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<strong>First</strong> <strong>Year</strong> <strong>of</strong> <strong>BABAR</strong>/<strong>PEP</strong>-<strong>II</strong><br />

— Measuring CP Violation at an Asymmetric B Factory —<br />

Masahiro Morii<br />

Stan<strong>for</strong>d Linear Accelerator Center


Outline<br />

Physics — What We Measure at <strong>BABAR</strong>/<strong>PEP</strong>-<strong>II</strong>, and Why<br />

◆ Why CP Violation?<br />

◆ CP Violation in the B System<br />

Signal — How We Detect the CP Violation<br />

◆ Event Signature<br />

Experiment — How It Works<br />

◆ <strong>PEP</strong>-<strong>II</strong> Accelerator<br />

Analysis — Where We Stand Today<br />

◆ Signal Reconstruction<br />

Summary and Outlook<br />

◆ CKM Matrix, Unitarity Triangle<br />

◆ How to measure sin2β<br />

◆ Experimental Challenges<br />

◆ <strong>BABAR</strong> Detector<br />

◆ Extraction <strong>of</strong> CP Violation<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Why CP Violation<br />

CP violation has been known <strong>for</strong> 36 years.<br />

◆ Discovered in → π + π − decays (Christenson et al., 1964).<br />

K L<br />

0<br />

It occurs naturally in the Standard Model.<br />

◆ Imaginary phase in the CKM matrix (Kobayashi, Maskawa, 1973).<br />

Needed to explain the matter-dominant universe.<br />

◆ What’s predicted by the SM, however, is not enough.<br />

All the experimental evidences are in the K 0 - K 0<br />

system.<br />

◆ Measurements are hard to interpret.<br />

➤Small effects (ε = 2.3x10 -3 ) and large hadronic uncertainties.<br />

◆ Theoretically clean signal → π 0 νν is hard to reach (BR ~ 10 -11 ).<br />

K L<br />

0<br />

Predictive power <strong>of</strong> the Standard Model has never been tested.<br />

Is CP violation fully explained by the Standard Model?<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


CP Violation in the B System<br />

“You can never tell with bees.”<br />

A.A. Milne, Winnie-the-Pooh<br />

The B 0 - B 0<br />

system is an excellent probe <strong>for</strong> CP violation studies.<br />

◆ Large CP violation expected in many channels.<br />

◆ “Gold-plated” channels exist with clean theoretical predictions and<br />

clean experimental signatures.<br />

The goal: Make enough independent measurements <strong>of</strong> CP violating<br />

effects to over-constrain the Standard Model.<br />

◆ NB: there is only one origin <strong>of</strong> CP violation in the SM.<br />

Two possible outcomes:<br />

◆ Everything is consistent and fully determines the CKM parameters.<br />

→ CP violation is fully explained by the SM.<br />

◆ No single choice <strong>of</strong> CKM parameters is possible.<br />

→ New physics beyond the SM.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


CKM Matrix<br />

Wolfenstein parametrization <strong>of</strong> the CKM matrix:<br />

V<br />

=<br />

V ud V us V ub 1 – λ 2 ⁄ 2 λ Aλ 3 ( ρ – iη)<br />

V cd V cs V cb<br />

≅<br />

– λ 1– λ 2 ⁄ 2 Aλ 2<br />

V td V ts V tb Aλ 3 ( 1 – ρ – iη) – Aλ 2 1<br />

◆ Approximation good to O(λ 3 ). λ = sinθ C ~ 0.22.<br />

◆ CP violation due to imaginary phases in V td and V ub .<br />

CP violation in mixing occurs through<br />

*<br />

◆ <strong>for</strong> - .<br />

V td V ts<br />

= – A 2 λ 5 ( 1 – ρ – iη)<br />

K 0 K 0<br />

*<br />

◆ and<br />

V td<br />

V tb<br />

*<br />

V ud V ub<br />

=<br />

Aλ 3 ( 1 – ρ – iη)<br />

= Aλ 3 ( 1 – λ 2 ⁄ 2) ( ρ + iη)<br />

B 0 B 0<br />

<strong>for</strong> - .<br />

Suppressed by λ 4 in the K system. No suppression in the B system.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


The Unitarity Triangle<br />

Unitarity condition V † V = VV † = 1 gives<br />

*<br />

V ud<br />

V ub<br />

* *<br />

+ V cd<br />

V cb<br />

+ V td<br />

V tb<br />

= 0<br />

The Unitarity Triangle:<br />

*<br />

V ud V<br />

----------------- ub<br />

*<br />

V cd V cb<br />

(ρ, η)<br />

α<br />

*<br />

V td V<br />

---------------- tb<br />

*<br />

V cd V cb<br />

γ<br />

β<br />

(1, 0)<br />

◆ All sides are O(1). Other triangles are squashed by λ 2 or λ 4 .<br />

The Unitarity triangle must close if the SM is correct.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Today’s Unitarity Triangle<br />

Top corner <strong>of</strong> the UT (95% C.L.) (S. Plaszczynski, Heavy Flavours 8, 1999)<br />

η _<br />

1<br />

0.9<br />

0.8<br />

0.7<br />

0.6<br />

0.5<br />

0.4<br />

0.3<br />

0.2<br />

0.1<br />

0<br />

ε Κ<br />

∆m(B d )<br />

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1<br />

∆m(B s )<br />

|V ub /V cb |<br />

◆ Measurements consistent with the SM with large errors.<br />

◆ Theoretical uncertainties dominate.<br />

ρ _<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


(ρ, η) → (sin2α, sin2β) plane.<br />

Today’s UT — Angles<br />

◆ –0.95 < sin2α < 0.50 (95% C.L.)<br />

◆ 0.50 < sin2β < 0.85 (95% C.L.)<br />

We know very little about sin2α.<br />

→ We need a measurement.<br />

We know sin2β rather well.<br />

◆ This is assuming the SM.<br />

→ Direct measurement can either<br />

➤Confirm the SM.<br />

➤Indicate New Physics beyond the SM.<br />

-1<br />

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1<br />

CP violation in the B system allows us (<strong>BABAR</strong> and the<br />

competitors) to measure these angles.<br />

sin(2β)<br />

1<br />

sin2β<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

-0.2<br />

-0.4<br />

-0.6<br />

-0.8<br />

-1<br />

-1<br />

0<br />

sin(2α)<br />

sin2α 1<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


CP Violation in the B system<br />

Consider B 0 → CP eigenstate f.<br />

◆ e.g. f = J/ψ K S .<br />

“Direct” and “mixed” decay amplitudes<br />

interfere with each other.<br />

→ Time-dependent CP asymmetry:<br />

Decay<br />

B 0 B 0 f<br />

Mixing<br />

Decay<br />

A CP<br />

() t<br />

Γ( B 0 () t → f )–<br />

Γ( B 0 () t → f )<br />

= ------------------------------------------------------------------------<br />

Γ( B 0 () t → f ) Γ B 0 = Cf cos( ∆m⋅<br />

t)<br />

+ Sf sin( ∆m ⋅ t)<br />

+ ( () t → f )<br />

◆ B 0 - B 0<br />

mixing frequency ∆m = (0.47±0.02)/ps.<br />

If only 1 diagram contributes to the decay,<br />

C f<br />

= 0 S f<br />

= – Im( λ)<br />

λ<br />

=<br />

〈 f |H | B 0<br />

〉<br />

---------------------<br />

〈 f |H | B 0 〉<br />

A CP<br />

() t = – Imλ ( ) sin( ∆m⋅<br />

t)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>First</strong> goal: Measure sin2β.<br />

How to Measure sin2β<br />

◆ Establish CP violation outside the K system.<br />

◆ Test the predictive power <strong>of</strong> the Standard Model.<br />

Look <strong>for</strong> B decays in which:<br />

◆ Single diagram dominates, and<br />

◆ Im( λ) = ±sin2β, or Arg( 〈 f |H | B 0 〉) = +− β.<br />

Three types <strong>of</strong> decays:<br />

◆ Color-suppressed modes:<br />

◆ Cabibbo-suppressed modes:<br />

◆ Penguin-dominated modes:<br />

b → ccs<br />

b → ccd<br />

b → sss or b → dds<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


J/ψ, ψ', ...<br />

c c<br />

b<br />

s<br />

B 0<br />

d<br />

d<br />

Example: B 0 → J/ψ K S<br />

b → ccs Modes<br />

K 0 s , K0 L , K0 *<br />

“Penguin”<br />

b<br />

d<br />

c<br />

c<br />

s<br />

d<br />

* V td<br />

* V cb<br />

* V cs<br />

⎛<br />

λ (–<br />

1) V tb ⎞⎛V --------------- cs ⎞⎛V ⎜ ⎟ ---------------- cd ⎞<br />

=<br />

⎜ ⎟⎜----------------⎟<br />

* * *<br />

⎝V tb<br />

V td<br />

⎠⎝V cs<br />

V cb<br />

⎠⎝V cd<br />

V cs<br />

⎠<br />

A CP () t = sin2βsin( ∆m⋅<br />

t)<br />

B-mix B-decay K-mix<br />

Is this single-diagram decay?<br />

◆ Good approximation: the dominant penguin diagram has the same<br />

phase as the tree diagram.<br />

Gold-plated mode: theoretically clean.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Example: B 0 → D + D -<br />

b → ccd Modes<br />

c<br />

D* - , D - ,...<br />

d<br />

b<br />

B 0 c<br />

d<br />

d<br />

D + , D* + ,<br />

λ<br />

=<br />

*<br />

V tbV<br />

--------------- td<br />

*<br />

* V cb<br />

⎛ ⎞⎛V cd ⎞<br />

⎜ ⎟⎜-----------------<br />

⎟<br />

*<br />

⎝V tb<br />

V td<br />

⎠⎝V cd<br />

V cb<br />

⎠<br />

A CP () t = – sin2βsin( ∆m ⋅ t)<br />

B-mix<br />

B-decay<br />

The tree diagram is Cabibbo suppressed.<br />

◆ Penguins with different phases contribute significantly.<br />

Theoretically less clean (tin-plated?)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


→ sss and dds Modes<br />

b<br />

B 0<br />

d<br />

s<br />

s<br />

s<br />

d<br />

φ,η'<br />

K 0 , K*<br />

b<br />

B 0<br />

d<br />

s<br />

d<br />

d<br />

d<br />

K 0 , K*<br />

π 0 ,ρ 0 ,ω<br />

Example: B 0 →φ K S , or φ K*<br />

* V td<br />

* V cb<br />

* V cs<br />

⎛<br />

λ (–<br />

1) V tb ⎞⎛V --------------- cs ⎞⎛V ⎜ ⎟ ---------------- cd ⎞<br />

=<br />

⎜ ⎟⎜----------------⎟<br />

* * *<br />

⎝V tb<br />

V td<br />

⎠⎝V cs<br />

V cb<br />

⎠⎝V cd<br />

V cs<br />

⎠<br />

A CP () t = sin2βsin( ∆m⋅<br />

t)<br />

B-mix Penguin K-mix<br />

How about the cleanliness?<br />

◆ B 0 →φ K S , or φ K* are pure penguin → OK.<br />

◆ B 0 → dds modes: the tree is both color- and Cabibbo-suppressed →<br />

Penguin probably dominates. Limitation: small branching ratios.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Event Signature: B 0 → J/ψ K S<br />

B 0<br />

e - ϒ e +<br />

4S<br />

B 0<br />

µ +<br />

µ - J/ψ<br />

∆z<br />

K S<br />

e +<br />

π +<br />

π -<br />

A CP<br />

() t<br />

Γ( B 0 () t → f )–<br />

Γ( B 0 () t → f )<br />

= ------------------------------------------------------------------------<br />

Γ( B 0 () t → f ) Γ B 0 = sin 2β<br />

+ ( () t → f )<br />

⋅<br />

sin( ∆m<br />

⋅ t)<br />

Three steps to measure A CP (t):<br />

◆ Reconstruct one B 0 (or anti-B 0 ) from J/ψ and K S .<br />

◆ Measure time from ∆z = βγ∆t.<br />

◆ Determine the flavor <strong>of</strong> the B 0 by tagging the other B 0 .<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Experimental Challenges<br />

Exclusive reconstruction <strong>of</strong> CP eigenstates.<br />

◆ Low branching ratios.<br />

Br( B 0 → J/ψK S<br />

) ⋅ Br( J/ψ → l + l − ) ∼ 5 × 10 – 5 ,<br />

Br( B 0 → φK S<br />

) ⋅ Br( φ → K + K − ) ∼ 5 × 10 – 6 ,<br />

Br( B 0 → D∗ + D∗ −<br />

) ⋅ Br( D∗ + → D 0 π + ) 2 ⋅ Br( D 0 → K − π + ⁄ K − π + π 0 ) 2<br />

∼ 1.4 × 10 – 5 .<br />

→ High statistics, high efficiency.<br />

◆ High multiplicity.<br />

→ Good detector hermeticity.<br />

e.g.: B 0 → D + D - K - π + π -<br />

◆ Combinatorial Background.<br />

K 0 π + π 0<br />

→ Good p T resolution. (Multiple scattering)<br />

→ Particle identification.<br />

→ Continuum suppression. (Γ bb /Γ qq ~ 1/3 at ϒ 4S .)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Experimental Challenges<br />

Precise time measurement<br />

B 0<br />

ϒ 4S<br />

µ +<br />

µ +<br />

B 0<br />

◆ B 0 B 0<br />

boosted along z with βγ = 0.56 → ~ 250µm.<br />

◆ Vertex resolution:<br />

➤<strong>for</strong> B CP : σ z ~ 50 µm.<br />

➤<strong>for</strong> B Tag : σ z ~ 90 µm (inclusive vertex reconstruction).<br />

→ σ(∆z) ~ 100 µm.<br />

∆z<br />

K S<br />

µ - J/ψ<br />

π +<br />

π -<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Experimental Challenges<br />

B flavor (B 0 or anti-B 0 ) tagging.<br />

◆ 3 basic “handles”:<br />

➤primary lepton tag: b → c l - ν l<br />

➤secondary lepton tag: b → c X; c → X l + ν l<br />

➤kaon tag: b → c X; c → s X; s → K -<br />

◆ Statistical combination <strong>of</strong> all the in<strong>for</strong>mation using<br />

➤Likelihood methods<br />

➤Fisher discriminant<br />

➤Artificial neural networks<br />

with perfect PID ε tag (%) (1-2ω) 2 (%) ε effective (%)<br />

Lepton 39.5 58.0 22.9<br />

Kaon 21.3 60.8 13.0<br />

Combined 60.8 35.9<br />

“Effective” efficiency<br />

as if there were no mis-tags<br />

How <strong>of</strong>ten you can tag<br />

Dilution due to mis-tags<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Experimental Challenges — Summary<br />

High statistics → High luminosity<br />

◆ Accelerator per<strong>for</strong>mance comes first.<br />

◆ Detector must cope with rate and background.<br />

High efficiency and good mass resolution<br />

◆ Maximize coverage. Minimize material.<br />

Vertex resolution<br />

◆ High-resolution, wide-coverage vertex detector.<br />

Flavor tagging<br />

◆ Particle identification.<br />

<strong>PEP</strong>-<strong>II</strong> / <strong>BABAR</strong> designed to meet these demands.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>PEP</strong>-<strong>II</strong> Storage Ring<br />

◆ High Energy Ring (HER)<br />

➤9 GeV e − beam, up to 1 A with 1658 bunches.<br />

➤refurbished <strong>PEP</strong>-I.<br />

◆ Low Energy Ring (LER)<br />

➤3.1 GeV e + beam, up to 2 A with 1658 bunches.<br />

➤newly built in 1998.<br />

◆ SLC linac injects beams to <strong>PEP</strong>-<strong>II</strong>.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>PEP</strong>-<strong>II</strong> Storage Ring<br />

◆ LER sits on top <strong>of</strong> the HER.<br />

◆ 9 GeV e - + 3.1 GeV e + → βγ ∼ 0.56.<br />

Design luminosity = 3 x 10 33 cm -2 s -1<br />

→ 3 x 10 7 B 0 ’s / year.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>First</strong> collision on May 26, 1999.<br />

<strong>PEP</strong>-<strong>II</strong> Per<strong>for</strong>mance<br />

Design Achieved<br />

Luminosity (cm -2 s -1 ) 3 x 10 33 1.7 x 10 33 cf. 0.83x10 33<br />

No. <strong>of</strong> bunches 1658 829<br />

LER current (mA) 2140 960<br />

HER current (mA) 610 750<br />

LER lifetime (min.) 240 145<br />

HER lifetime (min.) 240 400<br />

Beam size x (µm) 222 200<br />

Bean size y (µm) 6.7 6.3<br />

@ Cornell<br />

Record day luminosity: 80 pb -1 /day (cf. 42 pb -1 /day @ Cornell)<br />

Integrated luminosity: 3.2 fb -1<br />

<strong>PEP</strong>-<strong>II</strong> has made a beautiful start-up.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>PEP</strong>-<strong>II</strong> Integrated Luminosity<br />

3.8 fb -1 delivered<br />

3.2 fb -1 recorded<br />

Vacuum repair<br />

Detector improvements<br />

Current run continues through August 2000<br />

→ accumulate 10 fb -1 on-peak<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


<strong>BABAR</strong> Detector<br />

Instrumented Flux Return<br />

1.5T Solenoid<br />

Electromagnetic<br />

Calorimeter<br />

DIRC<br />

(Cerenkov)<br />

9GeV e - 3.1GeV e +<br />

Drift Chamber<br />

Silicon Vertex Tracker<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Silicon Vertex Tracker (SVT)<br />

◆ 5-layer double-sided Si detector. 143k readout channels.<br />

Layers Radii (cm) Readout pitch (µm) Resolution (µm)<br />

1, 2, 3 3.2, 4.0, 5.4 50 (φ) / 100 (z) 10 (φ) / 12 (z)<br />

4, 5 12.5, 14.0 80–100 (φ) / 210 (z) 10–12 (φ) / 25 (z)<br />

◆ Radiation-hard up to 2 Mrad.<br />

◆ Target resolution σ xy = σ z = [50/p T (GeV/c) ⊕ 15] µm.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


SVT Per<strong>for</strong>mance<br />

(µm)<br />

60<br />

40<br />

SVT Hit Resolution vs. Incident Track Angle<br />

Layer 1 - Z View B ABAR<br />

Data - Run 7925<br />

Monte Carlo - SP2<br />

20<br />

0<br />

-50 0 50<br />

(deg)<br />

(µm)<br />

60<br />

40<br />

Layer 1 - φ View<br />

Data - Run 7925<br />

Monte Carlo - SP2<br />

B ABAR<br />

20<br />

0<br />

-50 0 50<br />

(deg)<br />

◆ Achieved the target resolution <strong>for</strong> high-p tracks (15 µm at 0°)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


SVT Radiation Dose<br />

Radiation on the SVT is<br />

monitored by PIN diodes<br />

◆ Abort the beams in case <strong>of</strong><br />

severe radiation<br />

◆ Monitor the accumulated<br />

dose vs. “budget”<br />

→ Still com<strong>for</strong>table headroom under the radiation budget<br />

◆ May have to replace the modules in the horizontal plane in a few years<br />

◆ The rest OK <strong>for</strong> 10 years<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Drift Chamber (DCH)<br />

324 1015 1749<br />

68<br />

35<br />

202<br />

17.19<br />

551 973<br />

9 GeV e − 3.1 GeV e +<br />

IP<br />

236<br />

469<br />

1618<br />

◆ 7104 hexagonal cells in 40 layers = 10 superlayers x 4 layers each.<br />

◆ 4 axial and 6 stereo superlayers: AUVAUVAUVA.<br />

◆ Low mass materials to reduce multiple scattering.<br />

➤He:iC 4 H 10 (80:20) gas. Al field wires.<br />

➤Be inner cylinder (0.28% X 0 ). Carbon fibre outer cylinder (1.5% X 0 ).<br />

➤Al endplates (12 mm <strong>for</strong>ward, 24 mm backward).<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Resolution (cm)<br />

0.025<br />

0.02<br />

Tracks with p>1GeV/c<br />

DCH Per<strong>for</strong>mance<br />

<strong>BABAR</strong><br />

◆ He-iC 4 H 10 (80/20) <strong>for</strong> low multiple<br />

scattering<br />

◆ Single hit resolution in data (●)<br />

reached design: 2σ up to<br />

700 MeV/c → DIRC covers<br />

p > 500 MeV/c<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


DIRC: the PID Device<br />

Detector <strong>for</strong> Internally Reflected Cerenkov light.<br />

Quartz<br />

n 3<br />

t z<br />

n 2<br />

Detector<br />

Surface<br />

t y<br />

n 1<br />

y<br />

z<br />

n 3<br />

θ D<br />

Particle Trajectory<br />

z<br />

Side View<br />

◆ Measures the angle <strong>of</strong> Cerenkov light trapped inside quartz bars due<br />

to total internal reflection.<br />

◆ Light emitted at the end <strong>of</strong> the bar expands in purified water.<br />

◆ An array <strong>of</strong> small-diameter PMTs detects the light.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


DIRC Design<br />

◆ 144 quartz bars. 1.7 cm thick, 4.9 m long. Arranged in 12 bar-boxes.<br />

◆ Stand-<strong>of</strong>f-box at the backward end contains 6 m 3 <strong>of</strong> purified water.<br />

◆ 11,000 PMTs in 12 sectors.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


DIRC Per<strong>for</strong>mance<br />

240<br />

220<br />

200<br />

180<br />

160<br />

140<br />

120<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

BaBar<br />

Resolution: 3.0 mrad<br />

Bhabha events<br />

810 820 830 840 850 860<br />

Cherenkov Angle (mrad)<br />

Principle proven. More work needed to achieve σ = 2 mrad.<br />

◆ Background from scattered photons. Association algorithm.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Number <strong>of</strong> events/10 MeV<br />

DIRC Per<strong>for</strong>mance<br />

2500<br />

8000<br />

6000<br />

4000<br />

2000<br />

Number <strong>of</strong> events/10 MeV<br />

2000<br />

1500<br />

1000<br />

500<br />

<strong>BABAR</strong><br />

D 0 → K - π + D 0 → K - π +<br />

Without DIRC<br />

With DIRC<br />

0<br />

0<br />

1.6 1.8 2 1.6 1.8 2<br />

K - π + Mass (GeV) K - π + Mass (GeV)<br />

Efficiency ~ 80%, Rejection factor ~ 5<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Electromagnetic Calorimeter (EMC)<br />

9 GeV e − 3.1 GeV e +<br />

◆ 6580 CsI(Tl) crystals. 5760 in the barrel, 820 in the endcap.<br />

◆ Quasi-projective geometry. Photodiode readout.<br />

◆ Material in front <strong>of</strong> EMC = 0.25 X 0 (at 90˚), 0.20 X 0 (at 20˚).<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Entries<br />

π 0 Mass E γγ<br />

> 500 MeV<br />

1800<br />

1600<br />

1400<br />

1200<br />

1000<br />

800<br />

600<br />

EMC Per<strong>for</strong>mance<br />

◆ CsI(Tl) with photodiode readout<br />

◆ E(Bhabha) and m(π 0 ) resolutions<br />

close to Monte Carlo<br />

BaBar<br />

σ = 5.3%<br />

π 0 -mass = 134.9 MeV<br />

π 0 -width = 7.2 MeV<br />

(MC 5.0%)<br />

Entries/Bin<br />

1800<br />

1600<br />

1400<br />

1200<br />

1000<br />

800<br />

600<br />

400<br />

200<br />

EMC Bhabha Clusters<br />

Constant = 1674<br />

σ = 2.2%<br />

Mean = 1.016<br />

(MC 2.0%)<br />

Sigma = 0.02214<br />

Forward Barrel<br />

BaBar<br />

0<br />

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3<br />

E measured / E expected deposited<br />

400<br />

200<br />

0<br />

0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24<br />

m γγ<br />

(GeV)<br />

Electron ID (p > 0.5 GeV)<br />

Electrons >90%<br />

Pions 1–2%<br />

Bhabhas<br />

γ conversions<br />

τ → 3π<br />

K S → 2π<br />

100%<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Instrumented Flux Return (IFR)<br />

◆ Bakelite-based Resistive Plate Chambers (RPCs).<br />

◆ Identifies muons above 0.5 GeV/c from their penetration through iron.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


IFR Per<strong>for</strong>mance<br />

Efficiencies <strong>for</strong> muons (e + e − µ + µ − ) and pions (τ and K S decays)<br />

eff.<br />

1<br />

0.8<br />

<strong>BABAR</strong> IFR Barrel<br />

0.6<br />

0.4<br />

muons from eeµµ<br />

pions from τ-3prong and Ks<br />

0.2<br />

0<br />

0 1 2 3 4<br />

p (GeV/c)<br />

>75% efficiency with ~3% π mis-id. probability.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Are We Ready to Measure sin2β?<br />

<strong>PEP</strong>-<strong>II</strong> had a terrific startup!<br />

◆ Collected 3.2 fb -1 on tape → But we need more<br />

<strong>BABAR</strong> works beautifully<br />

◆ Initial problems have been solved<br />

◆ Per<strong>for</strong>mance very promising → Still much to do (calibration!)<br />

Physics analysis has started<br />

◆ Signals are seen in key channels → Limited statistics<br />

→ We don’t have a CP measurement just yet<br />

What can I show you today?<br />

Examples <strong>of</strong> what we have done to demonstrate<br />

➤How well we understand the detector<br />

➤How well the data agree with our expectation<br />

➤How realistic is our plan to measure sin2β by Summer<br />

Everything is PRELIMINARY and improving as I speak<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Inclusive J/ψ Reconstruction<br />

<strong>First</strong> step: Find J/ψ →l + l − decays.<br />

1.Select b-like events with R 2 < 0.5. (Fox-Wolfram moment ratio W 2 /W 0 )<br />

2. Combine 2 oppositely-charged tracks.<br />

3. Form a vertex with P(χ 2 ) > 1%.<br />

4. Very loose lepton ID.<br />

Monte Carlo signal: σ(m) = 11 MeV/c 2 .<br />

Candidates / 10 MeV/c 2<br />

100<br />

75<br />

50<br />

J/ψ → e + e - m(e + e - )<br />

e + e −<br />

<strong>BABAR</strong><br />

Candidates / 10 MeV/c 2<br />

150<br />

100<br />

J/ψ → µ + µ -<br />

µ + µ −<br />

<strong>BABAR</strong><br />

25<br />

50<br />

0<br />

2.6 2.8 3 3.2 3.4<br />

m(e + e - )<br />

GeV/c 2<br />

0<br />

2.6 2.8 3 3.2 3.4<br />

m(µ + µ - )<br />

GeV/c 2<br />

◆ Large tail in the e + e − channel due to bremstrahlung.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Candidates / 10 MeV/c 2<br />

60<br />

40<br />

20<br />

<strong>BABAR</strong><br />

σ = 16 MeV<br />

J/ψ → l + l - Signal<br />

J/ψ → e + e - (~540 pb -1 ) J/ψ →µ + µ - (~380 pb -1 )<br />

Candidates / 10 MeV/c 2<br />

100<br />

75<br />

50<br />

25<br />

J/ψ → µ + µ -<br />

<strong>BABAR</strong><br />

σ = 15 MeV<br />

J/ψ → e + e - m(e + e - )<br />

0<br />

2.6 2.8 3 3.2 3.4<br />

m(e + e - )<br />

GeV/c 2<br />

→ Already reasonable, but can be improved<br />

0<br />

2.6 2.8 3 3.2 3.4<br />

m(µ + µ - )<br />

GeV/c 2<br />

Tail due to Bremsstrahlung Background due to muon ID<br />

◆ Efficiency consistent with expectation<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Inclusive K S →π + π − Reconstruction<br />

Selection:<br />

◆ 2 oppositely-charged<br />

tracks. → Fit a vertex.<br />

◆ P(χ 2 ) > 0.01.<br />

◆ Angle α between p(K S )<br />

and the flight path:<br />

cosα > 0.999.<br />

Double Gaussian fit:<br />

◆ m(K S ) = 498.94 ± 0.12<br />

MeV/c 2 .<br />

◆ σ 1 = 2.8 MeV/c 2 (69%).<br />

◆ σ 2 = 11 MeV/c 2 (31%).<br />

N(π + π - ) candidates/1.7 MeV<br />

300<br />

200<br />

100<br />

<strong>BABAR</strong><br />

0<br />

0.45 0.475 0.5 0.525 0.55<br />

M(π + π - ) (GeV)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


B 0 → J/ψ K S Reconstruction<br />

Combine J/ψ and K S into B 0 .<br />

◆ J/ψ mass cut: 2.5 < m < 3.28 GeV/c 2 <strong>for</strong> e + e − ; ±3σ <strong>for</strong> µ + µ − .<br />

◆ Looser “lepton ID” using only the EMC.<br />

◆ K S mass cut: ±3σ.<br />

◆ Looser K S flight direction cut: cosα > 0.9.<br />

Constrained fit using m(J/ψ) and m(K S ) from the PDG.<br />

Helicity angle cut:<br />

◆ θ h = angle between K S and l + momenta<br />

in the CMS <strong>of</strong> J/ψ.<br />

➤Signal distribution ∝ sin 2 θ h .<br />

➤Background peaks near cosθ h = ±1.<br />

◆ Cut at cosθ h<br />

< 0.95.<br />

K S<br />

θ h<br />

l +<br />

J/ψ<br />

l −<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


B 0 → J/ψ K S Signal — MC<br />

Define signal region using 2 variables:<br />

CMS CMS<br />

◆ ∆E = E B – .<br />

E beam<br />

◆ “Beam-constrained” B 0 mass:<br />

m B<br />

=<br />

CMS<br />

E beam<br />

( ) 2 CMS<br />

– ( p B ) 2<br />

Monte Carlo signal events →<br />

◆ σ(∆E) = 10 MeV.<br />

◆ σ(m B ) = 2 MeV.<br />

→ Define ±3σ signal box.<br />

Look at the real data now!<br />

∆E (GeV)<br />

0.5<br />

0.4<br />

0.3<br />

0.2<br />

0.1<br />

0<br />

-0.1<br />

-0.2<br />

-0.3<br />

-0.4<br />

-0.5<br />

5.2 5.22 5.24 5.26 5.28 5.3<br />

Inv. Mass J/PsiKs (GeV)<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


◆<br />

∆E<br />

=<br />

E B<br />

CMS<br />

–<br />

B 0 → J/ψ K S Signal<br />

CMS<br />

E beam<br />

◆<br />

m B<br />

=<br />

CMS<br />

( E beam<br />

) 2 CMS<br />

– ( p B<br />

) 2<br />

0.1<br />

0.05<br />

<strong>BABAR</strong><br />

0<br />

4<br />

<strong>BABAR</strong><br />

-0.05<br />

3<br />

-0.1<br />

2<br />

5.2 5.225 5.25 5.275 5.3<br />

Data ~620 pb -1<br />

1<br />

0<br />

5.2 5.225 5.25 5.275 5.3<br />

Signal 12 events<br />

Background 1.4 ± 0.2<br />

Expected signal 9.8 ± 1.1<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Crosscheck: B + → J/ψ K +<br />

Charged channel B + → J/ψ( l + l − )K + .<br />

◆ Similar to the gold-plated channel.<br />

➤Kinematics.<br />

➤J/ψ reconstruction.<br />

◆ Higher BR (9.9 x 10 -4 ).<br />

◆ Easier reconstruction.<br />

◆ Self-tagging, i.e. B + flavor known by K + charge.<br />

→ Useful <strong>for</strong> data-based studies <strong>of</strong><br />

➤J/ψ reconstruction efficiency.<br />

➤Tagging algorithms.<br />

➤Vertex resolution.<br />

Selection is identical to J/ψ K S except <strong>for</strong> the K + :<br />

◆ Any good (>20 DCH hits) track.<br />

◆ If available, Cerenkov angle within ±6 mrad. <strong>of</strong> expected value.<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Data ~620 pb -1<br />

Signal 41 events<br />

Background 5.0 ± 0.6<br />

Expected signal 39.0 ± 3.4<br />

B + → J/ψ K + Signal<br />

∆E(GeV)<br />

0.1<br />

0.05<br />

0<br />

N ev<br />

/2.5MeV<br />

10<br />

7.5<br />

5<br />

2.5<br />

<strong>BABAR</strong><br />

0<br />

5.2 5.225 5.25 5.275 5.3<br />

B ch<br />

mass(GeV)<br />

-0.05<br />

-0.1<br />

B A B AR<br />

5.2 5.225 5.25 5.275 5.3<br />

M b<br />

(GeV)<br />

→ J/ψ K signals under control<br />

◆ Yields agree with expectations<br />

◆ Resolution OK<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


20<br />

15<br />

10<br />

5<br />

0<br />

15<br />

10<br />

5<br />

0<br />

<strong>BABAR</strong><br />

Vertexing in J/ψ K + (K* 0 )<br />

Signal<br />

Sideband<br />

-0.2 0 0.2<br />

-0.2 0 0.2<br />

∆z(cm)<br />

∆z(cm)<br />

→ Machinery <strong>for</strong> vertexing works<br />

◆ Needs different test <strong>for</strong> per<strong>for</strong>mance<br />

Exercise vertexing using<br />

◆ B + → J/ψ K +<br />

◆ B 0 → J/ψ K* 0<br />

K + π −<br />

96±12 events in ~1 fb -1 data<br />

◆ S/B ~ 6<br />

∆z = z-distance between<br />

◆ fully-reconstructed B vertex<br />

◆ vertex <strong>of</strong> the rest <strong>of</strong> the event<br />

Signal - sideband agrees with<br />

MC (histogram)<br />

◆ Width comes from lifetime<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


D* Signal and Vertexing<br />

N events/200 keV<br />

200<br />

100<br />

<strong>BABAR</strong><br />

Be<strong>for</strong>e refit<br />

σ 1<br />

=354 keV (27%)<br />

σ 2<br />

=908 keV (73%)<br />

◆ ∆m resolution improves with beam<br />

pr<strong>of</strong>ile constraint<br />

Entries/0.1 ps<br />

60<br />

~220 pb-1<br />

<strong>BABAR</strong><br />

40<br />

N events/200 keV<br />

0<br />

300<br />

200<br />

0.14 0.145 0.15 0.155<br />

m(Kππ) - m(Kπ)<br />

GeV<br />

<strong>BABAR</strong><br />

After refit<br />

σ 1<br />

=280 keV (47%)<br />

σ 2<br />

=679 keV (53%)<br />

20<br />

0<br />

-3 -2 -1 0 1 2 3<br />

Candidate D 0 → kπ proper decay time (ps)<br />

100<br />

N(D 0 ) 668 ± 33<br />

N(bkg) 79 ± 11<br />

0<br />

0.14 0.145 0.15 0.155<br />

m(Kππ) - m(Kπ)<br />

GeV<br />

τ(D 0 )<br />

σ(t)<br />

407 ± 25 ps<br />

437 ± 25 ps<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


0.17<br />

0.16<br />

∆m, GeV<br />

0.15<br />

~390 pb-1<br />

B 0 → D* − e + ν Signal<br />

<strong>BABAR</strong><br />

events/1 MeV<br />

80<br />

60<br />

40<br />

20<br />

0<br />

<strong>BABAR</strong><br />

0.14 0.15 0.16 0.17<br />

m(D * )-m(D 0 ), GeV<br />

60<br />

<strong>BABAR</strong><br />

0.14<br />

-4 -2 0 2 4<br />

MM 2 , GeV 2<br />

◆ ∆m = m(D* − )−m(D 0 ), MM = missing mass<br />

◆ 124 ± 19 signal events on 92 ± 12 bkgd<br />

events/0.4 GeV 2<br />

40<br />

20<br />

0<br />

-4 -2 0 2 4<br />

MM 2 , GeV 2<br />

→ Will be used <strong>for</strong> a mixing measurement<br />

→ Tagging efficiency and purity → CP measurement<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Extraction <strong>of</strong> sin2β<br />

We know how to find the signal. Efficiency and resolution ~OK<br />

How do we measure the CP violation?<br />

1.Measure the distance ∆z<br />

➤Vertexing working. Studies <strong>of</strong> resolution in the data continue<br />

2. Tag the flavor <strong>of</strong> the other B<br />

➤Particle ID critical. Studies underway using post-October data<br />

➤Mixing analysis using exclusive final states (e.g. D*l ν) will measure<br />

efficiency and mis-tag probability in the data<br />

3. Fit the ∆z distribution signed by the tag<br />

Full MC analyses have been per<strong>for</strong>med <strong>for</strong> J/ψ K S (+ a few others)<br />

◆ Expected signal yield ~160 events/10 fb -1 (Agrees with data)<br />

◆ Effective tagging efficiency ~ 22%–29% depending on the technique<br />

→ σ(sin2β) ~ 0.26–0.30 <strong>for</strong> 10 fb -1 , depending on sin2β<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC


Summary and Outlook<br />

<strong>PEP</strong>-<strong>II</strong> had a terrific first year<br />

◆ Record luminosity: 1.7 x 10 33 cm -2 s -1 . 80 pb -1 /day<br />

◆ <strong>BABAR</strong> recorded 3.2 fb -1 → 10 fb -1 by the end <strong>of</strong> August<br />

<strong>BABAR</strong> is working great<br />

◆ Per<strong>for</strong>mance approaching the design goals<br />

Analyses are progressing rapidly<br />

◆ Signals seen in key channels. Efficiency & resolution OK<br />

◆ Vertexing resolution OK. More tests will follow<br />

◆ Tagging being studied using real data, e.g. B 0 → D*l ν<br />

→ All ingredients <strong>for</strong> the CP measurement getting ready<br />

More data is coming!!!<br />

◆ With 10 fb -1 : Expect ~160 J/ψ K S events →σ(sin2β) ≤ 0.3<br />

<strong>First</strong> <strong>Year</strong> <strong>of</strong> BaBar/<strong>PEP</strong>-<strong>II</strong> Masahiro Morii SLAC

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