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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Study of the decay of B0 → ωK 0

S at Belle

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling February 29th, 2012 Physical Motivation Analysis of the Decay = B0 → ωK0

S

Summary and outlook

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Introduction to CP Violation

◮ Universe today is matter dominated ◮ Violation of CP = C(charge) ×P(parity) symmetry necessary to explain

the matter-antimatter asymmetry

◮ CP violation in the Standard Model: Cabbibo-Kobayashi-Maskawa (CKM)

mechanism

◮ CKM mechanism desribes the relation between the weak and the mass

eigenstates of quarks

◮ CKM mechanism expressed through a complex, unitary 3 × 3 matrix

CKM Matrix

  d s b  

weak

= VCKM   d s b  

mass

≡   Vud Vus Vub Vcd Vcs Vcb Vtd Vts Vtb     d s b  

mass

Vij : quark flavor transition couplings CKM mechanism not enough to explain all the missing antimatter

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

CP Violation in the Standard Model

Wolfenstein parametrisation

VCKM =   1 − λ2 λ Aλ3(ρ − iη) −λ 1 − λ2/2 Aλ2 Aλ3(1 − ρ − iη) −λ2 1   + O(λ4) λ = sin θC ≈ 0.22, θC: Cabibbo angle 4 free parameters: 3 mixing angles and 1 complex phase

1

φ

2

φ

1

φ

3

φ

|

cb *

V

cd

|V |

tb *

V

td

|V

(0,0) (1,0) ) η , ρ (

CKM matrix is unitary ⇒ Vud · V ∗

ub + Vcd · V ∗ cb + Vtd · V ∗ tb = 0

O(λ3) O(λ3) O(λ3) relevant for the B meson system Sides with similar size ⇒ large angles, precise determination of the observables (3 angles and 2 sides) possible problem over-constrained ⇒ leaves room for New Physics Decays via charmless b → sq¯ q (like B0 → ωK 0

S) transitions sensitive to φ1

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

CP Violation in the B Meson System

Time-dependent CP asymmetry

aCP(∆t, fCP) = NB0(∆t, fCP) − NB0(∆t, fCP) NB0(∆t, fCP) + NB0(∆t, fCP) = ACP cos(∆m∆t) + SCP sin(∆m∆t)

ACP measure for the direct CP violation B0 → fCP = B0 → fCP ACP = 1 SCP = 0 SCP measure for the indirect CP violation B0 → B0 → fCP = B0 → B0 → fCP ACP = 0 SCP = 1

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

CP Violation Measurement

mΥ(4S) = 10.58 GeV/c2 ≈ 2 × mB mB = 5.28 GeV/c2 B Meson production

◮ Υ(4S) resonance decays

almost exclusively into a B0B0 pair

◮ Υ(4S): JPC = 1−−

B: JPC = 0−− ⇒ B meson pair in a p-wave ⇒ asymmetric wave function ⇒ B mesons have opposite flavour B0B0 pair coherent

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

CP Violation Measurement

e e+

• Y(4S)

B B

l l

+

ν ν K-

• X

+

π π+

• π+

π

• tagside

CP side Δz

coherent B-B pair

high energy beam

t t

1 2

B0 or B0? → Look at the other B (tag-side): If l− ⇒ B0 on the tag-side and B0 on the CP-side If l+ ⇒ B0 on the tag-side and B0 on the CP-side ∆t measurement Asymmetric beam energies at the Belle experiment: Ee− = 8 GeV, Ee+ = 3.5 GeV ⇒ Boost in the center of mass system Measurement of ∆z ∼ 100 µm instead of ∆t ∼ ps Obtain ∆t = ∆z/cβγ

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

The Decay B0 → ωK 0

S t W − d

B0

b d

ω

d d K 0

S

s Vtb V ∗

ts

d

B0

b d

K 0

S

s u ω u Vub V ∗

us

Matrix elements for the two Feynman diagrams

◮ Mtree ∝ Vub · V ∗ us ∝ λ3 · λ ∝ λ4 ◮ Mpeng ∝ Vtb · V ∗ ts ∝ 1 · λ2 ∝ λ2

⇒ Decay is dominated by the penguin contribution Measurement of The branching fraction BR(B0 → ωK 0

S)

The CP parameters ACP and SCP = sin φeff

1

(pollution from the tree diagram)

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Physical Motivation

Why exactly the decay B0 → ωK 0

S? ◮ Theory predicts in the Standard

Model that sin 2φeff

1

from b → sq¯ q should be larger than for b → c¯ cs (sin 2φeff

1

− sin 2φ1 ǫ (0.0; 0.2))

◮ But the measurement may be

systematically lower, giving a hint of New Physics

◮ Could be caused by unknown new

particle in the loop carrying different weak phase

◮ Leads to a measured shift from

sin 2φ1 t ? d B0 b d ω d d K 0

S

s Vtb V ∗

ts

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Approach

Goal 1: Determination of BR(B0 → ωK 0

S), ACP and SCP

Goal 2: Minimize the statistical and and systematic uncertainties Approach to Goal 1

◮ Build an algorithm to reconstruct the events of interest ◮ Study the different backgrounds ◮ Build a model to separate the signal from the background

(multidimensional fit)

◮ Test the model

So far: Blind. Study only from Monte Carlo (MC) samples

◮ Apply model to the real data ◮ Determine BR(B0 → ωK 0 S), ACP and SCP and the uncertainties

Approach to Goal 2

◮ Build a better model than the previous analysis ◮ Use the full available data

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Prevoius Measurements of B0 → ωK 0

S B0B0-pairs BR(B0 → ωK 0

S)

ACP SCP Belle 388 × 106 (4.4+0.8

−0.7 ± 0.4) × 10−6

• Belle

657 × 106

• −0.09 ± 0.29 ± 0.06 0.11 ± 0.46 ± 0.07

BaBar 535 × 106 (5.4 ± 0.8 ± 0.3) × 10−6 −0.52+0.22

−0.20 ± 0.03

0.55+0.26

−0.29 ± 0.02

Challenging analysis BR(B0 → ωK 0

S) ∼ 10−6 small

Large background contribution Our method Use loose cuts on the observables to collect maximum signal Multidimensional fit to the observables including the event shape to separate signal and background

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Measurement of BR(B0 → ωK 0

S)

Extract BR(B0 → ωK 0

S) by a 6D extended unbinned maximum likelihood fit

Fit variables: ∆E, FB¯

B/q¯ q, m3π, H3π, q, ∆t

∆E = EBrec − Ebeam FB¯

B/q¯ q Fisher discriminant, event-shape dependent

q = 1 for B0 and q = −1 for B0 New in this analysis: H3π, powerful observable for background discrimination Multidimensional analysis ⇒ model for signal and background necessary

◮ signal ◮ misreconstructed signal ◮ continuum (e−e+ → q¯

q)

◮ charmed and charmless

B0B0 (B+B−) decays

◮ peaking background

(5π final states): B0 → D∗−π+, B0 → D−π+, B0 → D−ρ+ MC MC sideband data (high Erec, low Mbc =

• (E cms

beam)2 − (pcms B

)2) MC MC

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Toy MC studies for B0 → ωK 0

S

Test the model with Toy MC Expected number of events signal ∼ 230 q¯ q ∼ 5300 B ¯ B ∼ 130

Events / (0.005 GeV)

20 40 60 80 100 120 140 160 180 E (GeV) ∆

• 0.15
• 0.1
• 0.05

0.05 0.1 Residuals Normalised

• 2

2

Expectations for B(B0 → ωKS)

Fit

BF

1 2 3 4 5

• 6

10 ×

Events / (1e-07)

5 10 15 20 25 30 0.03 ± =2.49 µ 0.02 ± =0.25 σ

Uncertainty 9.2% Error scaled to final data sets Belle (previous): 13% , BaBar: 13% ⇒ Our method is better Pull distribution of B(B0 → ωKS)

Fit

Pull BF

• 5
• 3
• 1

1 3 5

Events / (0.2)

2 4 6 8 10 12 14 16 0.11 ± =-0.09 µ 0.08 ± =1.07 σ

No bias, correct error estimation

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Results from the Fit to the Data

Events / (0.005 GeV)

5 10 15 20 25 30 35 E (GeV) ∆

• 0.15
• 0.1
• 0.05

0.05 0.1 Residuals Normalised

• 2

2

Black: Full PDF Total background B ¯ B background

Preliminary Result from 135 × 106B ¯ B Pairs

B(B0 → ωK 0) = [4.94+1.28

−1.14] × 10−6

World average B(B0 → ωK 0) = [5.0 ± 0.6] × 10−6

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Toy MC studies for ACP and SCP

Expectations for ACP

Fit CP

A

• 1
• 0.5

0.5 1

Events / (0.04)

2 4 6 8 10 12 14 16 18 0.02 ± =-0.04 µ 0.01 ± =0.19 σ

Uncertainty ±0.19 Error scaled to final data set Belle (previous): ±0.24, BaBar: ±0.20 Pull distribution of ACP

Fit CP

Pull A

• 5
• 3
• 1

1 3 5

Events / (0.2)

2 4 6 8 10 12 14 16 0.1 ± =0.02 µ 0.07 ± =0.99 σ

No bias, correct error estimation Expectations for SCP

Fit CP

S

0.5 1 1.5

Events / (0.04)

2 4 6 8 10 12 14 16 18 0.03 ± =0.69 µ 0.02 ± =0.28 σ

Uncertainty ±0.28 Error scaled to final data set Belle (previous): ±0.38, BaBar: ±0.26 Pull distribution of SCP

Fit CP

Pull S

• 5
• 3
• 1

1 3 5

Events / (0.2)

2 4 6 8 10 12 14 16 18 20 0.11 ± =0 µ 0.08 ± =1.1 σ

No bias, correct error estimation

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Why will our statistical uncertainties be similar to BaBar’s final with almost twice the data? Our expected signal yield is NSig = 233 ± 15 events, BaBar’s final yield was NSig = 163 ± 18. BaBar has also more than twice the amount of continuum Ntotal = 17422 while

• urs is Ntotal = 6461.

The reason is our way of choosing the best B out of the possible event candidates: based on Mbc. BaBar can use best vertex without biasing their lifetime. Then they can use Mbc as a fit variable which provides better discrimination against background. Outlook:

◮ Find a way to add Mbc to the fit ⇒ 7D fit ◮ Further reduce the uncertainties by performing a simultaneous 7D fit to

the charged decay with the same kinematics B+ → ωK +

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Summary and outlook

◮ The decay B0 → ωK 0 S can provide us with knowledge of New Physics ◮ We have built a model which will provide better results than the previous

Belle analysis

◮ The method is about be improved even to further reduce the statistical

and systematic uncertainties for ACP and SCP

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle

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Physical Motivation Analysis of the Decay = B0 → ωK0 S Summary and outlook

Thank you for your attention

Veronika Chobanova, Jeremy Dalseno, Christian Kiesling Study of the decay of B0 → ωK0 S at Belle