Studies of the decay B c K at the BaBar experiment

Nick Barlow

University of Manchester

December 2003

2

The BaBar Collaboration

• The BaBar collaboration consists of– Nearly 600 physicists– From 10 countries– 75 Institutions

• The c analyses presented here were mainly performed by:– Me– Witold Kozanecki (Saclay)– Stefania Ricciardi (RHUL)– Frank Jackson (Manchester)– Gautier Hamel de Monchenault (Saclay)

3

Outline

• The PEP-II B-factory, and BaBar detector• CP violation in the quark sector of the

S.M.– CKM matrix and Unitarity Triangle– 3 Types of CP violation in B decays

• Measuring the CKM parameter sin2 using neutral B decays to charmonium + K.(Specifically Bc KS)– Reconstructing B candidates– B flavour tagging– Time-independent and time-dependent fits

4

Motivation for the BaBar experiment

• CP violation is interesting because– It is one of the least tested areas of the

Standard Model– Has some relevance to the matter/antimatter

asymmetry in the Universe

• Before BaBar and Belle, CP violation had only been observed in the kaon system– Hadronic uncertainties make comparison with

Standard Model parameters difficult

• High luminosity of PEP-II facilitates numerous other precision measurements in B physics

• (and tau physics…)

5

The PEP-II B-factory

e+e-(4S) BBbar

9 GeV electron beam

3.1 GeV positron beam

Boost (4S) in lab frame: 0.55

6

PEP-II performance

• Design peak luminosity = 3.0*1033cm-2s-1

• Record peak luminosity = 6.8* 1033cm-2s-1

• 24 hr record integrated = 484.3 pb-1

• 7 day record integrated = 2.49 fb-1

• Records improving all the time, largely thanks to trickle injection…

• Results in this talk use 81fb-1 (Runs 1 and 2).

7

Trickle injection

• Inject charge continuously into LER, while BaBar is taking data

• Will increase integrated luminosity by at least 15%– If backgrounds

are under control…

8

The BaBar detector

e+ (3.1GeV)

e- (9GeV)

Silicon Vertex Tracker

Drift CHamber

Electro-Magnetic Calorimeter

Detector of Internally Reflected Cherenkov light

Instrumented Flux Return

1.5T solenoid

9

Tracking and Vertex finding

• SVT consists of 5 layers of double-sided silicon strip detectors– Single hit resolution ~80 microns.

• DCH consists of 40 layers (10 superlayers) of wires, arranged in axial and stereo layers in helium/isobutane gas mixture– Measures momentum

and position of charged particles

(pT)/PT=0.13%PT0.45%

10

Particle Identification

• Charged hadrons (, K) identified using dE/dx in the DCH and SVT, and Cherenkov angle in the DIRC– K- separation>3.4 for

P<3.5GeV/c

• Leptons identified using the IFR () and EMC (electrons)

11

Neutral particles

• EMC consists of 6480 CsI(Tl) crystals, arranged in a barrel+endcap configuration.

• Measures energy of electrons and photons over a wide energy range.

12

CKM matrix and Unitarity Triangle

• Quark mixing is described by the CKM matrix

)(O

1A)i1(A

A21

1

)i(A21

1

VVV

VVV

VVV

V 4

23

22

32

tbtstd

cbcscd

ubusud

Wolfenstein parametrization

Complex phase CP violation0*** tbtdcbcdubud VVVVVV

Unitarity condition:

Can be represented as a triangle on complex plane:

Im

Re1

(,)

*

*

cbcd

tbtd

VV

VV

*

*

cbcd

ubud

VV

VV

13

Unitarity triangle cont.

• Angles are given by:

Im

Re1

(,)

*

*

cbcd

tbtd

VV

VV

*

*

cbcd

ubud

VV

VV

*

*

argubud

tbtd

VV

VV

*

*

argcbcd

ubud

VV

VV

*

*

argtbtd

cbcd

VV

VV

14

Existing constraints on Unitarity Triangle

Errors are dominated by

theoretical uncertainties!

Constraints come from- CP violation in K0 mesons - Vub and Vcb measurement - Bd and Bs mixing http://ckmfitter.in2p3.fr

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3 Types of CP violation in B decays

1. CP violation in decay (direct CP violation)

– Amplitude for a decay is not same as amplitude for CP conjugate decay

2. CP violation in mixing (indirect CP violation)

– Mass eigenstates cannot be chosen to be CP eigenstates

3. CP violation in interference between decays with and without mixing

– Time-dependent asymmetry in decays of and to common CP eigenstate.

0B 0B0B

0B

fCP

mixing

decay

tfA

fA

0t

t

16

CP violation in neutral B decays

• Consider neutral B decays into a CP eigenstate fCP that is accessible to both B0 and B0:

• Can define measurable quantity :

CP

CP

CP

f

ff A

A

p

q

violationCP 2 Type1p

q

violationCP 1 Type1CP

CP

f

f

A

A

violationCP 3 Type0Im

17

CP violation in neutral B decays

• Time dependent asymmetry given by:

• If || = 1 (no direct CP violation), this reduces to:

2

2

00

00

1

)sin(Im2)cos(1

))(())((

))(())((

CP

CPCP

CP

f

BfBf

CPCP

CPCPf

tmtm

ftBftB

ftBftBa

)sin(Im tma Bff CPCP

18

BCharmonium K0

• For BCharmonium K0, both ‘tree’ and dominant ‘penguin’ diagrams have same weak phase, so no direct CP violation, || = 1

d

c

dB

J/c

K

b

s

c

d

c

d

b

c

s

= chK sin2

Kf

f

BchK p

q

A

A

p

q

CP

CP

0

*

*

*

*

*

*

00 ImImcdcs

cdcs

cscb

cscb

tdtb

tdtbchKchK VV

VV

VV

VV

VV

VV

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BCharmonium K0

• BJ/KS is the ‘Golden channel’ at BaBar• The J/ decays to leptons – clean reconstruction with

good efficiency

• The c has a much larger intrinsic width than the J/, and only decays hadronically• Need to work harder to reduce background

• Less than half the total width is accounted for by known decay channels

• Although J/ and c have opposite intrinsic CP, the J/ is a vector, so (-1)L term in CP of 2-body state means that BJ/KS and BcKS decays have the same CP eigenvalue (-1)

20

Experimental method

• BBbar pair evolve as coherent P-wave state.– Always exactly 1 B0 and 1 B0

• Boosted wrt lab frame: can measure distance z between B decay vertices.

e+

(4S)

Fully reconstructe

d B decay

(CP eigenstate)

e.g. BJ/Ks, B cKs

Tag flavour of B-meson using e.g. charge of a primary lepton

t z/( c)

Btag

BCP

e-

21

Time-dependent asymmetry

• In principle, just measure time difference t for events where Btag is tagged as a B0 or a B0.

• In practice, tagging and t determination are not perfect – need to modify Probability Density Functions (PDFs) with mistag fractions (w) and resolution functions. B0

tagsB0 tags

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Time dependent PDF

P(t) exp(–|t|/B) ( 1 ± (1-2w) CP sin2sin(mt) ) R(t)

dilution Resolution function

• Can extract sin2b from time-dependent fit to t distributions of tagged events, BUT• Need to know mistag fraction and resolution

function• Need to add other PDFs for background events,

which in general will have different time dependence

CP eigenvalue

23

Neutral B-meson mixing

• Neutral B mesons can mix– Btag and Bflav can

be opposite flavour (no mixing)or same flavour (mixing)

e+

(4S)

Fully reconstructed B

decay

(flavour eigenstate)e.g. BD- +

Tag flavour of B-meson using e.g. charge of a primary lepton

t z/( c)

Btag

Bflav

e-

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B-mixing (2)

• PDF for B-mixing is given by:

tRtmDeBBBBP t

cos14

)( 0000

Resolution function

Dilution = 1-2w, where w is fraction of wrongly tagged events

Real probability of B-mixing

unmixed

mixed

25

B flavour tagging

• Fully reconstructing Btag not practical – would be too inefficient.

• Instead, use Neural Net tagging algorithms based on different physics processes– Primary lepton from W in bc transition– Charged kaons from bcs transition– Charge of slow pions from D* decays– Charge of highest pT track

• Divide into 4 hierarchical, mutually exclusive ‘Tagging Categories’, – Lepton, Kaon1, Kaon2, Other

• Each tagging category has its own mistag fraction, t resolution function, measured on real data using B mixing sample.

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Tagging performance

• Important parameter is effective tagging efficiency Q=eff(1-2w)2

– This is what goes into the error on sin2b

Category Efficiency

(%)

Mistag

(%)

QualityQ(%)

Lepton 10.2 0.2

2.7 0.3 9.2 0.2

Kaon1 17.8 0.2

8.8 0.4 12.1 0.3

Kaon2 16.3 0.2

20.4 0.6

5.7 0.2

Other 22.7 0.2

31.3 0.6

3.2 0.2

Total 67.1 0.5

30.2 0.5

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Measuring t

• Resolution on t is dominated by determination of z decay position of Btag

– Use constraints from knowledge of the beamspot position, and the momentum of Brec

t= z /( c) + small correction due to small momentum of Bs in Y(4S) frame

28

t resolution

• Different PDF used for each tagging category– All use triple Gaussian (core, tail, outliers),

and event-by-event errors. t resolution usually better for lepton

category8 free parameters:•Relative fractions of tail and outliers

•Scale factor for width of core Gaussian

•Bias for tail Gaussian

•Bias factors for core Gaussians in each tagging category

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Data samples

• BCP sample – reconstructed B0 c KS events– Used for measuring sin2b

• Bflav sample – events where 1 B is reconstructed into a flavour eigenstate e.g. D*0-

– Used for measuring mistag fractions and t resolution

• Bsig+ sample - reconstructed B+ c K+ events– Used for cross-checks, higher statistics than neutral

mode.

• Numerous Monte Carlo samples– Used for cross-checks, and tuning the fit – can vary

generated value of sin2b.

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Reconstruction of B c K candidates

• Use decay channels c KSK+- and c K+K-0

• Most BaBar (and Belle, CLEO) B analyses use the variables mES and E, which are (nearly) uncorrelated. E is the measured – expected energy of the

reconstructed B candidate

– mES is the beam-energy substituted B-mass

(assumes E=0)**beamB EEE

2*2*BbeamES pEm

mES (GeV5.35.2

31

Fighting background

• First run a ‘skim’ – loose cuts on topological variables, charged track multiplicity etc. to reduce dataset to a manageable size

• Apply cuts on reconstructed masses of KS, 0 etc.– All cuts optimized to maximise S/sqrt(S+B)

• Estimated from signal MC and E sidebands of on-resonance data

• Use a ‘Fisher Discriminant’ to fight background from u,d,s,c quark events– Linear combination of ‘event shape’ variables– Light quark events tend to have jet-like structure in CM

frame, while BBbar events tend to be more spherical, as B mesons are produced almost at rest in CM frame

32

Types of background

• Combinatorial background, from light quark events or BBbar events where Brec is reconstructed using tracks from both Bs– Can be studied using off-resonance data, MC,

and E sidebands of on-resonance data– Modeled using a threshold function (ARGUS

function) in mES

• Peaking background from BBbar events– In our case, studies on high-statistics BBbar MC indicate that

this is dominated by decays to the same final state as our signal, but without an intermediate c resonance:

• Neither of these backgroundsis expected to have any structure in mX – the mass of the

reconstructed c candidate.

B+

K+

K-

0

K+

C

B+

K*+

33

Time independent fit

• Extract signal and background yields using 2D unbinned maximum likelihood fit to mES and mX

c signal: Breit-Wigner Gaussian2 in mX Gaussian1 in mES

J/ signal: Gaussian2 in mX Gaussian1 in mES

Peaking background: Gaussian1 in mES linear in mX

Combinatorial background: Argus shaped in mES, linear in mX

34

Results of time independent fit

• KsKPi neutral channel: Tagging

categoryFraction of CP signal

Fraction of peaking bg

Lepton 0.38 ±0.09

0.18±0.06

Kaon 1 0.11±0.03

0.04±0.01

Kaon 2 0.16±0.04

0.05±0.02

Other 0.12±0.03

0.03±0.01

35

The sin2 fit

• The value of sin2 is obtained from a simultaneous fit to the Bflav and BCP samples – Takes into account potential small correlations

between sin2b and resolution functions etc., and uses statistical power of BCP sample.

• Altogether there are 34 free parameters– Mainly due to resolution functions for each

tagging category for signal and background, and the composition and time-dependence of the backgrounds in both the BCP and Bflav samples.

36

Results on MC

B0 c KS signal, generated with sin2b=-0.7

sin2b=-0.744±0.074

B+ c K+ signal, no CP violation expected

sin2b=-0.062±0.059

37

Results on Bsig+ sample

• No CP violation expected:

sin2b=0.11±0.16

38

Results on BCP sample

• i.e. the actual result!

sin2b=0.51±0.33

39

More cross-checks

• Can use ‘Toy MC’ experiments, where events are generated to according to a PDF, to check validity of result:– Compare maximised likelihood found in fit to data

with distribution of values found in 500 Toy MC experiments

– Compare statistical error in fit to data with those found in 500 Toy MC experiments

40

Systematic errors

Source Error on sin2

Fraction of peaking bg 0.02

CP of peaking bg 0.04

Dilution & t resolution 0.03

Combinatorial bg description 0.03

(mES) 0.0015

(mX) 0.02

mB B (PDG 2000) 0.01

Total systematic error 0.08

41

Comparison with other decay modes

CP =-1 CP =+1

sin2 = 0.755 0.074

sin2 = 0.723 0.158

42

Constraints on the Unitarity triangle

http://ckmfitter.in2p3.fr

BcKS only All BaBar results

43

Conclusions

• BaBar is a good place to do B physics– Very high luminosity of PEP-II– Clean environment of e+e- collisions

• BaBar and Belle have made precision measurements of CP violation in B mesons– Values of sin2b agree well with S.M. expectations

• It is (just about) possible to measure sin2b using the decay channel BcKS – Helped reduce the statistical error by 0.01 for the

last publication!!– Studies on MC indicate it will be advantageous to

include this channel in the main sin2b measurement for the forseeable future

44

45

Backup slides

46

What is an c ?

c is the ground state of the charmonium system– Mass = (2979.7 1.5) MeV/c2 (PDG 2002)– Width is very poorly known:– BaBar has just made a new measurement

using c from 2 photon production:

KK

eeee

SC

C

0

Use J/psi from ISR to measure detector resolution

2MeV/c8.05.23.33 TOTC

This is the value we use in our analyses