Transcript Document

CP Asymmetry
in B0->π+π– at Belle
•B0->π+π– and CP asymmetry in CKM
• e+e– -> (4S) at KEKB and Belle
• Belle data
Measurement of CP asymmetry in B0->π+π–
Interpretation vis-a-vis CKM
•Future
Kay Kinoshita
University of Cincinnati
Belle Collaboration
B0->π+π– involves f2 (a) of CKM:
CKM: matrix of W-quark couplings - 3x3, unitary
Vub*Vud
Vtb*Vtd
One condition of unitarity:
+ 1 + V *V = 0
Vcb*Vcd
cb
cd
-(rih)
Represented in
complex plane as
"unitarity triangle"
(rh)
1.0
f
VudVub*
VcdVcb*
a
f

0.0

-(1-r-ih)
Vtd Vtb*
VcdVcb*

f1
1.0
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B0->π+π–
2 paths, each w/wo mixing:
Tree
Penguin
VtdVtb*
f2 =arg
–VudVub*
(rh)
1.0
aVub*Vud
aVtb*Vtd
mixing+ "
aVtb*2Vtd2VubVud*
f
VudVub*
VcdVcb*
f

Vtd Vtb*
VcdVcb*
f1
0.0
1.0
aVtb*2Vtd2VtbVtd*
Bottom line: CP-asymmetric time-dependent rate from x-terms
"direct" asym
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Uncertainty: relative amplitudes of Tree, Penguin
• if T dominates,
• if P, T comparable,
Direct CP violation
Aππ=0, Sππ=sin2f2
Aππ≠0, Sππ~sin(2f2+2q)•2/(|l|2+1)
difference of
strong phase
≠1 if direct CP
violation
Previous Belle result {PRL 89, 071801 (2002)} (42 fb–1 ~45M B pairs)
Sππ= –1.21 +0.38+0.16
–0.27–0.13
Aππ= +0.94 +0.25+0.09
–0.31 –0.09
Each 2.9s from zero;
note physical region is
Now:
• more data - 78 fb–1
• improved analysis - tracking,
Dt resolution, event selection
• statistical analysis
(total 126 fb–1, ~1.3x108 B events)
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B production:
}B
(4S)
BB threshold
+e-
D
t=0
e+ B2
e
CP=–1,
conserved
-> (4S) { = 0.425}
Dz≈Dtc
~200 µm
B1
CP mode
@ t=Dt
As with sin2f1 via J/yK:
• reconstruct CP mode
• tag flavor
• reconstruct vertices
• unbinned max.
likelihood fit to Dt
flavor tag @t=0: e, µ, K±, ...
first B decay
(t=0), break CP
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Belle detector
Charged tracking/vertexing
- SVD: 3-layer DSSD Si µstrip
– CDC: 50 layers (He-ethane)
Hadron identification
– CDC: dE/dx
– TOF: time-of-flight
– ACC: Threshold Cerenkov (aerogel)
Electron/photon
– ECL: CsI calorimeter
Muon/KL
– KLM: Resistive plate counter/iron
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…the people
274 authors, 45 institutions
many nations
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B0->π+π– reconstruction
final selection:
DE E*cand–E*beam: 0±0.057 GeV(E*beam s 1/2/2) {Kπ shift –45 MeV}
Mbc (E*beam2-p*cand2 )1/2: 5.271 –5.287 GeV/c2(Beam-constrained)
… but less clean than B0->J/yKs:
• "physics bg" B0->K+π– => hadron ID, kinematics
dE/dx, TOF, Aerogel – “positive ID” eπ=91%, eK=10%
• continuum => event shape {qq “jet-like” vs BB “spherical”)
Fisher discriminant from modified Fox-Wolfram moments
B candidate direction relative to beam axis
Construct Likelihood ratio LR=LBB/[LBB+Lqq], 2 selections:
LR > 0.825 {eBB=53%, eqq=5%}
0.825 > LR > LRmin (cut depends on flavor tag classification)
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B0->π+π– Candidates
760 in signal box - 391 B0, 369 B0
0.825>LR>LRmin
LR>0.825
ππ
ππ
ππ:106±16
Kπ:41±10
qq:128±6
Total signal 163±24
ππ:57±8
Kπ:22±6
qq:406±17
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Flavor tagging: same as for sin2f1
l-
or
π–
l+
b
c
s
D*+ + D0
π
high-p lepton (p*>1.1 GeV): b->lnet K charge b->K–
medium-p lepton, b->c-> l+
*+->D0π+}
–
soft
π
b->c{D
K
hard π b->{c}π–X
* multidimensional likelihood, e>99%
• wrong-tag fraction w
classify events based on
expected w (MC) - 6 bins.
(B0 mixing amplitude in data) =>
• effective efficiency
= e(1-2w): net (28.8±0.5)%
mixing amplitude <-> w
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Dz vertex reconstruction: same as for sin2f1
sDt~1.43 ps (rms)
z
m
m-
K-
Dz
K-
Resolution function:
B0->Dp-, D*p-, D*r-, validate via lifetime
J/yKS, J/yK*0
<= tB0 = 1.551±0.018 ps
(PDG02: 1.542±0.016)
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More checks of Dt resolution+flavor tag
tB0 = 1.42±0.14 ps
(PDG02: 1.542±0.016)
tB0 = 1.46±0.08 ps
B0->K+π– mixing
Dmd = 0.55±0.07 ps–1
(PDG02: 0.489±0.008)
Fitted bg agrees w sideband
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Check for flavor bias
Look where zero asymmetry expected:
"Sππ"= –0.045±0.033
"Aππ"= -0.015±0.022
SKπ= –0.03±0.11
AKπ= +0.08±0.16
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Fitting for CP asymmetry
Same technique as with sin2f1
• unbinned maximum likelihood fit
• resolution function event-by-event: tracking, misreconstruction,
physics, approximation of Dt=Dz/c
• wrong tag fraction w, backgrounds
Fit for Aππ, Sππ :
root
Diluted
+Kπ (set AKπ = 0)
+resolution + bg
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Fit Results
LR>0.825
display Dt projection (78 fb–1 ~85M B pairs)
-5
0
5
-5
Background
subtracted
raw
asymmetry
0
5
Likelihood not parabolic
-> statistical errors
estimated numerically
via MC ensemble
+0.08
Sππ= –1.23±0.41 –0.07
Aππ= +0.77±0.27±0.08
(stat)
(sys)
still outside physical region => investigate …
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Fit Results: statistical analysis
• MC ensemble - 30k expts, 760 events ea, Aππ=0.569, Sππ=–0.822
probability of being outside physical boundary =60.1%
"
further (in s) from (0,0)=16.6%
data
physical boundary
Aππ
x
check linearity:
generated
vs fitted Aππ, Sππ
Sππ
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Confidence regions
•
•
•
•
Feldman-Cousins frequentist approach.
Acceptance regions from MC ensembles.
Systematic errors included.
Confidence Level (CL) calculated at each point.
interpret as evidence for
(3.4s) } CP non-conservation in B0->π+π–
(2.2s) } hint of direct
CP non-conservation
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Constraints on the CKM angle f2
• Sππ, Aππ depend on 4 parameters:
f2, f1[21.3°-25.9°], |P/T|[0.15-0.45], d
-> plot confidence contours in (f2, d)for various |P/T|
e.g.
|P/T|=0.3
Find:
f1 =23.5°
78°≤ f2 ≤ 152°
(95% C.L.)
insensitive to d
f2
d
From other CKM (CKM fitter group, 2002):
78.3°≤ f2 ≤ 121.6° (95% C.L.)
=> consistent
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Summary
Belle, 2000-2:
• peak L= 9.5x1033cm–2s–1 - nearly at design (1x1034cm–2s–1)
• passed 100 fb-1 in Oct. 2002
• with 78 fb–1 on (4S), sensitive to large values of sin2f2
• measure CP asym in B0->π+π–
constraints on f2, consistent with other CKM constraints.
hint of direct CP non-conservation
result submitted to PRD.
Next
• ->150 fb–1 by summer, 500 fb–1 by 2005
• Luminosity >@ design
• the CP challenge: stay tuned on f2
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additional slides
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Systematic uncertainties*
Aππ
Sππ
source
+error
-error
+error
-error
Background fractions
+0.058
-0.048 +0.044
-0.055
Vertexing
+0.044
-0.054 +0.038
-0.012
Fit bias
+0.016
-0.021
+0.052
-0.020
Wrong tag fraction
+0.026
-0.021
+0.015
-0.016
tB, Dmd, AKπ
+0.021
-0.014
+0.022
-0.022
Resolution function
+0.019
-0.020 +0.010
-0.013
Background shape
+0.003
-0.015
+0.007
-0.002
Total
+0.08
-0.08
+0.08
-0.07
* blind analysis: actual estimations done before seeing fit result.
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Constraints on the CKM angle f2
A( B 0  p p - )  -(| T | eid T eif3 ) | P | eid P ),
id T
A( B  p p )  -(| T | e e
0
lpp

-
- if 3
) | P | e
id P
i (d f3 )
1

|
P
/
T
|
e
 e if 2
1 | P / T | ei (d -f3 )
convention taken from
), M.Gronau & J.L.Rosner
Phys Rev D65, 093012 (2002)
4 parameters
Spp  [sin 2f2  2 | P / T | sin(f1 - f2 ) cosd
- | P / T |2 sin 2f1 ] / Rpp ,
App  -[2 | P / T | sin(f1  f2 ) sin d ] / Rpp ,
Rpp  1 - 2 | P / T | cos(f1  f2 ) cosd  | P / T |2
|P/T|
f1
d  d P - dT
0.15-0.45 (representative) Theory ~0.3
21.3 - 25.9deg (Belle & BaBar combined)
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