Measurements of sin2α from B-Factories

Measurements of sin2αfrom B-Factories

Masahiro MoriiHarvard University

The BABAR Collaboration

BEACH 2002, Vancouver, June 25-29, 2002

BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 2

Introduction! CP violation in B0 decays gives access to the angles

of the Unitarity Triangle

! sin2β measured to ±0.08 dominated by B0 → J/ψ KS

! Where does this leave us?

*

2 *

*

1 *

*

3 *

arg

arg

arg

td tb

ud ub

cd cb

td tb

ud ub

cd cb

V VV V

V VV V

V VV V

α φ

β φ

γ φ

= ≡ −

= ≡ −

= ≡ − ρ 1

η

α

βγ

See D. Marlow’s talk


Unitarity Triangle and sin2β

! Measured sin2β agrees with indirect constraints! Shrinking σ(sin2β) alone

may not reveal new physics

! Must measure the sidesand the other angles

*

*ud ub

cd cb

V VV V α

βγ

*

*td tb

cd cb

V VV V

Next possibility at the B Factories?


Measuring sin2α! Time-dependent CP asymmetry in B0 → fCP is

0 0

0 0

( ( ) ) ( ( ) )sin( ) cos( )

( ( ) ) ( ( ) ) CP CP

phys CP phys CPf d f d

phys CP phys CP

B t f B t fS m t C m t

B t f B t fΓ → − Γ →

= ∆ + ∆Γ → + Γ →

2

2 2

12 Im1 1

CP

CP CP

CP

ff f

f

AqS Cp A

λλ λλ λ

−= − = =

+ +

CKM phase appears here

b cc

s

W

cbVb u

u

dW

ubV

0SB J Kψ→ 0B π π+ −→

sin 2β sin 2α

Easy!


Penguin Pollution

! Unlike J/ψ KS, π +π − mode suffers from significant pollution from the penguin diagrams with a different weak phase

! To estimate αeff – α, we need:! P/T ratio – about 1/3 from BR(B → Kπ)/BR(B → ππ)! δ = strong phase difference between P and T

b uu

dW

ubVeffsin 2α

b uu

d

W

T = Tree P = Penguin

gtbV *

tdV


Belle BR×106BABAR BR×106Mode0

0 1.11.0

0 0 0

2 0 2 5.4 0.7 0.5 5.1 1.1 0.41 1 0 4.1 0.8 7.0 2.2 0.80 1 1 3.3 5.6

T C P

BBB

α α α

π ππ ππ π

+ −

+ + +−

→ ± ± ± ±→ ± ± ±→ − < <

Taming Penguins! Take advantage of the isospin symmetry

T C PT C Pα α α= ⋅ + ⋅ + ⋅A

b uu

d

ddb u

u

d

dd

b u

u

d

dd

All preliminary


B0 " π0π0 Branching Ratio

! BABAR: Preliminary 54 fb-1

! BR(π 0π 0) < 3.3×10–6 (90% CL)

! Belle: Preliminary 31.7 M BB! 2.2σ “bump” in the signal! Fitted BR= (2.9 ± 1.5 ± 0.6)×10–6

! BR(π 0π 0) < 5.6×10–6 (90% CL)

! CLEO: 9.13 fb-1

! BR(π 0π 0) < 5.7×10–6 (90% CL)

BELLE

Expect first observation in the near future


0 0

0 0

( ) ( )sin( ) cos( )

( ) ( )tag tag

d dtag tag

N B N BS m t C m t

N B N B ππ ππ

−= ∆ ∆ − ∆ ∆

+

z c tγβ∆ = ∆

CP Asymmetry in B0 " π+π−

! Same method as sin2β measurements! Difference: the direct CP term cannot be neglected

9 GeV3.1 GeVϒ4S

Btag

BCP

Tagusing l±, K±

Moving withβγ = 0.55

e− e+

π −

0 0orB B

π +CP final

state

# of events with 0 0 tagB B


Challenges

! Specific to B0 → π +π −

! Topology B0 → h+h− simple to reconstruct! Particle ID must separate π ± from K±

! DIRC (BABAR) and Aerogel (Belle)! Significant background from continuum

! Event-shape variables " Fisher discriminant

! Common with other CP measurements! Flavor tagging! Vertex reconstruction

! And, of course, as much as possibledt∫L

( ) ( )BR BR Kπ π π+ − + −<


B0 Reconstruction

! mbc (or mES) and ∆E peak cleanly for the two-body signal! Kπ and KK peaks shifted in ∆E " Additional discrimination

2 2ES bm E pππ= −

!

CME E Eππ∆ = −

bE =

π+π− MC

off-resonance data

π+π− MC

Κ+π− MC

BELLE BELLE

2 2( 2) ( )bc CMm Eπ π+ −= − +p p 2CME E E E

π π+ −∆ = + −


Whole event is jettyThe other B decays spherically

Continuum Background! Most of the background come from continuum

! Use event shape variables that represent “jettiness” to suppress them

π +

π −

π +

π −

Signal udsc background

Examples


Sphericity Angle! Angle θS between the sphericity axes of the B candidate and

the rest of the event

! Cut at 0.8 removes 83% ofthe continuum background

π +

π −

SθBABAR

π+π− MC

cos Sθ

background

0.8

reject


Fisher Discriminant! BABAR uses the “CLEO” Fisher

! Momentum flow in 9 cones around the candidate axis

! Output of Fisher goes into the likelihood fit

π +

π −

π+π− MC

D0π+ data

Bkg MC

mES sideband data


Bkg MC

off-res. data

Fisher Discriminant! Belle’s Fisher discriminant uses:

! Modified Fox-Wolfram moments! B flight direction

! Output is turned intoa likelihood ratio R! Cut at 0.825 removes

95% of continuumbackground

π+π− MC

D0π+ data

reject


Event Sample – BABAR! BABAR 55.6 fb-1 preliminary

! π+π− enhanced for these plots with a cut on Fisher

Kπ + continuum

16 715 9( ) 124N π π+ − + +

− −=


Event Sample – Belle! Belle 41.8 fb-1

Kπ

Continuum

( ) 78.5 13.8(stat)N π π+ − = ±


Maximum Likelihood Fit! Start from the physics function:

! Fold in ∆t resolution and mis-tag probabilities! Multiply by PDFs for mES, ∆E! BABAR uses particle ID and Fisher in the fit

! Belle uses these variables in event selection! Add PDFs for background (Kπ, KK, continuum)

! Feed the candidates and turn the crank…

[ ]( ) 1 sin( ) cos( )4

t

d def t m C m tS tππ ππ

τ

τ

∆

±

−

∆ = ∆ ∆ ∆ ∆± ∓0

0

tagtag

BB

+−


BABAR

BELLE

CP Fit Results

! BABAR and Belle disagree by >2σ! Belle 1.2σ outside the physical

boundary

! Is there any problem?! Crosscheck systematics

–0.02 ± 0.29 ± 0.07Cππ

–0.01 ± 0.37 ± 0.07Sππ

Belle (hep-ex/0204002)BABAR (preliminary)0.38+0.161.21 0.27 0.13

+− − −0.310.94 0.090.25

+− ±−

Belle usesA Cππ ππ≡ −

2 2 1S Cππ ππ+ =


CP Asymmetries – BABAR

! π+π− enhanced for these plots with a cut on Fisher! No significant asymmetry


CP Asymmetries – Belle

! Rate difference (= Cππ)! ∆t-dependent asymmetry (= Sππ and Cππ)

Subtract bkg0 tagB0 tagB


Crosschecks! Both experiment made extensive crosschecks, e.g.

! Asymmetry in background?! Look for asymmetries

in Kπ or mass sideband! Vertex resolution of the

2-body decays?! Measure B lifetime with ππ, Kπ! Measure mixing with Kπ

! Likelihood values and errors?! Toy Monte Carlo studies

BELLE


Monte Carlo Fit Test! Generate ~1000 “toy” experiments

! Belle used (−0.7, −0.7)for the central values

! Fit and compare:! Likelihood values! Pull distributions! Errors

! Lowest probability: 5.4%

BABAR

Sππ Cππ

σ(Sππ) σ(Cππ)

MC

Measured

Sππ Cππ

BABAR

BELLEBELLE

Measured

Everything looks reasonable


BABAR

BELLE

Interpretation

! How well do we know α?(*Gronau and Rosner, PRD65, 093012)

! Average BABAR and Belle! Assume β = 26°, P/T = 0.28

–0.49 ± 0.21

–0.66 ± 0.26

Average*

–0.02 ± 0.29 ± 0.07Cππ

–0.01 ± 0.37 ± 0.07Sππ


+− − −0.310.94 0.090.25

+− ±−

NB: Large uncertainty


! Measured Sππ ±1σcorresponds to

Interpretation

Gronau and RosnerPRD65, 093012

[89 ,138 ]α ∈ ° °

Indirect:

BABAR + Belle

3021(97 )α +

−= °

Accuracy comparableto the indirect constraints

We are starting to measure α


Summary

! BABAR and Belle measured sin2αeff using B0 → π +π −

! Direct constraint on α is reaching useful accuracy! Things to watch out for:

! sin2αeff with higher statistics " Resolve “discrepancy”! BR(B0 → π 0π 0) " Better bound on αeff – α

–0.02 ± 0.29 ± 0.07Cππ

–0.01 ± 0.37 ± 0.07Sππ


+− − −0.310.94 0.090.25

+− ±−

Measurements of sin2α from B-Factories

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