Post on 02-Feb-2016
description
Non-Mesonic Weak Decays of 5ΛHe and
12ΛC Hypernuclei formed by ( +,K+) Reaction
RIKEN H. OutaRIKEN H. Outa
22. Aug. 2006 FB18 (P28)
for KEK-PS E462 / E508 collaborationsOsaka Univ.a, KEKb, GSIc, Seoul Univ.d, Tohoku Univ.e,
Univ.of 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
Γ(Λn→nn)/Γ(Λp→np) ratio A= 5 B.H. Kang et al. PRL 96 (2006) 062301 A=12 M.J. Kim et al. nucl-ex/0601029 : PLB in press n/p spectra from A=5,12 S. Okada et al. PLB 597 (2004) 249-256 Asymmetry of proton from polarized hypernuclei T. Maruta et al. nucl-ex/0509016 Mesonic & non-mesonic decay widths ⇒ may be omitted in this talk S. Kameoka et al. Nucl. Phys. A754 (2005) 173c-177c S. Okada et al. Nucl. Phys. A754 (2005) 178c-183c
Weak decay of Weak decay of hypernucleus hypernucleus
ΓΓππ__ (→ p + π - )
ΓΓππ00 ( → n + π 0 )
ΓΓpp ( +“ p”→ n + p )
ΓΓnn ( +“ n”→ n + n )Γ2N (ΛNN →NNN)
Mesonic
q ~ 100MeV/c
Non-Mesonic(NMWD)
q ~ 400MeV/c
1/HY =Γtot
Γm
Γnm
Weak decay mode of Weak decay mode of hypernucleus hypernucleusWeak decay mode of Weak decay mode of hypernucleus hypernucleus
weak decay in free spaceweak decay in free space weak decay in free spaceweak decay in free space
→ p + π - : 63.9±0.5 %
→ n + π0 : 35.8±0.5 %
= 263.2±2.0 ps
→ Well known.
Study of the mechanism of baryon-baryon weak interaction
[1]Γ(Λn→nn) /Γ(Λp→np) Ratio- Long standing puzzle solved
10 0.5 1.5n / p
Th
eore
tica
l
N N
NΛ
Direct Quark mechanism
Meson Exchangemechanism
Λ N
NN
π,K,η,ρ,ω,K*
0.93±0.55 (Szymanski et al.)Exp. (for 5He)
n n / / pp ratio ratio : The most important observable used to study the isospin structure of the NMWD.
n / p ratio puzzle
ΓΓpp (Λ+“ p”→ n + p )
ΓΓnn (Λ+“ n”→ n + n )n n / / pp ~~ 0.10.1
Λ N
NN
W S
π
One Pion Exchange(OPE)
Simple theoretical model
Tensor-dominant requires the final Nn pair to have isospin 0.
strong tenser coupling (L=2, S=2)→ dominant term 3S1→3D1 (amplitude “d”)
nn np
pp ppn
n
n p
rescattering
Final state interaction (FSI)
effect
Experimental difficulties in the nucleon measurementExperimental difficulties in the nucleon measurement
Difficulty in detecting neutrons. There is no experiment to observe both of the protons a
nd neutrons simultaneously with high statistics. Final state interaction (FSI) effect not well established Distinguish between the FSI and ”NNnNN” process
NN→NNN(2N-induced process)
ΛN
NN
Wπ
N
N
(One of the theoretical model)
Expected single nucleon spectrum
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
np
np
nnp
n
p
np
npn
Coincidence
Coincidence
NMWD
NMWD
The present experimentThe present experimentKEK-PS E462/E508
1) Angular correlationAngular correlation ( back-to-back, cos<-0.8 ) 2) Energy correlationEnergy correlation ( Q ~ E(N1)+E(N2) ~ 152MeV )
1) Angular correlationAngular correlation ( back-to-back, cos<-0.8 ) 2) Energy correlationEnergy correlation ( Q ~ E(N1)+E(N2) ~ 152MeV )
NMWD : N→NN
* cosθ< - 0.8 * E(N1)+E(N2) cut
n
p
p
n
coinpairnpN
coinpairnnN
FSIpn
.avpn
R1
nppN
FSI
2n
.avnn
R1
nnnN
Direct measurement of the n / p ratio
Select N→NN events w/o FSI effect & NN→NNN.
Select light hypernuclei to minimize FSI effect, 5He and 12
C
π+
K+
Target: 6Li,12C
Excitation-energy spectra for Excitation-energy spectra for 66Li and Li and 1212
CC
6.2×104
events
4.6×104
events
decay counter
6Li (g.s.) 55
He He + p
The ground state of 6Li is
above the threshold of 5He + p.
55HeHe
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
Charged particles from 5He
PID function
Charged PIDCharged PIDNeutral PIDNeutral PID
Constant background very small
Neutron energy resolution→7MeV(FWHM) at 75MeV
1 / spectra
Neutral particles from 12C
Good n separation Good p d separation
Decay particle identificationDecay particle identification
The g.s. peak is clearly seen in all spectra with coincident decay particles.
previous experiment at BNL
inclusive
w/ proton
w/ π±
w/ neutron
w/ γ
Excitation spectra w/ coincident decay particles for Excitation spectra w/ coincident decay particles for 55HeHe
S. Kameoka et al. Nucl. Phys. A754 (2005) 173-177S. Okada et al. Nucl. Phys. A754 (2005) 178-183
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
Coincidence Measurement (A=12)
cos
n + pn + p
n + nn + n
p + pp + p
EEn +n +EEpp
EEn +n +EEnn
EEp +p +EEpp
MeVMeVNNNN
Co
un
tsC
ou
nts
12ΛC
npnn / np = 0.50±0.13±0.05
10 0.5 1.5n / p
0.93±0.55 (Szymanski et al.) for 5He
Exp
.
Λ N
NN
W S
π
One Pion Exchange(OPE)
Th
eo.
N N
NΛ
Direct Quark mechanism
Meson Exchangemechanism
Λ N
NN
π,K,η,ρ,ω…
n n / / pp ratio ratio
Previous exp. (at BNL)
Nnn / Nnp (5He)= 0.45±0.11±0.0355
He (E462)He (E462)
Kang et al. PRL 96 (2006) 062301
Γn / Γp (12C)= 0.50±0.13±0.051212
C (E508)C (E508)
Kim et al. nucl-ex/0601029 PLB in press
[2] Asymmetry of proton emission from polarized hypernuclei
Asymmetry measurement of decay proton
N() = N0(1 + Acos)
Asymmetry
Asymmetry : Volume of the asymmetric emission from NMWD
A = (R - 1)(R + 1)
R =N(-) N(+)
,
Asymmetryparameter
= N0(1 + Pcos)
Difference of acceptance & efficiency is canceled out !
R =N(+(-))×N(-(+)) N(+(+))×N(-(-)) 1/2
K >0
+ K+
/p
K
K <0
+K+
/p
K
P
P
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 =1/2 rule)
Importance of αnm measurement
NM for 5ΛHe NMWD
A=PA:Asymmetry of PionAsymmetry Parameter of Pion (= - 0.642±0.013)PPolarization of Lambda:Attenuation factor
・Polarization of
・ Asymmetry Parameter of ProtonAp=NMPp
Estimated from mesonic decay
We can calculate NM without theoretical help !p
Polarization of
ー : E462
ー : E278
: Motoba et al. NPA577 (1994) 293c
NOTE:Calculation by Motoba et al. considers excited state at E=4.5 MeV
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) 168-172nucl-ex/050916 ; submitted to PRLInstrumental Asymmetry <0.003
Asymmetry parameter of 12C, 11B
NM=-0.14±0.28+0.18 pst
atis
tical
conta
mi
-0.00
E160 : - 0.9±0.3
Comparison with recent calculations
OPE
+K
+K+DQ
OME
+K,OME can reproducen/p ratio but predict large negative NM
+K+
+K++DQ
n/p and NM can bereproduced only by+K++DQ model
Sasaki et al.PRC71 (2005)035502
(1) Large b(1S0→3P0) and f(3S0→3P1) amplitude(2) Violation of ΔI=1/2 rule considered
[3] Precise measurement of π-mesonic / non-mesonic decay widths - Test of Λ-nucleus potential
Kameoka et al., HYP03 procNPA754 (2005)
Lifetime measurements withLifetime measurements with(π(π++,K,K++) reaction & SKS) reaction & SKS
psHe 11278)( 5 psC 6212)(12 47
Other Results-1: Other Results-1: 0 0 and Λ-α potentialand Λ-α potential
He0 / = 0.201±0.011
Lifetime : 278+11 ps (E462)-10
HYP03 proceesingsNucl. Phys. A754 (2005)
Other results-2: A dependence of ΓOther results-2: A dependence of ΓNMNM
HYP03 proceedingsNucl. Phys. A754
SummarySummary N→NN was directly observed for the first time !! 5
ΛHe : Γn / Γp ratio ~ Nnn / Nnp = 0.45±0.11±0.03 Kang et al. PRL 96 (2006) 062301
12ΛC : Γn / Γp ratio ~ 0.50±0.13±0.05
Kim et al. PLB, in press
◆ Asymmetry parameter measured with improved accuracy !! 5
ΛHe :
11ΛB and 12
ΛC : Maruta et al. nucl-ex/0509016; thesis
◆ Mesonic and Non-mesonic decay widths are precisely measured - Test of Λ-nucleus potential
NM=0.08±0.08+0.08
p
[1] Importance of shorter-range mechanism OPE ⇒ Heavy meson & DQ exchange [2] Significant contribution from ΛN initial spin-singlet initial state - σ-meson exchange ? / Violation of ΔI=1/2 rule ?
-0.00
NM= - 0.14±0.28+0.18
p -0.00
Spare OHP
ΛNN→NNN considerationΛNN→NNN consideration
Λn
np
Wπ-
p
n
Λn
nn
Wπ0
p
p
4
1
Seen as….
n
n
p
p
n
n
Λn→nn like
Λp→np like
Garbarino
exp
erim
enta
lex
per
imen
tal
dat
ad
ata
theo
reti
cal
theo
reti
cal
calc
.ca
lc.
Comparison with theoretical calc.for angular correlation
Garbarino’s calc.
assuming Gn/Gp = 0.46 (for 5He ), 0.34 (for 12
C )considered 2N-induced( ~ 20%), FSI
55He (E462)He (E462) 1212
C (E508)C (E508)
n+p coincidencen+n coincidence
n+pn+n
cos cos
n+p coincidencen+n coincidence
n+pn+np+pp+p
Pair numberPair number /NMWD/NMWD
Pair numberPair number /NMWD/NMWD
n+p coincidencen+n coincidencep+p coincidence
n+p coincidencen+n coincidencep+p coincidence
Phys. Rev. Lett. 91 (2003) 112501Phys. Rev. Lett. 91 (2003) 112501 21
E462
- / of He
- branching ratio0 branching ratio
Lifetime
Other resultsOther results
- decay width for 5He
Errors were much improved !!
0 decay width for 5He and 12
C
HYP2003 proceedingsNucl. Phys. A754 (2005)
Kameoka et al., HYP03 procnucl-ex/0402023
Very recent measurements withVery recent measurements with(π+,K+) reaction & SKS(π+,K+) reaction & SKS
psHe 10278)( 5 psC 6212)(12 47
Preliminary
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
np coincidence analysis
Asymmetryθ <0 θ >0 Parameter
23/ 26 30/ 19 0.176± 0.122 0.306± 0.215
Asymmetry
6<|θ |<15°
Horizontal Scattering Angle
Nupper/ Nlower
np
np
nnp
n
p
Coincidence
NMWD
np back-to-back event
NM=0.31±0.22p
One and only(?) 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
Angular correlationAngular correlation Energy sumEnergy sum
n +
pn
+ p
n +
nn
+ n
estimated contamination from π - absorption
Coincidence analysis (5ΛHe)
Q-Value ~153MeV
cos θ <- 0.8
90 event
30 event
(1.34)
(4.38)
n/p estimation from Coincidence Measurement
Coincidence Measurement(E462/E508)
Pair Pair numbers/NMWDnumbers/NMWD
Estimated contamination from
Energy sum Energy sum distributiondistribution
7.0cos NNfor 1cos1 NNfor
n + pn + p n + nn + n
0034.00113.0
)018.0060.0()32
6(
a
0038.00156.0
)0204.00833.0()32
6(
b
Before subtraction After subtraction
)7.0(cos npnpN
)7.0(cos nnnnN
0138.01384.0
0144.00826.0 0149.00670.0
0142.01272.0
Uniform components subtractionUniform components subtraction
FSI consideration using pp-pairs
rrn,pn,p fraction ratio of the neutron and
proton induced channels.ffn,pn,p is reduction factor due to FSI.
ggn,n,pp is cross over influx of neutron(proton) from proton(neutron) due to FSI.p,q,q’p,q,q’ are angular acceptance factor.
fff np ggg np 02 gpn rrx /
0)(2 ''''2 ppnnnppp NNxNNx
,,,, ,,
Nnp(bb) 0.1272 0.0142
Nnn(bb) 0.0670 0.0149
Npp(bb) 0.0047 0.0017
MeVEthNN 30,7.0cos At
'2'
'
2'
2
qgrqgfrN
qgfrpffrN
qgfrpfrN
pnppppp
pnnpnpnp
nnpnnnn
))(( 'NNNN NbbN
Before FSI correctionBefore FSI correction
13.053.0)(
)(
bbN
bbN
np
nn4%4%
Systematic error calculationSystematic error calculation
05.013.051.0
p
n
1)1) Intentionally add 2Intentionally add 2 pp pp events events inin
-0.8<cos-0.8<cos<-0.7 <-0.7 %4:13.049.0
2)2)
3)3)Uniform b.g. level for different angular regions:6.6%Uniform b.g. level for different angular regions:6.6%
Neutron Efficiency Correction
Production of Polarized Hypernuclei
E462/E508 experiment 1.05GeV/c + beamis injected.
Distribution of polarization in the n(+,K+) reaction
2o 15oK+ scattering angle(K)
1.05GeV/c +
In large scattering angle, is much polarized.
P
+
K+
/p
K
6Li Hypernuclear mass spectra
6Li + +→ 6Li + K+
6Li → 5He + p
5He
6Li
5Li0MeV
8.3MeV
18.3MeV
(Pn-1,S)
(Pn-1,P)
(Sn-1,S)
p decay
decay
inclusive
coin
p coin
5.2×104 events
3.2×103 events
1.6×103 events
Instrumental Asymmetry
(,pC) reaction : Only Strong InteractionAsymmetry = 0 expected
6Li target 12C target- 0.000± 0.002 0.000± 0.0020.003± 0.002 - 0.003± 0.0030.003± 0.002 0.001± 0.002- 0.001± 0.001 - 0.002± 0.002
0.003± 0.001 0.002± 0.0020.000± 0.001 - 0.003± 0.002
Proton
Pion
6<|θ |<9°
9<|θ |<15°
2<|θ |<6°
Scattering Angle2<|θ |<6°
Horizontal Asymmetry
6<|θ |<9°
9<|θ |<15°
Instrumental Asymmetry < 0.3%
NNn n / N/ Npp Ratio Ratio
ΓΓnn (Λ+“ n”→ n + n )
ΓΓpp (Λ+“ p”→ n + p )
If Γn / Γp = 1 → Nn / Np = 3
If Γn / Γp = 0.5 → Nn / Np = 2
If Γn / Γp = 0 → Nn / Np = 1
Naive estimationNaive estimation(without considering FSI and ΛNN→NNN)
To avoid suffering from FSI effect & ΛNN→NNN,
High energy threshold High energy threshold
Γn:Γp Nn Np
1 : 1 … 3 : 1 1 : 2 … 2 : 1 0 : 1 … 1 : 1
Nn / Np = 2×n / p + 1
Neutron and Proton energy spectra of Neutron and Proton energy spectra of 55He and He and 1212
CC
Nn / Np (E>60MeV)~ 2.00±0.09±0.14
Nn / Np (60<E<110MeV) ~ 2.17±0.15±0.16
5He → n + : Q ~
135MeV (rate : 0.049±0.01-)
135MeV
apply upper energy limit of 110MeV !
apply a simple relation n / p = (Nn / Np - 1) / 2
n / p ~ 0.5
To avoid suffering from FSI effect & ΛNN→NNN,
High energy threshold High energy threshold
Corrected proton energy loss inside the target !!
(submitted PLB. nucl-ex/0406020)
inclusive
w/ proton
w/ pion
w/ neutron
w/ gamma
Excitation spectra w/ coincident decay particles for Excitation spectra w/ coincident decay particles for 1212CC
Spin / isospin dependence
p p n Λ
He4Λ
n n p Λ
H4Λ
p p n n Λ
He5Λ
Non-mesonic weak decay of 4He and 4
H
4He (K-,-) 4He or
4He (+,K+) 4He
n+n back-to-back4He (K-,0) 4
H p+n back-to-back (0 spectrometer )
see S.Ajimura : J-PARC LOI 21
To J-PARCTo J-PARC
RNS … N : nnn, pnp S : spin = 0 or 1
nm(4H) = ( 3Rn1+ Rn0 + 2Rp0 ) ×4 / 6
nm(4He) = ( 2Rn0 + 3Rp1 + Rp0 ) ×4 / 6
nm(5He) = ( 3Rn1+ Rn0 + 3Rp1 + Rp0 ) ×5 / 8
Need one-order higher statistics. J-PARC
π+
K+
decay counter
K6/SKS setup
Identification of hypernuclear formation
21
p
M scattered
K+
T1 target
Z-vertexMass
Z
Neutral decay particle IDNeutral decay particle ID
Good n separation
Good S/N ratio ( ~30) ( previous exp. S/N ratio (5
He) ~ 1 )~ 30 times (for 5
He) higher than previous exp.
High statistics ( ~ 5000 neutrons)~ 200 times (for 5He)
~ 30 times (for 12C)
higher than previous exp.
5MeV< En < 150MeVConstant
background very small
Neutron energy resolution(estimated from width) →7MeV(FWHM) at 75MeV
(TOF spectra)1 / spectra
Neutral particles from 12C
Charged particles from 5
He
derived from• dE/dx (at T2)• Total energy deposit (sequentially fired counters (T2,T3,T4).)• TOF (between T2 and T3)
PID function
Deuterons were separated from the protons.(for the first time!)
Charged decay particle IDCharged decay particle ID
Gaussian fit(±2 cut)
Non-mesonic weak decayNon-mesonic weak decay
strong tenser coupling (L=2, S=2)→ dominant term 3S1→3D1 (amplitude “d”)
+ N→ N + N + p → n + p: p = a2+b2+c2+d2+e2+f2
+ n → n + n : n = a2+b2+f2
OPE : n / p ~ 0.1
Λ N
NN
W S
π
One Pion Exchange (OPE) model
n / p ratio: The most important observable to study the isospin structure of the NMWD.
Exp. : n / p ~ 1
n / p ratio puzzle
Simple theoretical model
with large error
Mass number dependence Mass number dependence of neutron energy spectra ( A=5,12,89 )of neutron energy spectra ( A=5,12,89 )
( previous experiment )
As the mass number become lager, the number of neutron become lager in the low energy part, and smaller in the high energy part.
Q / 2 = 76 MeV
No peaking at Q / 2 (76MeV)
even 5ΛHe
suggested larger contribution of ΛNN→NNN or FSI than theoretical prediction.
Theoretical calc.
5ΛHe
w/ FSI
Q/2
-nucleus overlap for 5He
0 / = 0.201±0.011 (He)0 / = 0.201±0.011 (He)
nm/ = 0.395±0.016 (He)nm/ = 0.395±0.016 (He)
Γtotal (Γtotal (5656ΛΛFe) Fe) ~~ Γnm(A→∞) Γnm(A→∞) ~ ~ 1.2Γ1.2ΓΛΛ ( ( E307E307 ))
Γnm (Γnm (55ΛΛHe) = 0.4ΓHe) = 0.4ΓΛΛ ( ( Present)Present)
1/3 of Λ is inside α 1/3 of Λ is inside α
Both results are consistent,Both results are consistent,preferred larger overlap than YNG prediction.preferred larger overlap than YNG prediction.
55ΛΛHe (ORG) He (ORG) ~ ~ 40%40%
55ΛΛHe (YNG) He (YNG) ~ ~ 20%20%
1/3 of Λ is inside α 1/3 of Λ is inside α
0 / locates in between ORG and YNG.