CP Violation Measuring matter/anti-matter asymmetry with BaBar

Wouter Verkerke, UCSB

CP ViolationMeasuring matter/anti-matter asymmetry with BaBar

Wouter Verkerke

University of California, Santa Barbara


Outline of this talk

• Introduction to CP violation– A quick review of the fundamentals.

– CP-violating observables

• Experiment and analysis techniques– Accelerator and detector (PEP-II and BaBar)

– Event selection, measuring time dependent CP asymmetries

• Selection of (recent) BaBar CP violation results– The angle

– The angle

– The angle


Why is CP violation interesting?

• It is of fundamental importance– Needed for matter/anti-matter

asymmetry in the universe

– Standard Model CP-violation in quark sector is far too small to explain matter asymmetry in the universe

• History tells us that studying symmetry violation can be very fruitful

• CP violating processes sensitive to phases from New Physics

• Can CP-violation measurements at the B factories break the Standard Model in this decade?

– Measure phases of CKM elements in as many ways as possible


• In the Standard Model, the CKM matrix elements Vij describe the electroweak coupling strength of the W to quarks

– CKM mechanism introduces quark flavor mixing

– Complex phases in Vij are the origin of SM CP violation

The Cabibbo-Kobayashi-Maskawa matrix

u

d

t

c

bs

CP The phase changes sign under CP.

Transition amplitude violates CP if Vub ≠ Vub*, i.e. if Vub has a non-zero phase

Mixes the left-handed charge –1/3 quark mass eigenstates d,s,b to give the weak

eigenstates d’,s,b’.

3 2

2

3

=cos(c)=0.22


The Unitarity Triangle – Visualizing CKM information from Bd decays

• The CKM matrix Vij is unitary with 4 independent fundamental parameters

• Unitarity constraint from 1st and 3rd

columns: i V*i3Vi1=0

• Testing the Standard Model– Measure angles, sides in as many ways possible– SM predicts all angles are large

β

-i

-i

γ1 1

1 1 1

1 1

e

e

CKM phases (in Wolfenstein convention)

u

d

t

c

bs


Observing CP violation

• So far talking about amplitudes, but Amplitudes ≠ Observables.

• CP-violating asymmetries can be observed from interference of two amplitudes with relative CP-violating phase

– But additional requirements exist to observe a CP asymmetry!

• Example: process Bf via two amplitudes a1 + a2 = A.

weak phase diff. 0, no CP-invariant phase diff.

Bf

A

A

Bf

a1 a1

a2

a2

A=a1+a2A=a1+a2

+

-

|A|=|A| No observable CP asymmetry


Observing CP violation

• Example: process Bf via two amplitudes a1 + a2 = A. weak phase diff. 0, CP-invariant phase diff. 0

BfBfA=a1+a2

A=a1+a2

+-

|A||A| Need also CP-invariant phase for observable CP violation

a1 a1

a2

a2

AA


CP violation: decay amplitudes vs. mixing amplitudes

• Interference between two decay amplitudes gives two decay time independent observables

– CP violated if BF(B f) ≠ BF(B f)

– CP-invariant phases provided by strong interaction part.

– Strong phases usually unknown this can complicate things…

• Interference between mixing and decay amplitudes introduces decay-time dependent CP violating observables

– Bd mixing experimentally very accessible: Mixing freq md0.5 ps-1, =1.5 ps

– Interfere ‘B B f’ with ‘B f’

– Mixing mechanism introduces weak phase of 2 and a CP-invariant phase of /2, so no large strong phases in decay required

2 md 2B

N(B

0)-

N(B

0)

N(B

0)+

N(B

0)


ACP(t) from interference between mixing+decay and decay

• Time dependent CP asymmetry takes Ssin(mdt)+Ccos(mdt) form

• C=0 means no CP violation in decay process

• If C=0, coefficient S measures sine of mixing phase

mixing decay

If only single real decay amplitude contributes

20

02

0

0

)(

)(

)(

)( iif e

fBA

fBAe

fBA

fBA

p

q

00 BpBqB


CKM Angle measurements from Bd decays

• Sources of phases in Bd amplitudes*

• The standard techniques for the angles:

*In Wolfenstein phase convention.

Amplitude Rel. Magnitude Weak phase

‘bc’ Dominant 0

‘bu’ Suppressed

B=2 (mixing) Time dependent

B0 mixing + single bc decay

B0 mixing + single bu decay

Interfere bc and bu in B± decay.

The distinction between and measurements is in the technique.

β

-i

-i

γ1 1

1 1 1

1 1

e

e

bu

td


The PEP-II B factory – specifications

• Produces B0B0 and B+B- pairs via Y(4s) resonance (10.58 GeV)

• Asymmetric beam energies– Low energy beam 3.1 GeV

– High energy beam 9.0 GeV

• Boost separates B and B and allows measurement of B0 life times

• Clean environment– ~28% of all hadronic interactions is BB

BB threshold

(4S)


The PEP-II B factory – performance

• Operates with 1600 bunches– Beam currents of 1-2 amps!

• Continuous ‘trickle’ injection– Reduces data taking interruption

for ‘top offs’

• High luminosity– 6.6x1033 cm-2s-1

– ~7 BB pairs per second

– ~135 M BB pairs since day 1.

• Daily delivered luminosity still increasing

• Projected luminosity milestone– 500M BB pairs by fall 2006.

Wouter Verkerke, UCSBSilicon VertexDetector (SVT)

Drift chamber (DCH)

ElectromagneticCalorimeter (EMC)

1.5 T Solenoid

InstrumentedFlux Return (IFR)

SVT: 5 layers double-sided Si. DCH: 40 layers in 10 super- layers, axial and stereo.

DIRC: Array of precisely machined quartz bars. .

EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID, 0 and reco.

IFR: Layers of RPCs within iron. Muon and neutral hadron (KL)

The BaBar experiment

• Outstanding K ID• Precision tracking (t measurement)• High resolution calorimeter• Data collection efficiency >95%

Detector forInternally reflectedCherenkov radiation(DIRC)


Silicon Vertex Detector

Beam pipe

Layer 1,2Layer 3

Layer 4Layer 5

Beam bending magnets

Readoutchips


Čerenkov Particle Identification system

• Čerenkov light in quartz– Transmitted by internal reflection– Rings projected in standoff box

– Thin (in X0) in detection volume, yet precise…


Selecting B decays for CP analysis

• Exploit kinematic constraints from beam energies– Beam energy substituted mass has better resolution than invariant mass

– Sufficient for relatively abundant & clean modes

(mES) 3 MeV

(E) 15 MeV

mes>5.27 GeV

N = 1506Purity = 92%

mes (GeV)mes

E

2

SKccB )(0


Measuring (time dependent) CP asymmetries

• B0B0 system from Y(4s) evolves as coherent system– All time dependent asymmetries integrate to zero!

• Need to explicitly measure time dependence

– B0 mesons guaranteed to have opposite flavor at time of 1st decay

• Can use ‘other B0’ to tag flavor of B0CP at t=0

B-Flavor Tagging

Vertexing

t=1.6 ps z 250 mz

170 m

z 70 m

z/c

Tag-side vertexing~95%efficient

Exclusive B Meson

Reconstruction


Flavor tagging

Leptons : Cleanest tag. Correct >95%

Kaons : Second best. Correct 80-90%

b c

e

W

b c

e+

W+

b

Wc s

u

d

K

W+b

W+c s

u

d

K+

W

Full tagging algorithm combines all in neural network

Four categories based on particle content and NN output.

Tagging performance

= 28%

Determine flavor of Btag BCP(t=0)from partial decay products

efficiency mistake rate


B0(t) B0(t) ACP(t) = Ssin(mdt)+Ccos(mdt)

sin2

Dsin2

Putting it all together: sin(2) from B0 J/ KS

• Effect of detector imperfections

– Dilution of ACP amplitude due imperfect tagging

– Blurring of ACP sine wave due to finite t resolution

• Measured & Accounted for in simultaneously unbinned maximum likelihood fit to control samples

– measures t resolution and mistag rates.

– Propagates errors

Actual sin2 result on 88 fb-1

Imperfect flavor tagging

Finite t resolution

t t


• Interference between mixing and single real decay– Interfering amplitudes of comparable magnitude

the observable asymmetry is large (ACP of order 1)

• Extraordinarily clean theory prediction (~1% level)– Single real decay amplitude all hadronic uncertainty cancel

– ACP(t) = sin(2) sin(md t)

• Experimentally easy– ‘Large’ branching fraction O(10-4)

– Clear signature (J/ l+l- and KS +-)

B-factory ‘flagship’ measurement: sin2 from J/ KS

B0 Mixing……followed by………Decay Decay

B0

b

d ds

ccW+

Vcb

Vcs

J/

Ks

*

cc

d

s

W+

Vcb

Vcs

J/

Ks

*


‘Golden’ measurement of sin2sin2 = 0.76 0.074

Combined result (88 fb-1, 2001)sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030

(stat) (syst)

B0 (cc) KS (CP=-1)

B0 (cc) KL (CP=+1)

sin2 = 0.72 0.16

No evidence for cos(mt) term


Constraints on the apex of the Unitarity Triangle.

Standard Model interpretation

Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001

= (1-2/2) = (1-2/2)


Standard Model interpretation

Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001

Latest results including the Belle experiment.

One solution for is very consistent with the other constraints.

4-fold ambiguity because we measure sin(2), not

1

2

4

3

The CKM model for CP violation has passed its first precision test!

There is still room for improvement: measurement is statistics dominatedSummer ’04 data 2-3 x 88fb-1


B-factory measurements of sin2

• Going beyond the ‘golden’ modes– Consistency requires S=sin2, C=0

for all B0 decay modes for which the weak phase is zero.

– Decay modes dominated by the bs penguin may meet these criteria

– Measure ACP(t) from interference between mixing + bs decayand bs decay

• Loop diagrams are sensitive to contributions from new physics– Look for deviations of S=sin2

Standard model expectation for sin(2) from bs penguins

B0K0 B0’K0 B00K0

Experimentally best modes:

SM contributions that spoil S = sin2• u-quark penguin (weak phase = !)

but relative CKM factor of ~0.02• u-quark tree (different phase)

u /

u /

*Grossman, Ligeti, Nir, Quinn. PRD 68, 015004 (2003) and Gronau, Grossman, Rosner hep-ph/0310020

I

II

III

(I) (I, II & III) (II & III)

SM sin2 from SU(3)

B0K0 <0.25

B0’K0 <0.35

B00K0 <0.20

these limits will improvewith additionaldata


bs penguin measurements

Mode BF(Bf)

x10-6

iBFi

x10-6

Reco. Efficiency

Purity Tagged signalEvents

81fb-1 115fb-1

J/Ks 440 36.0 44% 97% 940

’Ks 33 10.6 23% ~60% 110

Ks 4 1.4 42% ~80% ~34 ~48

0Ks 6 4.1 17% ~50% ~83

Experimentally more difficult• Branching fractions smaller, more irreducible background

B0 ’KS B0 KS B0 KS0


sin2 from bs penguin measurements

’ ’Ks

BaBar 0.02 0.34 0.03

Ks

BaBar 0.45 0.43 0.07

bs penguin average

Babar 0.27 0.22

0Ks

BaBar 0.48 (+0.38) 0.11–0.47

sin2 from B0 (cc) KS


sin2 from bs penguin measurements

’ ’Ks

BaBar 0.02 0.34 0.03Belle 0.43 0.27 0.05Ave 0.27 0.21

Ks

BaBar 0.45 0.43 0.07Belle –0.96 0.50 (+0.09)Ave –0.14 0.33

K+K-Ks non-resonantBelle 0.51 0.26 0.05 (+0.18)

bs penguin average

Babar and Belle 0.27 0.15

–0.00

–0.11

0Ks

Babar 0.48 (+0.38) 0.11–0.47

sin2 from B0 (cc) KS

(My naïve averages)


sin2 : bs penguin modes

• Current naïve world averages

S = 0.27 ± 0.15 (~3 below J/Ks S = 0.74 ± 0.05).

C = 0.10 ± 0.09

• Still very early in the game– Measurements are statistics limited.

Errors smaller by factor 2 in 2-3 years.

– Standard Model pollution limits from SU(3) analysis will also improve with more data.


The angle from B

• Determination of : Observe ACP(t) of B0 CP eigenstate decay dominated by bu

– Interference between mixing+bu decay and bu decay

– Textbook example is B0 +.

• If the above bu tree diagram dominates the decay

ACP(t)=sin(2)sin(mdt).

bu decay

Vub

B0 Mixing

sin2


The angle - the penguin problem

• Turns out the dominant tree assumption for is bad.– There exists a penguin diagram for the decay as well

– Magnitude of penguin can be estimated from B K+- (dominated by SU(3) variation of this penguin)

– Penguin amplitude is large, contribution to B could be ~30%!

• Including the penguin component (P) in

• Coefficients from time-dependent analysis

penguin decay

tree decay Vub

Vtd/Vts

s

Ratio of amplitudes |P/T| and strong phase difference can not be reliably calculated

)22sin(1,sin 2

CSC

Unknown phase shift


Disentangling the penguin: determining 2

• Gronau & London: Use isospin relations

– Measure all isospin variations of B

B0 +- , B0 +-, B0 00 , B0 00

B- -0 = B+ +0

– Weak phase offset 2 can bederived from isospin triangles

• Complicated…

-


Disentangling the penguin: the Grossman-Quinn bound

• Easy alternative to isospin: Grossman-Quinn bound – Look at isospin triangles and construct upper limit on

– Minimum required input: BF(B 0) and limit on BF(B0 00)

– Works best if B0 00 is small

– Experimental advantage: no flavor tagging in B00

• Measure B0 00!

‘~10-5’ ‘~10-6’

)()(

)()()2/(sin

00

0000002

BBBB

BBBB


the Grossman-Quinn bound on for B0

• B0 00 is observed! (4.2)

• GQ Bound using world averages

– 00: (1.9±0.5)x10-6

– ±0: (5.3±0.8)x10-6

• 00 large, thus GQ bound not very constraining– Isospin analysis required for 00!

Plots are after cut on signal probability ratio not including variable shown, optimized with S/sqrt(S+B) .

[BELLE: (1.7±0.6±0.2)x10-6, 3.4]

.).%90(47o LC

)22sin(1 2

CS


Alternatives to B for determination of

• There are other final states of bu tree diagram, e.g.– B (Dalitz analysis required)

– B (Vector-vector multiple amplitudes)

• B +- analysis– 3 helicity amplitudes: Longitudinal (CP-even), 2 transverse (mixed CP)

– Looks intractable, but entirely longitudinally polarized*!

– + is basically a CP-even state with same formalism as +.

*As predicted by G.Kramer, W.F.Palmer, PRD 45, 193 (1992). R.Aleksan et al., PLB 356, 95 (1995).


the Grossman-Quinn bound for B0

• The Grossman-Quinn bound for B0

(assuming full longitudinal polarization)

(BaBar)

(Belle)


Alpha summary

• The system: large penguin pollution– We have seen B000!

– Current GQ bound:

– Full isospin analysis required!

• The system: small penguin pollution– Polarization is fully longitudinal (as predicted).

– Current GQ bound:

– Bound may improve as additional data becomes available

– Time-dependent + results (measures sin(2+2)) coming soon.

• There are more techniques than and – e.g. Dalitz analysis of


The angle

• Measuring = Measuring the phase of the Vub

– Main problem: Vub is very small: O(3)

– Either decay rate or observable asymmetry is always very small.

• Conventional wisdom: measuring at B factories is difficult/impossible.– Gamma is the least constrained angle of the Unitarity Triangle

• Current attitude: we should try.– There are new ideas to measure Dalitz decays, 3-body decays,…)

– New experimental data suggest color suppression is less severe, which eases small rate/asymmetry problem somewhat

– B-Factories produce more luminosity than expected(BaBar & Belle approaching O(200) fb-1 by Summer ’04 time )


The angle : B DK

• Strategy I: interfere bu and bc decay amplitudes– D0/D0 must decay to common final state to interfere

• Ratio of decay B amplitudes rb is small: O(10-1)

• rb is not well measured, but important

– rb large more interference more sensitivity to

colorsuppression

Ru is the left side of the Unitarity Triangle (~0.4).

]2.008.0[)(

)(

)(

)(0

0

CSub FRcbA

ubAr

KDBA

KDBA

FCS is (color) suppression factor([0.2-0.5], naively1/3)


from B DK – Two approaches

• Approach I: D0/D0 decay to common CP eigenstate

– ‘Gronau, London & Wyler’

– D0/D0 decay rate same

• Approach II: D0/D0 decay to common flavor eigenstate

– ‘Atwood, Dunietz & Soni’

– Use D0/D0 decay rate asymmetry to compensate B decay asymmetry`

• Complementary in sensitivity

– GLW: large BF: O(1±rb), small ACP: O(rb)

– ADS: small BF: O(rb2), large ACP: O(1)

00000

000

,intoe.g.decays,2/)(

,intoe.g.decays,2/)(

SSCP

CP

KKDDD

KKDDD

Branching fractions

small (0.1%-1%)

KD

KD0

0CKM favored

Doubly Cabibbo suppressed (by factor O(100))


B DK Observables – Gronau-London-Wyler

• There are more observables sensitive to than ACP

– Absolute decay rate also sensitive to , but hard to calculatedue to hadronic uncertainties

– GLW: measure ratio of branching fractions: hadronic uncertainties cancel!

– Experimental bonus: many systematic uncertainties cancel as well

– Bottom line: 2 observables each for CP+ and CP- decays

• 3 independent observables (R+, R-, A+=-A-), 3 unknowns (rb, b, )


2

2

0

00

10)20.035.031.8(

10)5.06.18.8(

)(

)()(

KDBB

KDBBKDBBR CPCP

B DK : GLW results

• Result for B- D0 K- in 115 fb-1

• Results for CP-odd modesin progress (R-, A-)

06.017.007.0 A

D0

background

06.017.007.0 A

GLW method: large BF, small ACP


B DK : The Atwood-Dunietz-Soni method

• Two observables, similar to GLW technique

– Ratio of branching fractions and ACP

• D0 K+-: 2 observables (A, R), 3 unknowns (rb, b+d, )

– Insufficient information to solve for

– Can add other D0 decay modes, e.g. D0 K+-0 4 observables (2xA, 2xR), 4 unknowns (rb, b+DKp, b+DKpp0, )

• Expected BF is ~510-7 – very hard!– Expect observable O(10) events in 100M BB events

– Unknown values of , rb, b add O(10) uncertainty of BF estimate

– Measurement not attempted until now


from Atwood-Dunietz-Soni method: B- [K+ -]D0 K- : results

• Newly developed background suppression techniques give us sensitivity in BF = O(10-7) range

BF 5x10-7 ~10 events

• But we don’t see a signal!– Destructive interference,

rb is small, or just unlucky?

• Cannot constrain with this measurement…

– But BF proportional to rb2 results sets upper limit on rb

MC yield prediction with BF=7x10-5: 12 evts

Yield in 115 fb-1 of data:1.1 3.0 evts

No assumptions: rb < 0.22 (90% C.L.) from CKM fit : rb < 0.19 (90% C.L.)(95% C.I. region)

ADS method: small BF, large ACP


B DK : prospects for B-factories at 500 fb-1

• Combine information on from various sources

• Example study– Assume =75o,

b=30o, d=15o

– Consider various scenarios

• GLW alone

rb=0.3

=75o, b=30o, d=15o

3

2

1GLW

2


2



• Scenarios

– GLW alone

– GLW+ADS(K)

– GLW+ADS(K)+d from CLEO-c

• ADS/GLW combination powerful

• There are additional information not usedin this study, e.g.

– GLW: D*0K,D0K*,D*0K*

– ADS: K0,K3

– sin() from D*, D0K0, DK,…

rb=0.3

=75o, b=30o, d=15o

3

2

111o

GLW

GLW+ADS

GLW+ADS+CLEO-c




• rb is critical parameter

2

rb=0.1

rb=0.2

rb=0.3

=75o, b=30o, d=15o

12

3

3

3

2

2

1

111o

23o

67o


Gamma summary

• The B DK program is underway– Measurements for GLW methods in progress (B D(*)0 K(*)-)

– First measurement of ADS method (B [K+-]K-)

– ADS and GLW techniques powerful when combined

– Final results depends strongly on rb

• Other methods in progress as well– Dalitz analyses of B- D0(KS)K-, B DK

– Time dependent analysis of B D*- (mixing + Vub decay)• |sin(2+)|>0.57 (95% C.L.))

– Analysis of B0 D(*)0 K(*)0

• There is no ‘golden’ mode to measure – All techniques are difficult and to 1st order equally sensitive.

– Combine all the measurements and hope for the best


Concluding remarks

• The CKM model for CP violation passed it’s first test (sin2).

– Future measurements of sin2 from B0 (cc)KS will continue improve constraints on apex of unitarity triangle

• The bs penguin measurement of sin2 offers a window to new physics.

– Another 2-3 years worth of data will clarify current 3 discrepancy

• We are cautiously optimistic that we can measure now that B decay turns out have little penguin pollution

• Measurement of just starting. Success depends on many unknowns…

• BaBar is projected to double its current dataset by 2006

Documents

CP Violation Measuring matter/anti-matter asymmetry with BaBar