gabriella sciolla massachusetts institute of technology on behalf of the babar collaboration aps/dpf...

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Gabriella Sciolla Massachusetts Institute of Technology

On behalf of the BaBar Collaboration

APS/DPF Meeting

Philadelphia, April 5 - 8, 2003

Cracking the Unitarity Triangle:

B physics at BaBar

April 5, 2003 B physics at BaBar G. Sciolla – M.I.T.

All sides are ~ O(1) measure

independently: 2 sides:

B mixing, Vub, Vcb,…

3 angles: CPV in B decays

The Beauty of the Unitarity Triangle

This allows us to: test CP sector of SM quantitatively CPV as a probe for New Physics


Outline

Introduction The experimental apparatus: PEP-II and BaBar CP violation in the B system: theory and analysis technique

The angles: sin2: the golden mode, the penguin modes, D*+D*-

The angle : B0 B0 The angle : progress in B+D0

CPK+

The sides: Inclusive measurement of Vub Prospects for Vtd from bd

Summary and conclusion


The PEP-II accelerator

Asymmetric B factory 9.0 GeV e- beam 3.1 GeV e+ beam

Peak luminosity Design: 3 x 1033 cm-

2s-1 Achieved: 5 x 1033 cm-2s-1

Integrated Luminosity Delivered: 118 fb-1 Recorded: 113 fb-1

As of April 3, 2003:

Results based on

89 M BB pairs


The BABAR Detector

DIRC (PID))

1.5 T solenoid

CsI(Tl) EMC

Drift Chamber

Instrumented Flux Return

Silicon Vertex Tracker

e+ (3.1GeV)

e- (9GeV)

Tracking: pT)/pT = 0.13% pT 0.45% DIRC: K- separation: > 3.5 for p<3.5 GeV/c EMC: E/E = 2.3% E-1/4 1.9%


CP violation in B0 decays

)sin()cos(

))(())((

))(())(()(

00

00

mtSmtC

ftBNftBN

ftBNftBNtA

ff

CPCP

CPCPCP

2

2

1

1

f

f

f

C

2

2Im

1

ff

f

S

ff f

f

Aqλ η

p A

i2e~

0B

0B

CPf

mixin

g

decay

0t t

CPfA

CPfA

Scenario 1 Only one diagram contributes:

ACP(t) = - Im sin(mt)

CPV in interference between mixing and decaysIm=sin2sin2 etc.

Scenario 2 More than one diagram contributes both sin and cos terms will survive

- Sf: depends on Im sensitive to UT angles - Cf: depends on ||, direct CP

(f

)


How to measure CPV at BaBar

.

))(())((

))(())(()(

00

00

CPCP

CPCPCP ftBNftBN

ftBNftBNtA

Ingredient #1:Exclusivereconstruction

Ingredient #3:Flavor tagging(coherent state)

Ingredient #2: t determination

e- (4S)

B0

B0

e-

+

-

Breco

Btag

e+

z~ c t

+

-


The measurement of


B0charmonium K0

The “golden channels”: Theoretically well understood Experimentally clean Relatively large BF (~10-4)

0Kb

c

sc

d0B

/J

d

ACP(t) = sin2 sinmt

CP sample:Mode CP Nevent

s

J/Ks, (2s)Ks, cKs,

cKs

-1 1506

J/KL +1 988

J/K*0(Ks 0) mixed

147

Full CP sample 2641


B flavor tagging

Tagging parameters measured from data using B0flavor eigenstates in time dependent B0 mixing analysis.

Category

tag (%) w (%) eff (%)

Lepton 9.1 0.2 3.3 0.6

7.9 0.3

Kaon1 16.7 0.2 9.9 0.7

10.7 0.4

Kaon2 19.8 0.2 20.9 0.8

6.7 0.4

Inclusive

20.0 0.3 31.6 0.9

2.7 0.3

ALL 65.6 0.5 28.1 0.7

B0D*- D0

l +(soft)

K +

B0D*-

(hard)

W+

2)21( wtageff

eff /1)2(sin


sin2 CP fit

CP=+1

CP=-1

sin(2) = 0.741 ± 0.067stat ± 0.033syst

Unbinned maximum likelihood fit to t distribution


Testing the “usual assumptions”

Assumptions made in the sin2 measurement: Same lifetime for BH and BL

No CP/T/CPT violation in the mixing

11 12 11 12

12 22 12 222

M M iM M

H

0H L

12 122

12 122

1i

i

Mq

p M

11 22 11 222

2

( )0

i

i

M Mz

m

CP/T

CPT

12

2

t

b

m

m

m

SM

112

2

t

c

m

m

p

qSM

z=0SM

Very good approximation in SM

But can we test them?


Fit for the “usual assumptions”Fit BCP and B0flavor eigenstates floating /, q/p, z

and CP:sgn(Re ) 0.008 0.037(stat.) 0.018(syst.) [ 0.084,0.068]

1.029 0.013(stat.) 0.011(syst.) [1.001,1.057]

(Re ) Re 0.014 0.035(stat.) 0.034(syst.) [ 0.072,0.101]

Im 0.038 0.029(stat.) 0.025(syst.

CP

CP CP

q p

z

z

) [ 0.028,0.104]

90% C.L.

The “usual assumptions”work!

Direct limit on Test of CPT invariance Indirect CP in mixing


Unitarity Triangle constraints

sin2 vs indirect UT constraints: Excellent agreement

This means: New Physics contribution is small

OR New Physics is not small but does

not appear in this particular decay because of accidental cancellations

.

sin2 from BaBar vs. indirect constraints

Observed CP asymmetry consistent with CKM mechanism

being the dominant source of CPV at low energies

Can we test this hypothesis?


More measurements of sin2Why do we need them?

measure sin2 in many modes characterized by: completely different Feynman diagrams theoretically clean and well understood

and compare results. Significant differences will be indication of New Physics.

Modes under study: Bcharmonium K0 B0KS

B0’KS

B0D*D* B0D*D


B0KS: the theory

Pure penguin modes: bsss |A/A|=1 within 4% Another clean measurement of sin2!

ACP(KS)~ACP(J/KS) If difference > few% Clear signal of New Physics

ACP(t)= - sin2 sinmt

*

*

*

*

*

*

)1(cscd

cscd

cbcs

cbcs

tdtb

tdtb

VV

VV

VV

VV

VV

VV

s

s

s

d

KS

s

s

s

d

Grossman,Worah PLB 395, 241 (1997)Grossman, Isidori, Worah PRD 58, 057505 (1998)


sin2 in B0KS: the results

Results: Fit with both S and C terms:

Assuming no direct CP (C=0):

Hints of New Physics? Compare SKs with

sin2(J/KS)=0.73±0.05

2effect.

SKs = 0.18 ± 0.51 ± 0.07

CKs =0.80 ± 0.38 ± 0.12

SKs = 0.26 ± 0.51

64 signal events

BackgroundB0 tag

B0 tag

--- C+S__ S only


sin2 in B0’KS

Another penguin dominated mode Tree is Cabibbo and color suppressed

Compared to BKS

Larger Branching Fraction: BFBaBar= (5.540.52) x 10-5

More challenging final state: ’ and

~70 candidates

’

KS

s

s

s

d

b

ds

W–

u

u

d

’

KS


CP fit of B0 ’KS

Results of CP fit:

Hints of New Physics? Compare S’Ks with

sin2(J/KS)=0.73±0.05

2effect

S’Ks = 0.02 0.34 0.03 C’Ks = 0.10 0.22 0.03

BaBar

BackgroundB0 tag

B0 tag


B0D*+D*-

Tree measures sin2 from bccd transitions Penguin expected to be small in SM (<10%)

But could be enhanced by New Physics

Transversity analysis used to measure CP odd component:

R=0.07 0.06 (stat) 0.03 (syst)

Mainly CP even!


B0D*+D*-: the CP fit

CP fit:

Compare with sin2(J/KS)=0.730.05

Im()=-0.310.430.13

= 0.980.250.09

Nsignal= 126 12

Indication for large penguin contribution

2

2

1

1

f

f

f

C

2

2Im

1

ff

f

S


sin2 summary

Statistical fluctuation or first signs New

Physics?


The measurement of


from

Significant contribution from both diagrams: P/T ~ 1/3 from Br(BK)/Br(B)

S sin2eff related to angle through isospin C direct CP violation

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

))(())(()(

00

00

mtSmtCftBNftBN

ftBNftBNtA

CPCP

CPCPCP

b uu

d

W

ubVb d

u

u

W

T = Tree P = Penguin

gtbV *

tdV

C.L. @90%51ααeff


B0: analysis technique

K- separation is a must Br(B0K+-) ~ 4 x Br(B0+-) PID gives >3.5 up to 3.5 GeV

Continuum background rejected by Fisher discriminant

Low branching fraction BR(B0+-) ~ 5 x 10-6

PID and Fisher included in maximum likelihood fit

MC

D0data

Bkg MC

mES datasideband


B0: the CP fit

No asymmetry seen in yet…04.025.030.0

05.034.002.0

C

S

N=157 20


B0 the validation

Extensive cross-checks: Possible two-body vertexing systematics CP asymmetry in the background Fit method Likelihood function

Some examples: Measure mixing in B0K+–

md = (0.52 0.05) ps-1 OK! Measure B in B0+–

B = (1.56 0.07) ps OK!

B = (1.56 0.07) ps

mB = (0.52 0.05) ps-1


B0 and B0

Similar to B0… Measures both mixing-induced CP() and direct CP

…just more challenging! Not a CP eigenstate 4 configurations:

B0, B0 3 final state combinatorics

Analysis strategy Pseudo two-body analysis

avoid interference regions of Dalitz plot Continuum background suppressed by NN Simultaneous fit for and

b u

ud

W

d d


B0 and B0

Combined Maximum Likelihood fit measures: BF(B0)= (22.61.82.2) 10-6

BF(B0)= (7.31.31.3) 10-6

and 6 other parameters related to CP: SCSCAA KCPCP ,, ,, ,

Indication for small penguins

N(= 428 34 N() = 120 20


B0 ) the CP fit Time integrated direct CP:

Time dependent CP:

Asymmetry between rates:

08.017.028.0 KCPA

03.008.018.0 CPA

04.018.036.0 C

03.024.019.0 S

03.00.250.15 S

04.00.28 0.180.19-

C

-- B backg.-- all backg.

First step towards a measurement of in B decays.

Full Dalitz analysis in progress.


Prospects for Clean measurement will eventually come from BSD+K-

In the mean time B factories can constrain using: D0

CP K

Theoretically clean (Gronau & Wyler, Dunietz 1991) Interference between b u (cs) and b c (us) Small accessible Branching Fractions ~ 10-7

B0D(*)+ Theoretically clean measurement of 2 Interference between b u (cd) and b c (ud)

B K Interference between penguin and tree diagrams Take ratios of Br’s to eliminate dependence on strong phases Model dependent - relies on assumptions of factorizaton, SU(3)


2

DD

:seigenstate CP are Dwhere00

CP

0CP

D

)(2 0 KDBA CP

)(2 0 KDBA CP

Isospin analysis

Measurements:

Progress in BD0CPK

)%20.035.031.8()(

)(0

0

DBBF

KDBBFR

)%5.06.18.8()()(

)()(00

00

CPCP

CPCP

CP DBBFDBBF

KDBBFKDBBFR

06.017.006.0)()(

)()(00

00

KDBBFKDBBF

KDBBFKDBBFA

CPCP

CPCP

CP


The measurement of the sides

*

*

cbcd

tdtb

VVVV

B0


Why another measurement of Vub?

sin2 is now very well known (sin2) ~5% (BaBar+Belle)

Measurement of the “decay side” ~ |Vub/Vcb| is not as well known (Vcb) ~ 5% (Vub) ~16-20%

Better measurement of Vub

needed for a more stringent test of CP side of Standard Model

Indirect constraints vs sin2 world average


A new measurement of Vub

The idea: One B exclusively reconstructed (4K/fb-1) Inclusively measure Br(bul) on recoiling B

Many kinematical variables: p*lepton, Q2, MX Reduce systematics measuring: B(Xul)/B(Xl) Better purity and smaller model dependence!

b0B

u

X

l

Vub

pl*>1

GeV/c


The result

S/B ~ 1.7!

Vub

)(340.00t)0.0023(syst)0.0027(sta0.0197

XlB

lXB u u,cbB

B


Vub inclusive: summary

Vub

This measurement: total error 13%


B00

B++

B0

B and B

() + high energy Continuum suppressed by NN PID rejects BK*

EW penguin dominated by top quark: BF(bd) ~ |Vtd|2 constraints on UT

Mode 90% CL (10-6) SM Expectation (106)

<1.2 0.49 ± 0.21

<2.1 0.85 ± 0.40

<1.0 ~ 0.5

d

36.0V

V 047.0

)(

)(

ts

td*

KBBF

BBF

Ali and Parkhomenko (hep-th 0105302)

Vtd

*

*

cbcd

tdtb

VV

VV


Summary

BaBar is attacking the UT from all sides and angles:

…and much more in parallel sessions!

sin2 is well established golden mode used as calibration new modes: B0KS, ’KS, D*D*,…

is coming… CPV in B0 and B0

Vtd from EW penguins B complements mixing

New Vub measurement The power of statistics!

is tough! BDCPK-


Conclusion

Although, some interesting shifts observed in sin2 from penguin modes and D*D*: Hints of New Physics??? Statistics will tell…

Despite the fierce attack, the Standard Model is still holding remarkably well…

And much more data is coming: Run-3 in progress (>24 fb-1 so far) >500 fb-1 by 2006 >1 at-1 by 2009 if physics case remains compelling

gabriella sciolla massachusetts institute of technology on behalf of the babar collaboration aps/dpf...

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