1Mahalaxmi Krishnamurthy Beauty 2003

Measurement of time dependentCP violation in B → φKs

Measurement of time dependentCP violation in B → φKs

Mahalaxmi KrishnamurthyUniversity of Tennessee

BaBar CollaborationBeauty 2003

CMU, Pittsburgh

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• Decays dominantly through penguin amplitudes (gluonic & EW), tree small:

e.g.

• CP violation from interference of decay amplitudes into CP eigenstate: B0 → φKs and B0 → B0-bar → φKs

b-> sss Decays

• Estimate hadronic pollution (rescattering) using flavor SU(3) relations (30% breaking) between (measured) decay rates of charmless quasi-twobody decays e.g. [Grossman, Ligeti, Nir, Quinn, hep-ph/0303171] worst case scenario today: | sin2β(φKs) – sin2β(J/ψKs) | < 25%

• Decay exclusively through penguin : | sin2β(φKs) – sin2β(J/ψKs) | < 4%

With more data, smaller bounds: 1 ab-1 expect ~10%

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BaBar Detector

110 fb-1

Lum

inos

ity f

b-1

deliv

ered

reco

rded

Off Peak

EMC

DCH

SVT

IFR

1.5T Magnet

DIRC

e-

e+

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Measuring sin(2ββββ) at the Asymmetric B-Factory

Time dependent asymmetry:

µ+, e+, K++++

e- : 9 GeV

φ φ φ φ

K+

K−

KS

π−

π+

Btag

∆z

e+ 3 GeV B-Flavor Tagging

ExclusiveB Meson

Reconstruction

BCP : φKs, φKl, J/ψ Ks ..

BCP

Btag : Xl+, XK+, Xπs-

(B flavor sample)

)sin()cos()()( tmStmCtRR

RRta ddf ff ∆+∆−=

+−= C S

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Event Variables

Energy-substituted mass:MES= [s/2 + pΥpB)2/EΥ

2-pB2]1/2

Energy difference: ∆∆∆∆E = EB* - s0.5/2

signal events : 70 ±±±±9.0purity : 61 ±±±±0.1%

MES (GeV/c2) ∆E (GeV)

Number of signal events is consistent with published branching fraction.

(EΥ, pΥΥΥΥ), (EB,pB) 4 momenta of Υ(4S) and B in laboratory frame, EB* reconstructed B energy in center of mass frame

Eve

nts

/(2.

5 M

eV/c

2 )

abs(∆E)< 0.06 GeV 5.27<MES< 5.29 GeV/c2

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Event Variables

Helicity angle: - angle(K, B) in φ-rest frame

*Υ(4S) rest frame

Fisher discriminant: linear combination of - cosθT (thrust angle*) - cosθB (emission angle* w.r.t. z-axis) - momentum-weighted Legendre polynomials

side band data

SignalMC

F = a0cosθT+ a1cosθB + a2ΣpiL0(θi ) + a3ΣpiL2(θi )

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• Tagging performance evaluated from data using B flavor sample

• Tags grouped into 4 mutually exclusive categories:

Q = Σ ε(1-2ω)2

8.10.8 3.9 9.5Lepton

10.71.1 9.616.3Kaon I

7.33.419.319.4Kaon II

2.7 4.031.820.5Inclusive

65.7

ε (%) ω (%) ∆ ω (%)

28.7 ±±±± 0.7Total

Q (%)Categoryefficiency mistag rate tag difference

6.8

11.7

13.9

14.6

φKs signal events

Tagging

Tagging optimized for maximal value of Q

σσσσ(S) ~ 1/√√√√ N Q

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J/ψ Ks

φφφφKs

Vertexing (ΦΦΦΦK % J/ΨΨΨΨ K)

φ→ Κ+Κ−

J/ψ → l+l−

Opening angles quite different:

φ in B (Y(4S) )

K+

K-

θH

B->φ K : - large Q-value - helicity angle ~ cos2θ

B vertex z resolutions :Bcp ~ 50 µmBtag ~ 170 µm

Resolution dominated by the tag side

cosθH

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• Comparable σ(∆t) distributions in J/ψ Ks and φKs

• Background Resolution Function:

)}()1()/({

)];8,0()));((,([)(

tftExpf

tpspsGttscalebiasGtR

B

outliericore

∆•−+∆•⊗

∆+∆∆=∆

δτ

σ

∆∆∆∆t Resolution

• Signal Resolution Function:

);8,0()3,(

)));((,()1()(

tpspsGfpsbiasGf

ttscalebiasGfftR

outlieroutliertailtail

itagcoreoutliertail

∆++

∆∆−−=∆ σ

- Established by extensive test with Monte Carlo (MC) events- Final parameters obtained from B flavor data in simultaneous fit with CP data

σ(∆t) (ps)

J/ψKs φKs

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• Fix parameters (yields) that depend exclusively on CP sample• Float parameters that depend on B flavor sample• Total 40 parameters:

• Use 86200 B-flavor, 2138 φKs(π+π-)• Maximum likelihood fit:

L = L(MES) x L(∆E) x L(F ) x L(cosθH) x L(∆t)

4FractionsTag

Bkg. dilution asym.

Bkg. Dilution

Sig. dilution asym.

Sig. Dilution

Bkg. time evolution.

Bkg. Resolution

Sig. Resolution

S and C

40TOTAL

4

4

4

4

6

3

9∆t

2CP

Tested with J/ψ Ks data, MC data mockfit and Toy MC.

CP Fit Strategy

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charmonium modes world average

Zoom into the signal region (mES>5.27GeV, |∆E| < 0.06 GeV)

Time Dependent CP Fit for φφφφKs

Raw asymmetry:

WAS = 0.736 http://www.slac.stanford.edu/xorg/hfag

S = 0.45 ±±±± 0.43C = -0.38 ±±±± 0.37

statistical error only

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• Standard Model: expect S=0 and C=0

S = 0.23 ±±±± 0.24C = - 0.14 ±±±± 0.18

Time Dependent CP Fit for φφφφK+

signal events : 252 purity : 76 ±±±±0.1%

Use φK+ as control sample

MES (GeV/c2)

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Systematic Uncertainties

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Old result: Fit to φKs, Ks →π+π- and Ks → π0π0 sample combined(80 fb-1) S = -0.18 ± 0.51 C = -0.80 ± 0.38

Old Ks →π+π– only: S = -0.12 ± 0.52 C = -0.77 ± 0.41

Overlap between reprocessed (new) Run1+2 and old Ks →π+π– (80% overlap)

Old Ks →π+π– : S = 0.02 ± 0.55 C = -0.57 ± 0.44

New Ks →π+π– : S = 0.05 ± 0.51 C = -0.25 ± 0.48

New+Run3 Ks →π+π– : S = 0.45 ± 0.43(110 fb-1) C = -0.38 ± 0.37

The move ……..

∆∆∆∆S ~ 0.0∆∆∆∆C ~ 0.3

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The Understanding

% change in tag due to reprocessing : ~ 8%

~ 3 events flip tag around ∆t = 0

Low statistics channels are prone to fluctuations !Low statistics channels are prone to fluctuations !

old sample : F+ - F- = 5 eventsnew sample : if all 3 flip events are B0 or B0 tags (F+ - F- = 2 or 8 ) ∆C ~ ±±±±0.3 ∆S ~ 0.0- swap of the samples confirms the effect

=> change of result covered by statistical error.

-> F± integrated rate of B0/B0

tagged events-> average dilution <D>=0.61±0.04

-

cp

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More results

•B (B0→ φφφφK0) = (8.4 +1.5 ± 0.3) x 10-6

•B (B+ → φφφφK+) = (10.0 +0.9 ± 0.5) x 10-6

•B (B+ → φπφπφπφπ+) < 0.41 x 10-6

•Acp(B+ → φφφφK+) = 0.04 ± 0.09 ± 0.01

-1.3

-0.8

C (B+ → φφφφK+) = -0.14 ± 0.18 Acp = 0.05 ± 0.07

C (B0 → φφφφKs) = -0.38 ± 0.37 Acp = 0.14 ± 0.14

⇒ the two independent methods give the same result of Acp in φK+

in 80 fb-1

hep-ex/0309025

in 110 fb-1

time dependent:

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- Measured time dependent asymmetry in B0 -> φφφφKs(ππππ+π π π π -). - More CP events are needed by increase of luminosity and by including other channels e.g. φφφφKl, φφφφKs(ππππ 0π π π π 0).

- Expectation for the error in 300 fb-1 : σ σ σ σ (S) = 0.26 σ σ σ σ (C ) = 0.17

Summary / Outlook

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Current status of S

average

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S = 0.45 ±±±± 0.43

• 15% correlation

Time Dependent CP Fit for φφφφKs

S

C = -0.38 ±±±± 0.37

C