First Year of BAB - Harvard Universityusers.physics.harvard.edu/~morii/talks/Moriond.pdfFirst Year of BaBar/PEP-II Masahiro Morii SLAC CP Violation in B0 Decays CP violation has been

EP-IIc B Factory —

ter

First Year of BABAR/P— Measuring CP Violation at an Asymmetri

Masahiro MoriiStanford Linear Accelerator Cen

SLAC

CP Violation in B0 Decays

atrix

ard Model?

, β, γ

rediction

ween direct androduces CP asymmetry

ACP ∆m t⋅( )sin=

First Year of BaBar/PEP-II Masahiro Morii

CP violation has been known for 36 years◆ Occurs naturally in the Standard Model ← the CKM m◆ Limited experimental constraints from KL decays

→ Is CP violation fully explained by the Stand

CP violation in B0 decays comes to the rescue

Asymmetry ACP is related to the CKM angles α◆ E.g. for (gold-plated decay)

→ Powerful probe to verify/disprove the SM p

B0

B0

CP eigenstate

Mixing

Decay ◆ Interference betmixed decays ptime-dependent

ACP t( )Γ B

0f CP→( ) Γ B

0f CP→( )–

Γ B0

f CP→( ) Γ B0

f CP→( )+-------------------------------------------------------------------------=

t = decay time

ACP 2βsin= B0

J/ψ KS→

SLAC

How We Measure CP Violation

fficiency

non-CP) decayes orB

0B

0

CP decay


Step 1: Reconstruct a CP decay (e.g. B0 → J/ψ KS)✱ Low BR (5 x 10-5) → luminosity & detector e

Step 2: Tag the flavor of the other B0

✱ Leptons and kaons → particle identificationStep 3: Measure ∆z → t = ∆z/βγc

✱ Asymmetric collider (βγ = 0.56)✱ Vertex resolution → silicon vertex tracker

e- e+

B0

B0 (t = 0)

ϒ4S µ+

µ- J/ψπ+

π-KS∆z

ACP t( )Γ B

0f CP→( ) Γ B

0f CP→( )–

Γ B0

f CP→( ) Γ B0

f CP→( )+------------------------------------------------------------------------- ACP ∆m t⋅( )sin= =

} Tagging (determin

}

SLAC

PEP-II Storage Ring

peak scan


9 GeV e- + 3.1 GeV e+ → βγ ∼ 0.56

First collision May 26, 1999

>1/2 design luminosity achieved

Design Achieved

Luminosity (cm-2s-1) 3 x 1033 1.7 x 1033

No. of bunches 1658 829

LER current (mA) 2140 960

HER current (mA) 610 750 ϒ4S

SLAC

PEP-II Integrated Luminosity

k

reded

ents

repair


Current run continues through August 2000

→ accumulate 10 fb -1 on-pea

3.8 fb -1 delive3.2 fb -1 record

Detector improvem

Vacuum

SLAC

BABAR Detector

3.1GeV e+

Drift Chamberrtex Tracker

1.5T Solenoid


9GeV e-

Instrumented Flux Return

Electromagnetic

DIRC

Silicon Ve

(Cerenkov)

Calorimeter

SLAC

Silicon Vertex Tracker (SVT)

Si detectora (●) reachedo

o 2 Mrad


SVT Hit Resolution vs. Incident Track Angle

Monte Carlo - SP2

Layer 1 - Z View

(deg)

(µm

)

Data - Run 7925

BA B A R

Monte Carlo - SP2

Layer 1 - φ View

(deg)

(µm

)

Data - Run 7925

BA B A R

0

20

40

60

-50 0 50

0

20

40

60

-50 0 50

◆ 5 layer double-sided◆ Hit resolution in dat

design: ~15 µm at 0◆ Radiation hard up t

SLAC

Drift Chamber (DCH)for low multiple

in data (●)40 µm


0

0.005

0.01

0.015

0.02

0.025

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

BABAR

Signed distance from wire (cm)

Res

olut

ion

(cm

)

Design Goal = 140 µm

◆ He-iC4H10 (80/20) scattering

◆ Single hit resolutionreached design: <1

Tracks with p>1GeV/c

◆ dE/dx resolution ~7.5% forBhabhas (design 7%)

◆ K/π separation >2σ up to700 MeV/c → DIRC coversp > 500 MeV/c

SLAC

DIRC: the PID Device

0

500

1000

1500

2000

2500

1.6 1.8 2

C

GeV)

D0 → K- π+

With DIRC

K- π+ Mass (GeV)

Num

ber

of e

vent

s/10

MeV

BABAR

2

ov light in quartzmitted by inter-

ectioncted by PMTs resolutiond (target 2 mrad)aration atc is 6.5 mrad


Without DIR

K- π+ Mass (

Num

ber

of e

vent

s/10

MeV

D0 → K- π+

0

2000

4000

6000

8000

1.6 1.8

◆ Cerenk→ transnal refl→ dete

◆ 3 mradachieve

◆ K/π sep4 GeV/

◆ K efficiency ~80% for D0 → K+π-

inside the acceptance◆ Background rejection factor ~5

(NB: background contains kaons)

SLAC

Electromagnetic Calorimeter (EMC)

d / E expected deposited9 1 1.1 1.2 1.3

lusters

BaBar

)

100%

abhasonversions

3π→ 2π


mγγ (GeV)0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

En

trie

s

0

200

400

600

800

1000

1200

1400

1600

1800

π0 Mass E γγ > 500 MeV

BaBar

π0-mass = 134.9 MeV

π0-width = 7.2 MeV

E measure0.6 0.7 0.8 0.

Entri

es/B

in

0

200

400

600

800

1000

1200

1400

1600

1800Constant = 1674

Mean = 1.016

Sigma = 0.02214

EMC Bhabha C

Forward Barrel

Constant = 1674

Mean = 1.016

Sigma = 0.02214

◆ CsI(Tl) with photodiode readout◆ E(Bhabha) and m(π0) resolutions

close to Monte Carloσ = 2.2%(MC 2.0%

Electron ID (p > 0.5 GeV)

Electrons >90%

Pions 1–2%

Bhγ c

τ →KS

σ = 5.3%(MC 5.0%)

SLAC

Instrumented Flux Return (IFR)

GeV)

5%

%


BABAR

p (GeV/c)

eff.

muons from eeµµpions from τ-3prong and Ks

IFR Barrel

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4

◆ Bakelite-based RPCs sand-wiched between iron plates

◆ Muons above p = 0.5 GeV/cidentified

◆ Neutral hadrons (KL) detected

Muon ID (p > 0.5

Muons >7

Pions ~3

e+e− → e+e−µ+µ−

τ → 3πKS → 2π

SLAC

Are We Ready to Measure sin2β?

ration!)

te

mer

s I speak
PEP-II had a terrific startup!◆ Collected 3.2 fb-1 on tape → But we need more

BABAR works beautifully◆ Initial problems have been solved◆ Performance very promising → Still much to do (calib

Physics analysis has started◆ Signals are seen in key channels → Limited statistics

→ We don’t have a CP measurement just yet

What can I show you today?

Examples of what we have done to demonstra➤How well we understand the detector

➤How well the data agree with our expectation

➤How realistic is our plan to measure sin2β by Sum

Everything is PRELIMINARY and improving a

SLAC

J/ψ → l+l- Signal

(µ+µ-) GeV/c2

(µ+µ-)(µ+µ-)

BABAR

3.2 3.4

- (~380 pb-1)

V

e to muon ID


m(e+e-) GeV/c2

Can

dida

tes

/ 10

MeV

/c2

m(e+e-)

J/ψ → e+e-

m(e+e-)

BABAR

0

20

40

60

2.6 2.8 3 3.2 3.4

J/ψ → e+e- (~540 pb-1)

σ = 16 MeV

m

Can

dida

tes

/ 10

MeV

/c2

m

J/ψ → µ+µ-

m

0

25

50

75

100

2.6 2.8 3

J/ψ → µ+µ

σ = 15 Me

◆ Efficiency consistent with expectation

→ Already reasonable, but can be improved

Background duTail due to Bremsstrahlung

SLAC

K → π+π- Signal


S

M(π+π-) (GeV)

N(π

+π- )

can

did

ate

s/1

.7 M

eV

BABAR

0

100

200

300

0.45 0.475 0.5 0.525 0.55

◆ Mass resolution OK

Mass 498.94 ± 0.12 MeV

σnarrow 2.8 MeV (69%)

σwide 11 MeV (31%)

SLAC

B0 → J/ψ K Signal

BABAR

.25 5.275 5.3

12 events

1.4 ± 0.2

9.8 ± 1.1


S

-0.1

-0.05

0

0.05

0.1

5.2 5.225 5

BABAR

0

1

2

3

4

5.2 5.225 5.25 5.275 5.3

◆

◆

∆E EBCMS

EbeamCMS

–=

mB EbeamCMS( )

2pB

CMS( )2

–=

Data ~620 pb-1

Signal

Background

Expected signal

SLAC

B+ → J/ψ K+ Signal

Mb(GeV)5.25 5.275 5.3

s under control expectations

(calibration, PID)


B A B A R

∆E(G

eV)

-0.1

-0.05

0

0.05

0.1

5.2 5.225

Bchmass(GeV)

Nev

/2.5

MeV

BABAR

0

2.5

5

7.5

10

5.2 5.225 5.25 5.275 5.3

Data ~620 pb-1

Signal 41 events

Background 5.0 ± 0.6

Expected signal 39.0 ± 3.4

→ J/ψ K signal◆ Yields agree with◆ Resolution OK◆ Bkg will improve

SLAC

D* Signal and Vertexingroves with beam

0 1 2 3

π proper decay time (ps)

BABAR

ial ⊗ gaussian fitpectednt with PDG


0

100

200

0.14 0.145 0.15 0.155

0

100

200

300

0.14 0.145 0.15 0.155

GeV

N e

ven

ts/2

00

ke

V

m(Kππ) - m(Kπ)

BABAR

Before refitσ1=354 keV (27%)σ2=908 keV (73%)

GeV

N e

ven

ts/2

00

ke

V

m(Kππ) - m(Kπ)

BABAR

After refitσ1=280 keV (47%)σ2=679 keV (53%)

◆ ∆m resolution impprofile constraint

0

20

40

60

-3 -2 -1

Candidate D0 → kE

ntrie

s/0.

1 ps

~220 pb-1

◆ Simple exponent◆ Resolution as ex◆ Lifetime consiste

SLAC

B0 → D*−e+ν Signal

0.15 0.16 0.17

m(D*)-m(D0), GeV

BABAR

-2 0 2 4

MM2, GeV2

BABAR

asurement


0.14

0.15

0.16

0.17

-4 -2 0 2 4

BABAR

MM2, GeV2

∆m, G

eVBABAR

0

20

40

60

80

0.14

even

ts/1

MeV

0

20

40

60

-4

even

ts/0

.4 G

eV2

~390 pb-1

◆ ∆m = m(D*−)−m(D0), MM = missing mass◆ ~120 signal events

→ Will be used for a mixing measurement→ Tagging efficiency and purity → CP me

SLAC

Summary and Outlook

ugust

ion OK

D*l ν

ing ready

2 β) ≤ 0.3


PEP-II had a terrific first year◆ Record luminosity: 1.7 x 1033 cm-2s-1. 80 pb-1/day◆ BABAR recorded 3.2 fb-1 → 10 fb-1 by the end of A

BABAR is working great◆ Performance approaching the design goals

Analyses are progressing rapidly◆ Signals seen in key channels. Efficiency & resolut◆ Vertexing resolution OK. More tests will follow◆ Tagging being studied using real data, e.g. B0 →

→ All ingredients for the CP measurement gett

More data is coming!!!◆ With 10 fb-1: Expect ~160 J/ ψ KS events → σ(sin

Documents

First Year of BAB - Harvard Universityusers.physics.harvard.edu/~morii/talks/Moriond.pdfFirst Year of BaBar/PEP-II Masahiro Morii SLAC CP Violation in B0 Decays CP violation has been