First Year of BABAR/PEP-II - Harvard University Laboratory for ...
First Year of BABAR/PEP-II - Harvard University Laboratory for ...
First Year of BABAR/PEP-II - Harvard University Laboratory for ...
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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