cp-violating asymmetries in charmless b decays: towards a measurement of james d. olsen princeton...

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


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


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


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


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* ,


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


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)


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


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


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


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


)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)


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


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


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


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


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


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)


)()(

)()(

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 , , , , ,


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

qq


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

qq

background and qqBB

hep-ex/0207068


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


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!

cp-violating asymmetries in charmless b decays: towards a measurement of james d. olsen princeton...

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