Transcript Branching Fractions and CP Asymmetries in B0→pp,Kp,KK

CP-Violating Asymmetries in Charmless
B Decays: Towards a measurement of a
On behalf of the BaBar Collaboration
International Conference on High Energy Physics
Amsterdam, July 24-31, 2002
James D. Olsen
Princeton University
CP asymmetries in p+p- and K+p-
Submitted to Phys Rev (hep-ex/0207055)
Decay rates for p+p0 and p0p0
hep-ex/0207065 and hep-ex/0207063
CP asymmetries in r+p- and r+Khep-ex/0207068
CP Violation in the Standard Model


CP symmetry can be violated in any field theory with at least one
irremovable complex phase in the Lagrangian
This condition is satisfied in the Standard Model through the threegeneration Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix
 d   Vud Vus Vub  d 
 
  
 s   Vcd Vcs Vcb  s 
 b  V V V  b 
   td ts tb  
Unitarity Triangle
VudVub* + VcdVcb* + VtdVtb*  0
*
VudVub
*
VtdVtb
a(f2)

B0pp, rp
g(f3)
B0DK
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(f1)
The angles (a,,g) are related to
CP-violating asymmetries in
specific B decays

One down, two to go…
sin 2 BaBar  0.741  0.067  0.033
0
* B J/yKS
VcdVcb
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Observing CP violation at the U(4S)


At the (4S), BB pairs are produced in a
coherent P-wave
Three observable interference effects:



B
B
0
f CP
0
CP violation in mixing (|q/p| ≠ 1)
(direct) CP violation in decay (|A/A| ≠ 1)
(indirect) CP violation in mixing and decay (Iml ≠ 0)
q AfCP
λ fCP  
p AfCP
Observable in time evolution of B0B0 system (assume DG0)
0
phys
 f CP , Dt )  G4 e
- G Dt
0
f ( B phys
 f CP , Dt )  G4 e
- G Dt
f (B
direct CP violation  C ≠ 0
indirect CP violation → S ≠ 0
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1 + S
1 - S
Sf 

cos( Dm Dt )
f CP
sin( Dmd Dt ) - C f CP cos( Dmd Dt )
f CP
sin( Dmd Dt ) + C f CP
2 Im λ fCP
1 + | λ fCP |
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2
Cf 
d
1 - | λ fCP | 2
1 + | λ fCP | 2
3
CP Violation in B0 → p+pWith Penguins (P):
Tree (T) Level:
lpp
Vtb*Vtd Vud* Vub

VtbVtd* VudVub*
mixing
lpp  e
decay
i ig
2 ia 1+ P / T e e
1+ P / T e i e -ig
Cpp  sin(  )
2
Spp  1 - Cpp
sin( 2a eff )
lpp  e 2ia
Cpp  0
Need branching fractions for
p+p-, pp0, and p0p0 to get a
from aeff → isospin analysis
Spp  sin( 2a )
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Overview of Analyses

Analysis issues: charmless B decays


Rare decays! BR ~ 10-5-10-6 → need lots of data (PEP-II)
Backgrounds:





Need to determine vertex position of both B mesons → silicon
Need to know the flavor of “other” B → particle ID
We use maximum likelihood (ML) fits to extract signal yields and
CP-violating asymmetries



Ambiguity between p and K → need excellent particle ID (DIRC)
Time-dependent CP analysis issues:


Large background from e+e- → qq → need background suppression
Modes with p0 suffer backgrounds from other B decays
Kinematic and topological information to separate signal from lightquark background
Particle ID to separate pions and kaons
The data sample corresponds to 87.9 million BB pairs
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K/p Separation with the DIRC


Cherenkov angle c used in the maximum likelihood fit to
distinguish pions and kaons
Resolution and K-p separation measured in data
Kaon sample
p
hypothesis
K
hypothesis
D*+  D0p + , D0  K -p +
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Analysis of B → pp, Kp, KK

Analysis proceeds in two steps:



Time-independent fit for yields and Kp charge asymmetry
Time-dependent fit for Spp, and Cpp
Kinematically select B candidates with mES, DE
*2
mES  Ebeam
- p*B2

*
DE  EB* - Ebeam
Suppress qq background with Fisher discriminant
F  0.53 - 0.60   pi* + 1.27   pi* cos( i* )
i

*
i
Fit yields and charge asymmetry
- +
+ N
(
K
p
)
N
(
K
p )
Kp
ACP 
N ( K -p + ) + N ( K +p - )
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p*
2
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p-
p+
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Branching Fraction Results
Submitted to Phys Rev (hep-ex/0207055)
87.91.1 million BB
Mode
Yield
BR (10-6)
B0 → p+p-
157  19
4.7  0.6  0.2
B0 → K+p-
589  30
17.9  0.9  0.7
B0 → K+K-
1 8
 0.6 (90% CL)
ACP(Kp)
- 0.102  0.050  0.016
Preliminary
Projections in mES and DE
pp
pp
Kp
pp
Kp
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Kp
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Vertex Reconstruction
Dz 1
Dt 
g c
Exclusive Brec reconsctuction
BREC Vertex
BREC daughters
Interaction Point
Average Dz resolution ~ 180mm
Example in B → pp
Beam spot
z
Dz resolution dominated by
tag side → same resolution
function as charmonium
(sin2) sample
BTAG Vertex
BTAG direction
e+e- → qq
TAG tracks, V0s

Resolution function parameters obtained
from data for both signal and background


B → pp
Signal from sample of fully reconstructed B
decays to flavor eigenstates: D*(p, r, a1)
Background from data sidebands
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-
B Flavor Tagging

c
s
K-
New tagging algorithm with physics-based neural
networks




b
Inputs include leptons, kaons, slow-p (from D*), and
high-momentum tracks
Outputs combined and categorized by mistag prob (w)
5 mutually exclusive categories:





Lepton – isolated high-momentum leptons
Kaon I – high quality kaons or correlated K- and slow-p+
Kaon II – lower quality kaons, or slow-p
Inclusive – unidentified leptons, poor-quality kaons, highmomentum tracks
Untagged – no flavor information is used
~7% improvement in Q  e(1-2w)2
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Tagging in Charmless B Decays
81/fb B→ h+h- sample split by tagging category

Tagging efficiency is very
different for signal and bkg


Strong bkg suppression in
categories with the lowest
mistag prob (Lepton/Kaon)
Different bkg tagging
efficiencies for pp, Kp, KK
Tagging Efficiencies (%)
Background
Category
Signal
pp
Kp
KK
Lepton
9.1
0.5
0.4
0.6
Kaon I
16.6
8.9
12.7
7.8
Kaon II
19.8
15.5
19.4
14.4
Inclusive
20.1
21.5
19.2
21.7
Untagged
34.4
53.6
48.3
55.6
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Validation of Tagging, Vertexing, and ML Fit
Fit projection in sample of Kp-selected events

Kp decays are self-tagging


T = tag charge
Q = kaon charge
fTK,Qp (Dt ) 

e
- Dt / 
4
1 - TQ(1 - 2w) cos(Dmd Dt )
Float  and Dmd in same
sample used to extract CP
asymmetries:
  (1.56  0.07)ps
Dmd  (0.52  0.05)ps
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-1
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CP Asymmetry Results
Fit projection in sample of pp-selected events
Preliminary
Spp  0.02  0.34  0.05
qq + Kp
Cpp  -0.30  0.25  0.04
Submitted to Phys Rev (hep-ex/0207055)
App
(
)
(
)
(Dt ) 
N (B ) + N (B )
0
0
N Btag
- N Btag
0
tag
0
tag
 Spp sin( Dmd Dt ) - Cpp cos( Dmd Dt )
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Cross-checks

Inspect pp-selected sample


2-param fit consistent with full fit
asymmetry vs. mES




Asymmetry in yields consistent with
measured value of Cpp, but does not
suggest large direct CP violation
Toy MC generated over all allowed
values of Spp and Cpp


App vs. mES in sample of pp-selected events
Expected errors consistent with data
No significant bias observed
Validated in large samples of signal
and background MC events
Systematic errors dominated by
uncertainty in PDF shapes
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signal bins
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Taming the Penguins: Isospin Analysis
Gronau and London, Phys. Rev. Lett. 65, 3381 (1991)


The decays B p+p-, p+p0, p0p0 are related by isospin
Central observation is that pp states can have I = 2 or 0
(gluonic) penguins only contribute to I = 0 (DI = 1/2)

p+p0 is pure I = 2 (DI = 1/2) so has only tree amplitude
 (|A+0|  |A-0|)


Triangle relations allow determination of penguininduced shift in a
2 eff  2 + pp
a
a 
But, need branching
fractions for all three
decay modes, and for
B0 and B0 separately
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The Base of the Isospin Triangle: B+→p+p0

Analysis issues:


Usual charmless two-body;
large qq background, p/K
separation
Potential feeddown from r+p
r+p-
Minimize with tight cut on DE
Fit region
Simultaneous fit to pp0/Kp0
Mode
B+ → p+p0
e+e- → qq
B+ → K+p0
Yield
ACP
+1.0
5
.
5
125+-23
- 0.03+-00..18
- 0.9  0.6
21
17  0.02
+1.2
239+-21
22 12.8 -1.1  1.0 - 0.09  0.09  0.01
Preliminary
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BR (10-6)
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hep-ex/0207065
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Next Side Please: B0→p0p0

Analysis issues:



Small signal!
rp0 feeddown
Background suppression:


Event shape and flavor tagging
to reduce qq
Cut on M(p+p0) and DE to
reduce rp0 background, then fix
in the fit
Data after cut on probability ratio (e ~ 20%)
hep-ex/0207063
Np 0p 0  23+-10
9
Preliminary
B( B 0  p 0p 0 )  3.6 10 -6 @ 90% C.L.
p0 p0
rp0
Significance including systematic errors = 2.5s
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Setting a Bound on Penguin Pollution

Can still get information on a with only an
upper bound on p0p0:

For example: Grossman-Quinn bound (assume
only isospin)
sin (a eff
2

0
1
0
0 0
BR ( B  p p ) + BR ( B  p 0p 0 )
-a)  2
BR ( B   p p 0 )
 0.61@ 90% C.L.

Correlations and systematic errors included
a eff - a  51 @ 90% C.L.


Many other bounds on the market

Charles, Gronau/London/Sinha/Sinha, etc…
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CP-Violating Asymmetries in B0 → r+p-, r+KR. Aleksan et al., Nucl. Phys. B361, 141 (1991)

Opportunity and challenges


In principle, can measure a directly, even with penguins
Much more difficult than p+p



Three-body topology with neutral pion (combinatorics, lower efficiency)
Significant fraction of misreconstructed signal events and backgrounds
from other B decays
Need much larger sample than currently available to extract a cleanly
We perform a “quasi-two-body” analysis:



Select the r-dominated region of the p+p-p0/K+p-p0 Dalitz plane
Use multivariate techniques to suppress qq backgrounds
Simultaneous fit for r+p- and r+K-
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Not a CP eigenstate, (at least) four amplitudes contribute:
Time-integrated asymmetry:
0
B  r +p -
N (r + h- ) - N (r - h+ )
ACP 
N (r + h- ) + N (r -h+ )
B 0  r +p -

rh
Time evolution includes:
( S rh + QDS rh ) sin( Dmd Dt )
CP
B r p
0
-
+
CP
0
0
0
- r+-p + + B  r +p B 0  r +p - + B  r -p + and B 0 

(Crh + QDCrh ) cos( Dmd Dt )
B r p
Q is the r charge
rK is self-tagging:
direct CP violation → ACP and C ≠ 0
CrK  0, DCrK  -1, S rK  0, DS rK  0
indirect CP violation → S ≠ 0
Fit for:
DC and DS are insensitive to CP violation
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rp
rK
ACP
, ACP
, Crp , DCrp , S rp , DS rp
20
Analysis



signal
mES, DE, Neural Net (NN), c, Dt
Components


Signal rp and rK
Misreconstructed signal events




misreconstructed signal
Mostly due to wrong photon(s)
B backgrounds


qq
Multi-dimensional ML fit
from b → c and charmless B decays
Same lifetime as signal
e+e- → qq
Fix B background yields, fit for
signal yields and CP asymmetries
Validation:
  (1.59  0.12)ps
Dmd  (0.51  0.09)ps -1
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Yields and Charge Asymmetries
N rp  413
N rK  147
+34
-33
B B and q q background
+ 22
- 21
qq
hep-ex/0207068
rp
ACP
 -0.22+-0.08
0.08 ( stat )  0.07( syst )
rK
+0.14
ACP
 0.19
( stat )  0.11( syst )
B Band
q q background
0.14
Preliminary
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B0 rp time-dependent asymmetry
hep-ex/0207068
C r p  0.45+-0.18
0.19 ( stat )  0.09( syst )
S r p  0.16+-0.25
0.25 ( stat )  0.07( syst )
Preliminary
DC r p  0.38
+0.19
-0.20
( stat )  0.11( syst )
B B only
B B and q q
DS r p  0.15+-0.25
0.25 ( stat )  0.05( syst )
Systematic error dominated by
uncertainty on B backgrounds
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Summary


Hyperactive effort within BaBar to constrain,
measure, and otherwise determine a
Charmless two-body decays:


No evidence for large direct or indirect CP violation in pp
Beginning to piece together the necessary inputs to the
isospin analysis



Measurements of decay rates for pp0 and p0p0 (upper limit)
Too early for a significant constraint
Charmless three-body decays

First measurement of CP asymmetries in rp and rK
The next few years will be interesting indeed!
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