measurement of branching fraction and time- dependent cp asymmetry parameters in b 0 k 0 0 decays...
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Measurement of Branching Fraction and Time-dependent CP Asymmetry
Parametersin B0K00 Decays
奈良女子大学大学院 人間文化研究科 博士後期課程 3年
複合現象科学専攻 高エネルギー物理学研究室藤川 美幸希
• Introduction• Experimental Apparatus• Analysis Event Reconstruction Branching Fraction Measurement Time-dependent CP Analysis • Summary
2009/2/24 公聴会 2
Introduction - CP violation
=
≫
Big Bang
Present
C (Charge conjugation) :
P (Parity) :
CP violation is one of the necessary condition to explain the baryon asymmetry in universe
CP violation( , ) ( , )t x t x
Matter Anti-matter
Matter Anti-matter
Asymmetry btw Particle and anti-particle
Q Q
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• First observation of CP violation in neutral kaon system
Short lifetime : KS +1
Long lifetime : KL −1
CP eigenvalue
• If CP is conserved, KL is forbidden
KL experimentally observed J. H. Christenson et al. Phys. Rev. Lett. 13, 138 (1964)
But…
Introduction - CP violation
2009/2/24 公聴会 4
~
2 3
2 2 4
3 2
1 2 ( )
1 2 ( )
(1 ) 1CKM
A i
V A
A i A
Ο
Kobayashi-Maskawa Theory
Prog. Theor. Phys. 49, 652(1973)
• CP violation in Standard model can be explained by 3 quark generations
Irreducible complex phase ⇒ can violate CP
The 2008 Physics Nobel Prize
'
'
'
ud us ub
CKM cd cs cb
td ts tb
d d V V V d
s V s V V V s
b b V V V b
Introduction - CKM matrix
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3 3 3
0
( ) ( ) ( )
td tb cd cb ud ubV V V V V V
O O O
• Unitarity of CKM matrix ⇒
1
2
3
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
• Triangle lengths ⇒ large CP violation possible~ 3( )O
Related to B decays
• Independent measurement for 3 lengths and angles important for complete test of CKM model or for search of new mechanism
Introduction - CKM matrix
( , )
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Introduction – How to measure CPV in B0 system?• Two different amplitude is necessary to observe CP violating phase
• In B system, there is two sources of CP violation1 2| exp( )| CPObs i
1. Mixing induced CP violation
2. Direct CP violation
CPff f where fcp is CP eigenstate
Example of fcp; J/K0, K00,,,,
0 0 B Bff
Interference between B decays with and without mixing
2009/2/24 公聴会 7
B
mixing
B
B
①
②
fCP
fCP
Interference between B decays with and without mixing Mixing-induced CP Violation
Introduction – Mixing-induced CP violation
B0
B0
fCP
mixing
①
②
①
②
fCP
2009/2/24 公聴会 8
B
mixing
B
B
①
②
fCP
fCP
Interference between B decays with and without mixing Mixing-induced CP Violation
Introduction – Mixing-induced CP violation
B0
CP
B0
B0fCP fCP
B0
mixingmixing
①
②
①
②
fCP
sin dm t d Heavy Lightm m m
2009/2/24 公聴会 9
• Decay amplitudes:
• CP violating asymmetry (ACP) is defined as:
• A non-zero ACP requires the following 3 conditions:– more than 2 decay amplitudes– non-zero strong phase difference : I - j = ≠ 0– non-zero weak phase difference : φi -φj = φ ≠ 0
Introduction – Direct CP violation
| ( )| | ( )| B f B f Direct CP Violation
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0 0
0 0
[ ( ) ] [ ( ) ]( )
[ ( ) ] [ ( ) ]
cos sin
CP CPCP
CP CP
CP d CP d
B t f B t fa t
B t f B t f
A m t S m t
Direct CP Violation parameter
mixing-induced CP Violation parameter
CPA
CPS
1
1
-1
-1
Physical Region
Introduction –Direct + mixing-induced CP violation
Need to measure time-dependence
B0K00 case
2009/2/24 公聴会 11
Introduction – Coherent B0B0 production
0 0
0 0
(4 ) has L=1
system is
⇒ i n coherent state
e e S B B
B B
BTag
e e(4 ) S
0B
0B
CP eigenstate
0B
0B
0B fCPfCP
Flavour tag ;qFlavour tag ;q ( )
z
ct
~
( )
z
ct
~
z ~ 0.425
0 0B B
⇒ We need to ti me di ff eren e
c t
0
0
if one B meson identified as B at time t,
another B meson must be B a
Coherent Mixing
t same time.
(EPR paradox)
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Introduction – Status of CPV measurement in B0J/K0
Clear CP violation observed in mixing –induced CPV process
bc transition
B0 tag_B
0 tag
sin21= 0.642 ±0.031 (stat) ±0.017 (syst)
hep-ex/0608039, PRLhep-ex/0608039, PRL
A = 0.018 ±0.021 (stat) ±0.014 (syst)
B0J/K0
1Im sin2td tb cb csCP CP
td tb cb cs
V V V VS
V V V V: eigenvalueCP CP
B0-B0Mixing
2009/2/24 公聴会 13
Introduction – CP violation in B0K00
interferes with SMNP iie e
Penguin diagram with new physics
d d
sd
d
bKS
0 d
KS
d
sd
d
b ?0
Penguin diagram in Standard Model
1sin2 CP CPS 1sin2 CP CPS
bs transition proceeds via second order decay process
bs modes sensitive to New Physics
2009/2/24 公聴会 14
Introduction – CP violation in B0K00
bc
bs
B0J/K0
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• ACP(B0) = –0.094±0.018±0.008 4.8
• ACP(B+) = + 0.07 ±0.03 ±0.01
Introduction – Direct CP violation status
w/ 535MBB
Nature 452, 332-335(2008)B0K-+ B0K+-
First evidence of direct CP violation in B system
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Isospin sum rule among BKCP asymmetries
M. Gronau, PLB 672(2005)82-88)
Introduction – Prediction for direct CPV in B0K00
0 00 00
0 00 00( ) 2 ( )
(( )
( ) ( ) ( )( ) ( ) (
))
CPCP CP CP
B K B KA K A K A K
B K B K B KB K
A K
• Prediction from SM ACP(K00) = - 0.148±0.044• Breaking sum rule indicates New Physics• Good test for Standard Model
measurement
Standard Model prediction
Experimental Apparatus
KEKB Accelerator & Belle Detector
2009/2/24 公聴会 18
3km circumference
KEKB Accelerator
(4 ) e e S B B
BelleDetector
e-
e+
:
: .
: .
.
GeV
GeV
e
s GeV
e
8
35
1058
0425
• Why asymmetric collider?
Increase BB separation,
B flight length ~ 200m
~ /t z c
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• Integrated Luminosity
Performance of KEKB
• Peak luminosity L = 1.7118×1034 /cm/s
2006 summer605 fb-1 = 657MBBpair
year
Inte
grat
ed l
umin
osity
(fb
-1)
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/ KL detector
(KLM)
Central Drift Chamber (CDC)
CsI(Tl) Calorimeter (ECL)
Aerogel Cherenkov cnt.
(ACC)
Si vtx. detector
(SVD)
Time of Flight (TOF)
SC solenoid 1.5T
8 8 GeVGeV ee--
3.5 3.5 GeVGeV
ee++
Belle detector
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SVDSVD Silicon Vertex Detector High precision tracking for vertex reconstruction
CDCCDC Central Drift Chamber Tracking
Low momentum PID (<1GeV/c)
ACCACC Aerogel Cherenkov Counter
High momentum PID
(1-4GeV/c)
TOFTOF Time-Of-Flight counters
Low momentum PID(<1.2GeV/c)
Trigger timing
ECLECL Electromagnetic Calorimeter
Photon and electron ID
KLMKLM KL ,Muon detector Identify KL and muon
Belle sub-detector
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3 layers x-z planeSVD1
SVD2
23º<<139º
17º<<150º4 layers
• In summer 2003, SVD1 was replaced with SVD2
ee-- ee++
SVD – Configuration
• First 152×106BB pairs collected with SVD1• Remaining 505×106BB pairs collected with SVD2
Event Reconstruction
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Event Reconstruction
0B0
0SK -
+
B0K00
B0 KS0
B0 KL0 (mentioned later)
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KS is reconstructed from two charged
Event Reconstruction – KS0
0.480 < m(+-) < 0.516 GeV/c2
• Invariant mass
• The smaller of dr1 and dr2, the shortest distance between the two KS daughter tracks and the IP (dr)
• The azimuthal angle between the momentum vector and decay vertex vector of a KS candidate (d)
• The distance between the two KS daughter tracks at their point of interception (z_dist)
• The flight length of a KS candidate in the x-y plane (fl)
IP SKp
SKV
< 30 mrad
two angleshould be small
Long flight
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0 reconstructed from two
Event Reconstruction – 0
• Energy of photon
• Invariant mass
• Mass-constrained fit 2
• Angle between 0 in the B rest frame and momentum in the 0 rest frame
E > 50MeV in Barrel, E > 100MeV in End-cap region
0.115< M(0 )<0.152 GeV/c2
cos <0.95
2<50
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Event Reconstruction – B0KS0
2 kinematic variables
Y(4S)
B0
B0
e+e-
CMS : Υ(4S) rest frameEbeam: Beam energyEB : Reconstructed B energyPB : Reconstructed B momentum
2 2( ) ( )
CMS CMSbc beam B
CMS CMSB beam
M E p
E E E
Signal MC
2009/2/24 公聴会 28
Event Reconstruction – Continuum suppression
e+e-(4S)BB (Spherical)
e+e- qq (Jet-like)
Main background ⇒ continuum (e+e-qq)
Fox-Wolfram moments : Event shape variables cosB : polar angle of B in CMS
BBqq
0.3<LS/B
Combine variables
BBqq
signal retained : 91%
background rejected : 71%
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Event Reconstruction – B0
Signal
Branching Fraction Measurement
2009/2/24 公聴会 31
Branching Fraction – Basic formula
0( )
SigNB
N B
:SigN Signal yield
: Reconstruction efficiency0( ):N B 0Number of B
( )( )
( )Data
CFMC
ii
i
Consider difference between data and MC :
We use 657×106 BB events
Reconstruction efficiency obtained from Signal MC
2009/2/24 公聴会 32
Branching Fraction – Experimental Results
bcM E /S BL
• Mbc, E, LS/B distribution from data
How to estimation Signal yield?
need to determine the signal shape & background shape
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Branching Fraction – Signal PDF
• Mbc, E, LS/B correlated – Can’t fit with 1D×1D×1D functions– Use 3D histogram PDF
sig /P ( , , ) bc S BH M E L
bcM E /S BL
Probability Density Function
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Branching Fraction – BB background PDF
• Mainly from BK, BK* ,,,
• Use 3D histogram PDF
/BBP ( , , ) bc S BH M E L
bcM E /S BL
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Branching Fraction – Total PDF
• qq background PDF
• Total likelihood
– Extended unbinned maximum likelihood fit
– Free parameters• Branching Fraction( ), Nbkg
• Argus shape, Poly1st shape• fqq
qq 1 /P ( )× ( )× ( )bc S BArgus M Poly E H L
( )
1
1!
Sig BkgN N Ni i i
Sig Sig Bkg qq qq qq BBi
eL N P N f P f P
N
0( )
SigNB
N B
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Branching Fraction – Fit result
E/S BL
BBBB qq
Signal
bcM
- 0.15<E<0.1 ; 0.7<L
5.27 <Mbc <5.29 ; 0.7<L - 0.15<E<0.1 ; 5.27< Mbc <5.29
Signal yield ; 634±37
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Branching Fraction – Result
B (B0K00)=[8.72 (stat) (syst)]×10 - 6- 0.50 + 0.51
- 0.40 + 0.46
0 00 0 0 S
0S
B(B K )B(B K )=
B(K K )
-6(4.36 0.25) 10
0.5
• Agree with current world average B(B0K00)=(9.8±0.6)×10 - 6
• Systematic Uncertainty
Time-dependent CP Analysis
2009/2/24 公聴会 39
Evaluate CP asymmetry from the t and q
CP Analysis – Principle of Measurement
Smeared by • Detector resolution• Wrong flavor tag effect
t (ps)
True t
0
0
Tag
Tag
B
B
B
B
t (ps)
0
0
Tag
Tag
B
B
B
B
Measured t
e e(4 ) S
0B
0B
CP eigenstate
0B
0B
0B fCPfCP
Flavour tag ;qFlavour tag ;q ( )
z
ct
~
( )
z
ct
~
z ~ 0.425
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– IP (interaction point) tube constraint fit
B decays vertices are reconstructed using the tracks coming from their decay particles using kinematical vertex fit.
CP Analysis – Vertex reconstruction
• No primary tracks from B vertex
• Extrapolate KS track to the Interaction Point
• Events without the vertex can still be used to measure A
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• For the BCP-Btag coherency originated from (4S) decay, Btag flavour determines BCP flavour (B0 or B0) at Btag decay time.
• Flavour information is determined from Btag decay products.
CP Analysis – Flavour Tagging
0
0
tag-side
1 ( )
1 ( )
q B
q B
Inclusive Leptons:high-p l intermed-p l+
K - → B0
K + → B0
- → B0
+ → B0
High-p l - → B0 High-p l + → B0
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We understand the following resolution components:
– Detector resolution:
– Effect of non-primary particles:
– Kinematic approximation of B0 flight:
B
Bz
Btag
D
lNon-primary particles
Brec
Ks
0
Residuals=Zrec ― Zgen
Bgen
[NIM A533: 370,2004]
CP Analysis –Treatment of vertex resolution
CP TagR R
NPR
KR
( ) NP KCP TagR RR t RR
Tag-side only
2009/2/24 公聴会 43
0
0
| | /
( , )
1 (1 2 )( cos sin ) ( )4
B
Sig
t
CP d CP d
B
P t q
eq w q w A m t S m t R t
( , ; , ) ( , ) (1 ) ( , )CP CP Sig Sig Sig BkgP t q A S f P t q f P t q
Maximum Likelihood Fit
Wrong flavor tag effect
.
1
( , ) ( , ; , ) 0Neve
CP CP i i CP CPi CP CP
L LL A S P t q A S
S A
CP Analysis – Maximum Likelihood fit 1
Resolution
Signal Background
Signal fraction Next slide
Signal PDF
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qq qq
| |/
qqP ( ) (1 ) ( )2
t
et f f t R
Sideband region
+Beff
+ - + -
+eff
| |/
B B B BB
P ( )2
t
et R
+effB
=1.66 ±0.08 psCharged B MC
B+B-
00 SigP ( , ) P B B
t q 0 1.21±0.21 pseffB
Neutral B MC
B0B0
Finite lifetime of D meson and short lived particles
5.20 < Mbc < 5.26 GeV/c2
0.05 < E < 0.20 GeV
CP Analysis – Maximum Likelihood fit 2
Background PDF
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CP Analysis – Inclusion of B0KL0
2
No t information,
1 1( ; ) 1 (1 2 )
2 [1 ( ) ]
sig CP CPd
P q A q w q w Am t
• KLis detected by KLM hits and/or ECL hits
• Cannot measure decay vertex of KL0
• Time-integrated PDF is used to measure ACP
Wrong flavor tag effect
2009/2/24 公聴会 46
CP Analysis – Final Results
A N
B 0 N
B 0
NB 0 N
B 0
ACP = + 0.14±0.13(stat)±0.06(syst)
SCP = + 0.67±0.31(stat)±0.08(syst)eff
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CP Analysis – Systematic Uncertainty
Summary & Discussion
2009/2/24 公聴会 49
ACP = + 0.14±0.13(stat)±0.06(syst)
SCP = + 0.67±0.31(stat)±0.08(syst)
Summary
We have measured Branching Fraction and Time-dependent CP Violation Parameters of B0K00 from 657×106 BB pairs
B (B0K00)=[8.72 (stat) (syst)]×10 - 6- 0.50 + 0.51
- 0.40 + 0.46
eff
SCP ; Consistent with B0J/K0 (bc transition modes)
ACP ; No evidence for direct CP violation
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Discussion – Compare with other experiment
SCP
BaBar 0.55 0.20 0.03Belle 0.67 0.31 0.08Average 0.57 0.17CCP = - ACP
BaBar 0.13 0.13 0.03Belle 0.14 0.13 0.06Average 0.01 0.10
eff
Isospin sum rule expectation
Our ACP result ; 1.9 deviation from isospin sum rule
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Discussion – Future prospect
Now 605fb-1 Now
605fb-1
Super-B target 50ab-1
Super-B target 50ab-1
Total errorStatistical errorSystematic error
Expected error
• ACP=0.05 need for evidence direct CP violation
• Need further reduction of systematic errors
• Evidence for mixing-induced CP violation expected with 2ab-1
2ab-1
Backup
2009/2/24 公聴会 53
Previous result
515 32 0KS signal515 32 0KS signal
Raw symmetry
good tags
= 0.05 0.14(stat) 0.05(syst)
= 0.33 0.35(stat) 0.08(syst)
-
+ CP
CP
A
S
535MBBPRD 76 (2007) 090113
465 MBB
= 0.13 0.13(stat) 0.03(syst)
= +0.55 0.20(stat) 0.03(syst)
-CP
CP
A
S
2008 preliminary result
556 32 0KS signal556 32 0KS signal
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Correction Factor – How to get?( )
( )( )
DataCF
MC
ii
i
Consider difference between data and MC :
IP SKp
SKV
< 30 mrad
two angleshould be small
Long flight
0
0 0
0
0
0 0
00
0
00
0
( 3 )(2 ) ( 3 )
( )(2 )( )
( 3 )(2 ) ( 3 )
( )(2 )( )
(2 )( )
(2 )
Data
Data MC
DataMC
Data
Data
Data MC
DataMC
MC
DataCF
MC
NNNN
NN
NN
Compare KS selection criteria ON/OFF
BF
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Branching Fraction – Correction factors from B+K+0
• Using B+K+0, we obtain the correction factors for the signal shape
• Account for difference between data and MC
/( , , ) ( , )Sig Sig bc S B bcP H M E L S M E
Before correction After correction
E E
Mbc shift Mbc smear E shift E smear
-0.0006
±0.0004
0.904
±0.013
+ 0.007
±0.001
0.999 ±0.089
BBBB qq
Signal
5.27 <Mbc <5.29 ; 0.7<LS/B
BF
公聴会 56
• For the BCP-Btag coherency originated from (4S) decay, Btag flavour determines BCP flavour (B0 or B0) at Btag decay time.
• Flavour information is determined from Btag decay products.
-1 0 +1-1 0 +1Btag=B0 B0B0 B0
qr qr
Flavour information is parametrized using qr: q: MC determined discrete flavor (1 or -1) r: MC determined flavour ambiguity (0~1)
Eve
nt f
ract
ion
CP Analysis – Flavour Tagging
2009/2/24 公聴会 57
cos
OF SFd
OF SF
P Pm t
P P
tmw d cos)21(
Wrong tag effect for each r binis estimated from time-dependent B0-B0 mixing fitusing self-tagged control samples:B0D(*)-l+ D(*)-D*
Events are classified into 6 categories according to the r.
Measured time-dependent flavour asymmetry
Dilution due to mis-flavour tagging
Wrong flavour tag effect
Time-dependent flavour asymmetry:
Measured asymmetry:
CP Analysis – Wrong flavour tag effect
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Using event-by-event vertex quality: z and , (Vertex fit2 along the z direction only) Detector resolution is modeled using MC as:
- Event-by-event Double Gaussian
- Sigma of Gaussian ~ Error z
- Scaled up according to
Ent
ries
t (ps)
Neutral B
Lifetime fit
(B0 = 1.530 +/- 0.009ps [PDG2006])
All resolution parameters aredetermined from fits to data/MC samples.
We verify the fitted lifetime usingthis model is self-consistent to the onewe used in the sin21,A fit. J/ψK0 DataB0 = 1.544 +/- 0.016(stat) ps
CP Analysis – Resolution function
2009/2/24 公聴会 59
TCPV PDF
+ - + -
+ - 00
qq qq
qq B B B B
qq B B
/
qq qq /
P
1(1 )[ (1 )
21
(1 )(1 ) P2
1(1 )(1 )(1 )P ]
2( , , , )
( , , )
CPV
Out Sig sig Sig
Sig
Sig Out OutB B
Sig Sig bc S B
bc S B
f f P f f P
f f f
f f f f P
f f M E L r
f f M E L
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Tag Side Interference
• Interference between CKM-favored and CKM-suppressed BD transitions in tag side.
• Estimated by pseudo-experiments whose parameters are obtained from B0D*l samples
• No interference for semi-leptonic decay in tag side (It was included till last year)
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657 ± 37 KS0 signal
Mbc
657 MBB465 MBB
E
556 ± 32 KS0 signal
LS/B
Signal yield extraction– Comparison
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• Mbc calculated from direction of KL cluster• E can’t be calculated
Selection criteria– B0KL0
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Signal yield extraction– B0KL0
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Background subtraction
657 MBB
• First measurement
• KL0 signal
285 ± 52 (stat) ± 57 (syst)
3.7 (including systematics)
Mbc L
BB+qqqq
Signal
Signal yield extraction– B0KL0
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657 MBB465 MBB
A = +0.14 0.13 0.06S = +0.67 0.31 0.08
A = +0.14 0.13 0.06S = +0.67 0.31 0.08
A= C = 0.13 0.13 0.03S = +0.55 0.20 0.03
A= C = 0.13 0.13 0.03S = +0.55 0.20 0.03
KS0+KL0KS0
tCPV results– Comparison