
Outab, H. Parkh,P. K. Sahab, Y. Satob, M.
Sekimotob, T. Takahashie,H. Tamurae, K. Tanidai,
A. Toyodab, K. Tsukadae,T. Watanabee, H. J. Yimd
G(?n?nn)/G(?p?np) ratio B.H. Kang et al.
nucl-ex/0509015 PRL submitted Asymmetry
parameter T. Maruta et al. nucl-ex/0509016
PRL submitted Mesonic decay widths ? omitted in
this talk S. Kameoka et al. Nucl. Phys. A754
(2005) 173-177 S. Okada et al. Nucl.
Phys. A754 (2005) 178-183
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Weak decay of L hypernucleus
L weak decay in free space
L? p p- 63.90.5 L ? n p0 35.80.5
tL 263.22.0 ps
? Well known.
Study of the mechanism of baryon-baryon weak
interaction.
3
The most important observable used to study the
isospin structure of the NMWD.
Gn / Gp ratio
Gp (?p? n p) Gn (?n? n n)
Theoretical
Gn / Gp ratio puzzle
4
Experimental difficulties in the nucleon
measurement

• Difficulty in detecting neutrons.
• ? There is no experiment to observe both of
  the protons and neutrons simultaneously with high
  statistics.
• Final state interaction (FSI) effect
• ? not well established
• Distinguish between the FSI and LNN?nNN process

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The present experiment
KEK-PS E462/E508
Direct measurement of the Gn / Gp ratio
NMWD ?N?NN
1) Angular correlation ( back-to-back,
cosqlt-0.8 ) 2) Energy correlation (
QE(N1)E(N2) 152MeV )
Select light hypernuclei to minimize FSI effect,
5LHe and 12LC
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Excitation-energy spectra for 6LLi and 12LC
5LHe
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Decay-particle detection system
Neutral particle TOF (T1?T4) T3 VETO
Solid angle 26 9(T)9(B)8(S)
Charged particle dE/dx (at T2) total energy
deposit (T2T3T4) TOF (T2?T3)
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Neutral PID
Decay particle identification
Neutral particles from 12LC
1 / b spectra
Good p p d separation
Good g n separation
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Excitation spectra w/ coincident decay particles
for 5LHe
The g.s. peak is clearly seen in all spectra with
coincident decay particles.
S. Kameoka et al. Nucl. Phys. A754 (2005)
173-177 S. Okada et al. Nucl. Phys. A754
(2005) 178-183
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Angular correlation
Coincidence analysis
Energy sum
cos ?lt-0.8
n p
90 event
(1.34)
30 event
n n
(4.38)
Q-Value 153MeV
estimated contamination from p- absorption
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np- nn- angular distribution
Back-to-back
Back-to-back
systematic error is mainly come from efficiency
for neutron (6) acceptance(3)
Gn/Gp Nnn / Nnp 0.450.110.03
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Gn / Gp ratio
Previous exp. (at BNL)
0.930.55 (Szymanski et al.) for 5LHe
Exp.
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Importance of anm measurement
If assuming initial S state
We can know the interference between states
with different Isospin and Parity .
(Applying DI1/2 rule)
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Asymmetric proton emission from NMWD
N(?) 1 Acos? 1 aP cos?
L
P
Asymmetry Parameter
Asymmetry
N
Decay counter Acceptance ? gt 0.7
1
-1
1
0
SKS Acceptance jK 15o-15o
cos?
Difference of acceptance efficiency is canceled
out !
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aNM for 5?He NMWD
Estimated from mesonic decay
Polarization of L
Asymmetry Parameter of Proton
ApaNMPLe
p
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Asymmetry parameter of 5?He
T.Maruta et al. HYP03 Nucl.Phys.A754 (2005)
168-172 nucl-ex/050916 submitted to PRL
Final !!
statistical
p contamination
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np coincidence analysis
np back-to-back event
NMWD
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Summary

• LN?NN was directly observed for the first time
  !!
• 5?He Gn / Gp ratio Nnn / Nnp
  0.450.110.03
• nucl-ex/0509015
• 12?C Gn / Gp ratio Nnn / Nnp
  0.500.140.03
• Kim (Session IV-6)
• ? Asymmetry parameter measured with improved
  accuracy !!
• Single proton
• np coincidence
• 
  nucl-ex/0509016

1 Importance of shorter-range mechanism
OPE ? Heavy meson DQ exchange 2 Significant
contribution of ?N initial spin-singlet
state contribution - s-meson exchange ?
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Spare OHP
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Production of Polarized Hypernuclei
1.05GeV/c p beam is injected.
E462/E508 experiment
Distribution of L polarization in the n(p,K)L
reaction
1.05GeV/c p
P
K scattering angle(jK)
In large scattering angle, L is much polarized.
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6Li Hypernuclear mass spectra
L
inclusive
5.2104 events
6Li p? L6Li K
L6Li ? L5He p
(Sn-1,SL)
p coin
3.2103 events
18.3MeV
(Pn-1,PL)
p decay
L decay
8.3MeV
p coin
1.6103 events
(Pn-1,SL)
5Li
0MeV
5LHe
6LLi
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Instrumental Asymmetry
(p,pC) reaction Only Strong Interaction
Asymmetry 0 expected
Instrumental Asymmetry lt 0.3
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Nn / Np Ratio
GnGp Nn Np 1 1 3 1 1
2 2 1 0 1 1 1
Gn (?n? n n) Gp (?p? n p)
If Gn / Gp 1 ? Nn / Np 3 If Gn / Gp
0.5 ? Nn / Np 2 If Gn / Gp 0 ? Nn / Np 1
Naive estimation
(without considering FSI and ?NN?NNN)
Nn / Np 2Gn / Gp 1
To avoid suffering from FSI effect ?NN?NNN,
High energy threshold
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Neutron and Proton energy spectra of 5LHe and
12LC
(submitted PLB. nucl-ex/0406020)
To avoid suffering from FSI effect ?NN?NNN,
High energy threshold
Nn / Np (60ltElt110MeV) 2.170.150.16
135MeV
5LHe ? n a Q135MeV (rate
0.0490.01Gp-)
apply upper energy limit of 110MeV !
Nn / Np (Egt60MeV) 2.000.090.14
apply a simple relation Gn / Gp (Nn / Np -
1) / 2 ? Gn / Gp 0.5
Corrected proton energy loss inside the target !!
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Excitation spectra w/ coincident decay particles
for 12LC
inclusive
w/ proton
w/ pion
w/ neutron
w/ gamma
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Other results
Lifetime
p- branching ratio
p0 branching ratio
p - decay width for 5LHe
p0 decay width for 5LHe and 12LC
Statistical errors were much improved !!
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To J-PARC
Non-mesonic weak decay of 4LHe and 4LH
see S.Ajimura J-PARC LOI 21
Spin / isospin dependence
Gnm(4LH) ( 3Rn1 Rn0 2Rp0
) r4 / 6 Gnm(4LHe) ( 2Rn0 3Rp1
Rp0 ) r4 / 6 Gnm(5LHe) ( 3Rn1 Rn0 3Rp1
Rp0 ) r5 / 8
RNS N Ln?nn, Lp?np S spin 0
or 1
4He (K-,p-) 4LHe or 4He (p,K) 4LHe ? nn
back-to-back
4He (K-,p0) 4LH ? pn back-to-back (p0
spectrometer )
? Need one-order higher statistics. ? J-PARC
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K6/SKS setup
K
decay counter
p
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Identification of hypernuclear formation
K
p
T1
Z
target
Mass
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Neutral decay particle ID
Neutral particles from 12LC
Neutron energy resolution (estimated from g
width) ? 7MeV(FWHM) at 75MeV

• Good g n separation
• Good S/N ratio (30)
• ( previous exp.
• S/N ratio (5LHe) 1 )
• ?30 times (for 5LHe)
• higher than previous
• exp.
• High statistics
• ( 5000 neutrons)
• ?200 times (for 5LHe)
• 30 times (for 12LC)
• higher than previous
• exp.

5MeVlt En lt 150MeV
Constant background very small
1 / b spectra
(TOF spectra)
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Charged decay particle ID
Charged particles from 5LHe
Gaussian fit (2s cut)
Deuterons were separated from the protons. (for
the first time!)
PID function

• derived from
• dE/dx (at T2)
• Total energy deposit (sequentially fired
  counters (T2,T3,T4).)
• TOF (between T2 and T3)

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Non-mesonic weak decay

• N? N N

• p ? n p
• Gp a2b2c2d2e2f2

L n ? n n Gn a2b2f2
Gn / Gp ratio
The most important observable to study the
isospin structure of the NMWD.
Simple theoretical model
strong tenser coupling (DL2, DS2) ? dominant
term 3S1?3D1 (amplitude d)
One Pion Exchange (OPE) model
OPE Gn / Gp 0.1
Exp. Gn / Gp 1
Gn / Gp ratio puzzle
with large error
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Mass number dependence of neutron energy spectra
( A5,12,89 )
5?He
No peaking at Q / 2 (76MeV) even 5?He
suggested larger contribution of ?NN?NNN or FSI
than theoretical prediction.
( previous experiment )
Theoretical calc.
w/ FSI
Q/2
Q / 2 76 MeV
As the mass number become lager, the number of
neutron become lager in the low energy part, and
smaller in the high energy part.
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L-nucleus overlap for 5LHe
Gp0 / GL 0.2010.011 (5LHe)
Gp0 / GL locates in between ORG and YNG.
5?He (ORG) 40 5?He (YNG) 20
1/3 of ? is inside a

Gnm/ GL 0.3950.016 (5LHe)
Gtotal (56?Fe) Gnm(A?8) 1.2G? (E307)
Gnm (5?He) 0.4G? (Present)
1/3 of ? is inside a

Both results are consistent, preferred larger
overlap than YNG prediction.