Recent Results from BaBarAdrian Bevan

On the behalf of the BaBar collaboration

 HELLENIC SOCIETY FOR THE STUDY OF HIGH ENERGY PHYSICS

Workshop on RECENT DEVELOPMENTS IN

HIGH ENERGY PHYSICS AND COSMOLOGY

Athens, 17-20 April 2003

All results are preliminary unless otherwise stated

Athens, '03 2

Talk Outline

• Theoretical Motivation

• The PEP-II accelerator & BaBar Detector

• Experimental issues• Vertex reconstruction• Flavour tagging• Particle Identification• background suppression techniques

• Results• sin2, sin2, direct CP Violation searches, rare decays

•Summary and Outlook

Athens, '03 3

Physics Background – CP Violation• CP Violation is one of Sakharov’s conditions for obtaining a matter antimatter asymmetry in the universe.

• The observed CP Violation explained by the Standard Model of Particle Physics is 9 orders of magnitude too small to explain our universe

•There must be some kind of new physics to discover in the flavour sector

• Standard Model mechanism for CP Violation in weak interactions is given by a single complex phase in the CKM matrix.

32

4

23

2 2

(1 / 2 )

1 / 2 ( )

(1 ) 1

ub

cd cs

td t

ud u

ts

s

cb

b

A iV V

V V

V

V V

V A

A O

V AV i

CP Violating phase presentlarge uncertainty in magnitude and phase→related to a phase in the unitarity triangle

A. D. Sakharov (1967) JTEP Letters 5, 24

Athens, '03 4

Experimental constraints before the B-factory era

(CP Violation measurements prior to 1999)

• Observed effects have been entirely in the kaon system

• This defined our knowledge of CP Violation in the SM• 1964, Christensen et al. discovered CP Violation in K2 decay

• Following 37 years: measured several CP Violation effects in

• Semi-leptonic kaon decay, ~10-3

• Non-leptonic kaon decay, +-, 00: • CP Violation in mixing: ~10-3

• CP Violation in decay: ’/ ~10-3 (KTeV/NA48)• Observed a CP Violating asymmetry in KL→+-e+e- (13% effect)

(observed a T odd asymmetry in this decay as well)

Athens, '03 5

• CP-LEAR• measured T violation• obtained an upper limit on CPT Violation from

• There are still important measurements to do with kaons

• over-constraining the CKM matrix is the aim

• With the advent of the B factories, we were able to test this description for the first time!

0 0 0 0 0 0, , , L L SK K K e e K e e

0 0

182.1 10K K

m m GeV

Athens, '03 6

Unitarity of the CKM matrix gives 6 triangles in the complex plane; 9 relations in all– few triangles have all sides with the same order in – kaon system is not one of these– interesting one for the Bd/u system is “The Unitarity Triangle”:

| |

| |

itd td

iub ub

V V e

V V e

mixing

The CKM Matrix in terms of Bd/u decay

0 0B B

* * * 0ud cd cub tdb tbV V V VV V

(1,0)(0,0)

(,)*

*td tb

cd cb

V V

V V*

*ub ud

cd cb

V V

V V* , ,

,...

B D DK

K

, ,B

0

* *

/ ,

, ,...

SB J K

D D K

Athens, '03 7

For neutral K, B, D … mesons• strong eigenstates are not CP eigenstates

• mass eigenstates are an admixture of different strong eigenstates

• particle antiparticle (mixing), f = 2

Neutral Meson Phenomenology

0 0

0 0

2 21

H

L

B p B q B

B p B q B

p q

(mixing is very suppressed in the case of D mesons)

0 01 1( ), ( )

2 2H L H LB B B B B Bp q

mass eigenstates=CP eigenstatesif no CP Violation in mixing (q/p=1)

if q/p1 have CP Violation

in mixing; e.g. K=2.3x10-3

CP even

CP odd

Athens, '03 8

Direct CP Violation (f = 1)

• need interference between diagrams with different strong (i) and weak phases (i):

• Direct CPV only seen in ; ' ~ few 10-6

• theory predicts large asymmetries in B+/0 (few to ~80%)

00K K

2 2

,

| | | | 2 sin sinff i i jji ji j

A A A A

2 2| | | | 0ffA A direct CPV

Athens, '03 9

Observing CP violation at the (4S)

• Three observable interference effects:– CP violation in mixing (|q/p| ≠ 1)

– (direct) CP violation in decay (|A/A| ≠ 1)

– CP violation in interference of mixing and decay (Im ≠ 0)

0B

0B

CPf

CP

CP

CP

ff

f

Aqλ

p A

Analyse time evolution of BB system (assume )

CP

CP

2f

f 2f

1 | λ |

1 | λ |C

CP

CP

ff 2

f

2 Im λ

1 | λ |S

direct CP violation→ C ≠ 0

Indirect CP violation → S ≠ 0

0

4

04

( , ) 1 sin( ) cos( )

( , ) 1 sin( ) cos( )

CP

CP

CP

CP

tphys CP d d

tphys CP d

f

f

f

f d

f B f t e m t m t

f

S

SB f t

C

Ce m t m t

( ) ( )

( ) ( )CP

Br B f Br B fA

Br B f Br B f

00

0 0

( ( ) ) ( ( ) )( )

( ( ) ) ( ( ) )

sin( ) cos( )

physphysCP

phys phys

B t f B t fA t

B t f B t f

S m t C m t

Athens, '03 10

s,d

Searching for Direct CP Violation & probing New Physics with Bu/d

• Large ACP requires amplitudes of similar order– b→u: suppressed tree: i.e. charmless decays

• large predicted ACP

– b→s,d: penguins: radiative decays good for constraining BSM

• small predicted ACP

• Understand penguins

• Access to , and

• Sensitive to New Physics effects via loops – minimal SUGRA: B→Xs, K+, K0+ …

– R-parity Violating SUSY: KS …

– SUSY searches – K*

Athens, '03 11

Experimental Issues

• interesting modes have small branching fractions– large light quark continuum background qq– quarks below threshold are u,d,s & c– other B background

• Need good K/ separation

• Need to boost to do time dependent CP measurements

• require good vertex resolution for CP measurements

• need to ‘tag’ the flavour of the B (as a particle/anti particle)

• understand charge bias– detector: trigger, tracking; reconstruction– event selection, particle ID, analysis– physics: differences in (anti)particle interaction in matter (e.g. K)

Athens, '03 12

On the whole - very similar designs with many common features for both Belle and BaBar:

• e+e- colliders

• run on the (4S) resonance – produce ~100% BB pairs

• run 40 MeV below resonance for qq background studies

• To measure CP violation in the B system, need to measure vertex difference of the evolving BB system (t) silicon device

• need asymmetric beam energies

The Accelerator & Experiment

0 0

0.110.10~ 1.09 0.08B B

B B

f

f

(PDG 2002)

Athens, '03 13

E(e-) = 9.0 GeVE(e+) = 3.1 GeV

0.56

Design Achieved

Luminosity (cm-2 s-1) 3 x 1033 5 x 1033

Int. Lum / day (pb-1) 135 341

Int. Lum / month (fb-1) 3.3 7.2

The PEP-II e+e- collider

Most results use: 81 fb-1 on-resonance.

88 million BB events (as for ICHEP02).Most results use: 81 fb-1 on-resonance.

88 million BB events (as for ICHEP02).

Athens, '03 14

Silicon VertexDetector (SVT)

Drift chamber (DCH)

Detector forInternally reflectedCherenkov radiation(DIRC)

ElectromagneticCalorimeter (EMC)

1.5 T Solenoid

InstrumentedFlux Return (IFR)

SVT: 5 layers double-sided Si. Crucial for measuring t.

DCH: 40 layers in 10 super- layers, axial and stereo.

DIRC: Array of precisely machined quartz bars. Excellent Kaon identification.

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

Athens, '03 15

Measure angle of Cherenkov light

– Transmitted by internal reflection

– Detected by~10,000 PMTs

– excellent K/ separation

c

Particle

Quartz bar

Cherenkov light

Active Detector Surface

KDDD 00* ,

Particle Identification (PID)

Detection of Internally Reflected Cherenkov Light (DIRC)

Athens, '03 16

Measuring CP Violation from time dependence1. Start with data sample of BB pairs

2. Reconstruct one B in a CP eigenstate decay mode

3. “Tag” the other B to make the matter/antimatter distinction 4. Determine the time between the two B decay vertices, t

5. Plot t distribution and do CP fit for S and C.0/ SB J K

Athens, '03 17

Event Selection Techniques

Use beam energy to constrain mass & energy difference

*B beamE E E * 2 2( )ES beam Bm E P

MES ~ 3 MeV

B background

signal

E ~ 15-80 MeV; larger with neutrals

qq background

Athens, '03 18

Event Selection Techniques

B events are spherical u,d,s,c is jets

• shape variables

u,d,s,c

B event

Signalu,d,s,cbackground

Fisher Discriminant

Arb

itrar

y U

nits

• Maximum Likelihood fits or cut based analysis off-resonance & E sidebands are used to parameterise light quark background

• flavour-tagging (e, , K, slow from other B)

Athens, '03 19

B Flavour Tagging

• Tagging algorithm with physics-based neural networks– Inputs include leptons, kaons, slow- (from D*), and high-momentum

tracks– Outputs combined and categorized by mistag probability (w)

• 5 mutually exclusive categories:• Lepton – isolated high-momentum leptons

• Kaon I – high quality kaons or correlated K+ and slow--

• Kaon II – lower quality kaons, or slow-

• Inclusive – unidentified leptons, poor-quality kaons, high-momentum tracks

• Untagged – no flavor information is used

b sc

K-

Q = (1-2w)2 = (28.1 0.7)%

clea

ner

sign

alla

rger

m

ista

g p

rob

Athens, '03 20

Tagging: example of Charmless B Decays

• Tagging efficiency is very

different for signal and background

• Strong bkg suppression in categories with the lowest

mistag prob (Lepton/Kaon)

• plots shown are for h+h-,

a rare decay with significant backgrounds.

81/fb B→ h+h- sample split by tagging category

150

100

Athens, '03 21

Vertex Reconstruction

Resolution function parameters obtained from data for both signal and background– Signal from sample of fully reconstructed B

decays to flavor eigenstates: D*(, , a1)– Background from data sideband sample

Beam spot

Interaction Point

BREC Vertex

BREC daughters

Exclusive Brec reconstruction

BTAG direction

TAG tracks, V0s

zBTAG Vertex

c

zt

1

B →

Example in B →

e+e- → qq

t (ps)

z resolution dominated by tag side (other B)

•Average z resolution ~180m

•Average z ~260 m

without the boost would have z~20m …

e.g. CLEO

without the boost would have z~20m …

e.g. CLEO

Athens, '03 22

• Dominant amplitudes for b ccs decay:

• Both amplitudes have the same weak phase:

• For B0 J/Ks we obtain:

Golden Mode for CP Violation in B decay(theoretically clean)

b

d

ccsd

W+

b

d

s

c

c

dW+c

g

Tree Penguin

Measuring

*cb csA V V

VcdVcb arealmost real -only phase isfrom mixing

*2

*

* *

* *icb cs

CP Ctb t

P CPc

d

tb

cs cd

csb csK d cdt

V VV V

V

V V

V V

q A qe

p A p V VV

B0 mixing Decay K0 mixing

Athens, '03 23

sin 2Vtb

*Vtd

Vcb*Vcd

final statesb ccs

CP = +1•B J/ KL

0

CP = -1•B J/ Ks

0, Ks0 +-, 00

•B (2S) Ks0

•B c1 Ks0

•B c Ks0

•B J/ K*0, K*0 Ks0

mostly CP even

E=Eb-Ebeam (GeV)

J/KL signalJ/X backgroundother background

Athens, '03 24

Latest result on 88M BB events

Ks modes KL modes

hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)

sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030

sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030

2641 tagged events (78% purity; 66%

tagged )

tagging efficiency Q=(28.1 0.7)%

Still statistics limited…

Athens, '03 25need more statistics to see if the SM holds out

B0 Ks ( )b sss• pure penguin

• sensitive to new physics

• measured in this mode should

agree with

• In SM:

b ccs

84 million BB pairs 84 million BB pairs (supersedes ICHEP ‘02)(supersedes ICHEP ‘02)

84 million BB pairs 84 million BB pairs (supersedes ICHEP ‘02)(supersedes ICHEP ‘02)

0

0

0.18 0.51 0.07

0.80 0.38 0.12S

S

K

K

S

C

• c.f. world average: sin2 = 0.73 ± 0.06• >2 difference.• Belle measures -0.73±0.64±0.18

sin 2 , 0s sK KS C

Goodness-of-fit: 23%

C

~60 events

Athens, '03 26

Also have results fromD*D*, ’Ks, J/0

but interpretation of the measurement of S and C in terms of the weak phase is plagued by uncertainty …

“reference” sin2

pure penguin

mostly penguin?

colour- & CKM-suppressed tree competing penguin

CKM-suppressed treesmall penguin pollution

0

Everything is consistent

awit nd h / the SMSJ K

Athens, '03 27

Measuring

0B

0B

0B

Interesting modes to measure

need to perform an Isospin analysis of branching ratios of B0 and B0 to 2 final states to determine shift in due to the presence of penguin diagrams

22 eff

doing a ‘quasi 2 body analysis’ – will eventually have to analysethe whole Dalitz plot.

In analogy to these modes one needs to analyse the time dependence of this decay and extract from an isospin analysis of each of the three partial waves (L=0,1,2) in the final state (full angular analysis is required)

The shift in from penguin diagrams is expected to less than that in

Work is now in preparation for analysis of modes

the time evolution of gives the shifted value of

need a neutral B meson decay with several contributing processes that interfere in order to be able to measure the phase

Athens, '03 28

CP Violation in B→

mixingdecay

Tree (T) Level:

)2sin(

0

2

S

C

e i

* *

**tb td

tb td

ud ub

ud ub

V VV V

V V VV

)2sin(1

)sin(

eff2

/1

/12

CS

C

e ii

ii

eeTP

eeTPi

With Penguins (P):

0 0 0 0B & B • Need to measure the weak phase

• can use isospin relations to extract shift

• penguins are significant: P/T~0.2

Athens, '03 29

Can bound the shift on using BR(B→ 0) and upper limit on B0→00

0 0 0

2

0

Bsin ( )

Beff

Br

Br

The Isospin Analysis:

Need to measure decay of Band B to

final states in order to determine the shift

0 0 0, ,

( ),

( ) ( )

A A B f

A CP A A B f

)Br()Br(2

)Br()Br()Br(cos

0

00210

Measure ’ for B(B) from

Already have

Need to measure

Grossman Quinn bound

eff

Athens, '03 30

qq + K

157 19 7 events

)cos()sin(

)(00

00

tmCtmS

BNBN

BNBNtA

dd

tagtag

tagtag

Fit projection in sample of -selected events

0.02 0.34 0.05

0.30 0.25 0.04

S

C

6(4.7 0.6 0.2) 10BR

The first side of the Isospin triangle: B+→-

PRL 2002 89 281802  fit , K simultaneously

K e+e→qq

•large qq background/K separation

Athens, '03 31

The Base of the Isospin Triangle: B→• large qq background /K separation

• Potential background from

– Minimize with tight cut on E

hep-ex/0303028, submitted to PRL

Simultaneously fit for /K

Fit region

ee→ qq

0K

00K

0

ee→ qq

0 1.0 60.9

2321

0.180.17

( ) (5.5 0.6) 10

Yield = 125

0.03 0.02CP

BR B

A

Athens, '03 32

• Small signal; BR few 10-6

background

• Background suppression

– Event shape and flavour

tagging to reduce qq

– Cut on M() and E() to reduce background,

then fix in the fit

Significance including

systematic errors = 2.5

0 0

109

0 0 0 6

23

( ) 3.6 10 (90% . .)

N

BR B C L

The Missing Sides : B→

ee→ qq

hep-ex/0303028, submitted to PRL

Athens, '03 33

Bounding penguin

pollution:

0 0 0

2

0

Bsin ( )

B

0.61@90% . .

eff

Br

Br

C L

0| | 51 @90% . .eff C L

Need ~20 the data to do a better isospin analysis than using a bound in B→

Use Grossman-Quinn Bound:• assumes isospin• most conservative of several bounds in literature• account for correlated systematic uncertainties common to both analyses

0.7 0.6 60.6 0.3(1.6 ) 10

Athens, '03 34

0 0,B B K Not a CP eigenstate

0

| |/(1 ) ( )sin ( )cosXCP

tXB X XX

Xf t e SA Cm t C m tS

related to

direct CPV

K is self tagging

CK, SK, SK =0, CK =-1 •large expected:

ACP() & ACP(K) qq

signalSXF+BBg

6

1.3 61.2

( ) (22.6 1.8 2.2) 10

( ) (7.3 1.3) 10

B B

B B K

Athens, '03 35

B0→+-continuumB-backgroundtotal fit

cos( ) sin( )C m t S m t

~88106 B pairs

Dilution from background

2.1

0.18 0.08 0.03

0.36 0.18 0.04

0.28

0.19 0.24 0.03

0.17 0.08K

S

A

C

A

Athens, '03 36

Direct CP Violation Searches

• Looked for direct CP Violation in many modes

• so far no evidence

• most precise measurement ~4.6%

• charge asymmetry in • is 2.5 effect• is 2.1 effect

• look for updates using more data

Athens, '03 37

Rare Decays:(*)B K ll

• very rare process

• BR ~10-7

• penguin + FCNC process

• consistent with SM

• sensitive to new physics

0.24 0.11 60.20 0.18

* 6

( ) (0.78 ) 10

( ) 3.0 10 (90% C.L.)

BR B Kll

BR B K ll

Athens, '03 38

s,d

(Cabibbo favoured)

* and B K B (Cabibbo suppressed)

• radiative penguins• new physics probes• constrain |Vtd/Vts| (Rt)

Rt

B00

B++

B0

signal region

MES (GeV)

E (

Ge

V)

0 *0 5

* 5

0 *0

0 0 6

6

0 0 6

( ) (4.23 0.40 0.22) 10

( ) (3.82 0.62 0.22) 10

( ) 0.044 0.076 0.012

( ) 1.4 10

( ) 2.3 10

( ) 1.2 10

CP

BR B K

BR B K

A B K

BR B

BR B

BR B

@90% C.L.78fb-1

20.7fb-1

Phys Rev Lett 88 101805 (2002)

Athens, '03 39

Observation of a Narrow Meson Decaying to D+s0 at a Mass of 2.32 GeV/c2

hep-ex/0304021

0

D+s

Athens, '03 40

Other BaBar physics I’ve not covered

•BaBar has also produced results on• B lifetime and mixing; B, m• semi-leptonic B decays• D mixing• fB+B-/fB0B0

to name but a few

• in addition to B; have very large and D samples to study

• produced in excess of 35 publications

• several more preprints have been submitted for publication

http://www-public.slac.stanford.edu/babar/BaBarPublications.htm

Athens, '03 41

Summary• Since the start up of BaBar and Belle we have learnt?

• SM description of CP Violation passed its first real test• Observed CP Violation in the B System • CP Violation in B system consistent with that in kaons• Still not enough CP Violation to explain matter-antimatter

asymmetry in the universe

• Expect ~500 fb-1 by 2006 (5x current data set)

• With this we will:• start to test SM by doing alternate measurements of sin2• start to measure with greater precision in several modes• continue the search for direct CP Violation• work towards over constraining the unitarity triangle

• can we break it?• search for and constrain new physics in the flavour sector

Athens, '03 42

Constraints on (,)Dominated by resultsfrom B physics!

• main constraint: sin2

• fitting using Belle result and indirect constraints ~1.50

• measuring is next test of SM

CKM Fitter Group:

A. Hööcker, H. Lacker, S. Laplace,

F. Le Diberder, Eur. Phys. Jour.

C21 (2001) 225, [hep-ph/0104062]