CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a
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Transcript of CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a
CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of
James D. OlsenPrinceton University
International Conference on High Energy Physics
Amsterdam, July 24-31, 2002
On behalf of the BaBar Collaboration
CP asymmetries in and K+
Decay rates for and
CP asymmetries in and K
Submitted to Phys Rev (hep-ex/0207055)
hep-ex/0207065 and hep-ex/0207063
hep-ex/0207068
ICHEP 2002 J. Olsen 2
CP Violation in the Standard Model
b
s
d
VVV
VVV
VVV
b
s
d
tbtstd
cbcscd
ubusud
Unitarity Triangle
033.0067.0741.02sin BaBar
VtdVtb*
VudVub*
VcdVcb*
B,
BJ/KSBDK
* * * 0ud ub cd cb td tbV V V V V V
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 three-generation Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix
The angles (,,) are related to CP-violating asymmetries in specific B decays One down, two to go…
ICHEP 2002
ICHEP 2002 J. Olsen 3
Observing CP violation at the (4S)
At the S), BBpairs are produced in a coherent P-wave
Three observable interference effects: CP violation in mixing (|q/p| ≠ 1) (direct) CP violation in decay (|A/A| ≠ 1) (indirect) CP violation in mixing and decay (Im ≠ 0)
CP
CP
CP
ff
f
Aqλ
p A
Observable in time evolution of BB system (assume )
CP
CP
2f
f 2f
1 | λ |
1 | λ |C
CP
CP
ff 2
f
2Im λ
1 | λ |S
direct CP violation C ≠ 0
indirect CP violation → S ≠ 0
)cos()sin(1),(
)cos()sin(1),(
40
4
0
tmCtmSetfBf
tmCtmSetfBf
dfdft
CPphys
dfdft
CPphys
CPCP
CPCP
0B
0B
CPf
ICHEP 2002 J. Olsen 4
CP Violation in B→
)2sin(1
)sin(
eff2
/1
/12
CS
C
e ii
ii
eeTP
eeTPi
With Penguins (P):
mixing
*
*
ubud
ubud
VV
VV
*
*
tdtb
tdtb
VV
VV
decay
Tree (T) Level:
)2sin(
0
2
S
C
e i
Need branching fractions for , , and to get from eff → isospin analysis
ICHEP 2002 J. Olsen 5
Overview of Analyses
Analysis issues: charmless B decays Rare decays! BR ~ 10-10→ need lots of data (PEP-II) Backgrounds:
Large background from ee → qq → need background suppression Modes with suffer backgrounds from other B decays
Ambiguity between and K → need excellent particle ID (DIRC) Time-dependent CP analysis issues:
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 Kinematic and topological information to separate signal from light-
quark background Particle ID to separate pions and kaons
The data sample corresponds to 87.9 million BB pairs
ICHEP 2002 J. Olsen 6
K/Separation with the DIRC Cherenkov angle c used in the maximum likelihood fit to
distinguish pions and kaons Resolution and K- separation measured in data Kaon sample
K hypothe
sis
hypothe
sis
KDDD 00* ,
ICHEP 2002 J. Olsen 7
Analysis of B → , K, KK
Analysis proceeds in two steps: Time-independent fit for yields and K charge asymmetry Time-dependent fit for S, and C
Kinematically select B candidates with mES, E
Suppress qq background with Fisher discriminant
Fit yields and charge asymmetry
2*2beamES
*BpEm *
beam* EEE B
i i
iii ppF2*** )cos(27.160.053.0
*
p*
)()(
)()(
KNKN
KNKNAKCP
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Branching Fraction Results
Mode Yield BR (10-6) ACP(K)
B→
B→ K
B→ KK
016.0050.0102.0
2.06.07.4
7.09.09.17
CL) (90% 6.0
19157
30589
81
KK
Projections in mES and EPreliminary
K
87.91.1 million BBSubmitted to Phys Rev (hep-ex/0207055)
ICHEP 2002 J. Olsen 9
Vertex Reconstruction
Resolution function parameters obtained from data for both signal and background Signal from sample of fully reconstructed B
decays to flavor eigenstates: D*(, , a) Background from data sidebands
Beam spot
Interaction Point
BREC Vertex
BREC daughters
Exclusive Brec reconsctuction
BTAG direction
TAG tracks, V0s
zBTAG Vertex
B →
Example in B →
ee→ qq
z resolution dominated by tag side → same resolution function as charmonium (sin2) sample
Average z resolution ~ 180m
c
zt
1
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B Flavor Tagging
New tagging algorithm with physics-based neural networks Inputs include leptons, kaons, slow-(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-
Kaon II – lower quality kaons, or slow- Inclusive – unidentified leptons, poor-quality kaons,
high-momentum tracks Untagged – no flavor information is used
b sc
K-
~7% improvement in Q w
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Tagging in Charmless B Decays
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 , K, KK
81/fb B→ hh sample split by tagging category
Category Signal K 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
Tagging Efficiencies (%)
Background
ICHEP 2002 J. Olsen 12
Validation of Tagging, Vertexing, and ML Fit
)cos()21(14
)(/
, tmwTQe
tf d
tKQT
K decays are self-tagging T = tag charge Q = kaon charge
Float and md in same sample used to extract CP asymmetries:
1-ps)05.052.0(
ps)07.056.1(
dm
Fit projection in sample of K-selected events
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)cos()sin(
)(00
00
tmCtmS
BNBN
BNBNtA
dd
tagtag
tagtag
CP Asymmetry ResultsFit projection in sample of -selected events
04.025.030.0
05.034.002.0
C
Sqq + K
Preliminary
Submitted to Phys Rev (hep-ex/0207055)
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Cross-checks Inspect -selected sample
2-param fit consistent with full fit asymmetry vs. mES
Asymmetry in yields consistent with measured value of C, but does not suggest large direct CP violation
Toy MC generated over all allowed values of S and C
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
A vs. mES in sample of -selected events
signal bins
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Taming the Penguins: Isospin Analysis
The decays B, , are related by isospin Central observation is that states can have I = 2 or 0
(gluonic) penguins only contribute to I = 0 I = 1/2) is pure I = 2 I = 1/2) so has only tree amplitude
(|AA) Triangle relations allow determination of penguin-
induced shift in 22 eff
Gronau and London, Phys. Rev. Lett. 65, 3381 (1991)
But, need branching fractions for all three decay modes, and for B and Bseparately
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The Base of the Isospin Triangle: B→
Analysis issues: Usual charmless two-body;
large qq background, /K separation
Potential feeddown from
Minimize with tight cut on E
Mode Yield BR (10-6) ACP
B→
B→ K
2321125
6.05.5 0.19.0
2122239
0.18.12 2.11.1
02.003.0 18.017.0
01.009.009.0
Simultaneous fit to /K
Preliminary
Fit region
ee→ qq
hep-ex/0207065
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Next Side Please: B→
Analysis issues: Small signal! feeddown
Background suppression: Event shape and flavor tagging
to reduce qq Cut on M() and E to
reduce background, then fix in the fit
C.L. %[email protected])(
236000
10900
BB
N
Significance including systematic errors = 2.5
Data after cut on probability ratio ()
Preliminary
hep-ex/0207063
ICHEP 2002 J. Olsen 18
Setting a Bound on Penguin Pollution
Can still get information on with only an upper bound on : For example: Grossman-Quinn bound (assume
only isospin)
Many other bounds on the market Charles, Gronau/London/Sinha/Sinha, etc…
C.L. %[email protected]
)(
)()(21
)(sin0
000000
eff2
BBR
BBRBBR
C.L. %90@51eff
Correlations and systematic errors included
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CP-Violating Asymmetries in B→ ,
Opportunity and challenges In principle, can measure directly, even with penguins Much more difficult than
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 cleanly
We perform a “quasi-two-body” analysis: Select the -dominated region of the K Dalitz plane Use multivariate techniques to suppress qq backgrounds Simultaneous fit for and
R. Aleksan et al., Nucl. Phys. B361, 141 (1991)
ICHEP 2002 J. Olsen 20
)()(
)()(
hNhN
hNhNA hCP
0000 and BBBB
)cos()(
)sin()(
tmCQC
tmSQS
dhh
dhh
direct CP violation → ACP and C ≠ 0
indirect CP violation → S ≠ 0
C and S are insensitive to CP violation
CP
0
B
0
B
0B
0B
Not a CP eigenstate, (at least) four amplitudes contribute:
Time evolution includes: CP
Time-integrated asymmetry:
Q is the charge
K is self-tagging:
0 ,0 ,1 ,0 KKKK SSCC
Fit for:
SSCCAA KCPCP , , , , ,
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Analysis
1-ps)09.051.0(
ps)12.059.1(
dm
Multi-dimensional ML fit mES, E, Neural Net (NN), c, t
Components Signal and K Misreconstructed signal events
Mostly due to wrong photon(s) B backgrounds
from b → c and charmless B decays Same lifetime as signal
ee → qq
Fix B background yields, fit for signal yields and CP asymmetries
Validation:
signal
misreconstructed signal
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Yields and Charge Asymmetries
2221
3433
147
413
KN
N
background and qqBB
0.140.140.19 ( ) 0.11( )K
CPA stat syst 0.140.140.19 ( ) 0.11( )K
CPA stat syst
0.080.080.22 ( ) 0.07( )CPA stat syst 0.080.080.22 ( ) 0.07( )CPA stat syst
Preliminary
background and qqBB
hep-ex/0207068
ICHEP 2002 J. Olsen 23
BB00 time-dependent asymmetry time-dependent asymmetry
0.180.19
0.250.25
0.45 ( ) 0.09( )
0.16 ( ) 0.07( )
C stat syst
S stat syst
0.180.19
0.250.25
0.45 ( ) 0.09( )
0.16 ( ) 0.07( )
C stat syst
S stat syst
0.190.20
0.250.25
0.38 ( ) 0.11( )
0.15 ( ) 0.05( )
C stat syst
S stat syst
Systematic error dominated by uncertainty on B backgrounds
Preliminary
qqBB and
only BB
hep-ex/0207068
ICHEP 2002 J. Olsen 24
Summary
Hyperactive effort within BaBar to constrain, measure, and otherwise determine
Charmless two-body decays: No evidence for large direct or indirect CP violation in Beginning to piece together the necessary inputs to the
isospin analysis Measurements of decay rates for and (upper limit) Too early for a significant constraint
Charmless three-body decays First measurement of CP asymmetries in and K
The next few years will be interesting indeed!