1. 9th Vienna Conference on Instrumentation
B-Factory Detectors
9th Vienna Conference on Instrumentation
Daniel Marlow
Princeton University
February 23, 2001
February 2001
B Factory Detectors

2. Disclaimer
I am a member of Belle, but have been asked by the organizers to cover “B-Factory” detectors in general, which I took to mean the asymmetric e+e- B-Factory detectors, BaBar and Belle. I have done my best to be even handed. Any inaccuracies with respect to BaBar are entirely of my own creation.
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B Factory Detectors

3. Theme
Both collaborations have very recently submitted their first meaningful measurements of indirect CP violation for publication. Keeping in mind the timeliness of those results and the purpose of this conference, I thought it might of interest to examine the CP violation measurement paying special attention to the detector techniques involved.
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B Factory Detectors

4. Physics Goal
The principal physics goal of the B Factories is to test the KM picture of CP violation by measuring the angle (aka ) of the unitarity triangle.
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B Factory Detectors

5. Indirect CP Violation
Thus for a decay where is a CP eigenstate, we have two “indistinguishable” decay paths
This leads to a time-dependent CP asymmetry
where
February 2001
B Factory Detectors

6. The Measurement tag Need to: Measure momenta ID leptons & K’s
Measure vertices CP
CP eigenstate
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B Factory Detectors

7. The Measurement
The time-dependent asymmetry appears mainly as a mean shift in the distribution between events tagged
as decays and
events tagged as
decays.
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B Factory Detectors

8. A Specific Reaction (Case Study)
Hadron Identification
Lepton Identification
tag
CP eigenstate
Momentum Measurement
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B Factory Detectors

9. Asymmetric Colliders 25 fb-1 12 fb-1 KEK: KEK- 8.0 x 3.5 GeV/c
Lumi: Design
Achieved
SLAC: PEP II 9.0 x 3.1 GeV/c
Lumi: Design
Achieved
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B Factory Detectors

10. February 2001
B Factory Detectors

11. Detector Drawing
B=1.5 T
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12. Charged Particle Momentum Measurement
The basis of essentially every signal we study.
Uses drift-chamber technology, which has progressed from a “black art’’ to something approaching science over the past ~30 years.
Basic concept is the same, but there are some refinements, e.g., the use of low-Z gas (He:C2H6), detector-mounted readout electronics, and shaped endplates.
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B Factory Detectors

13. Drift Chamber Mechanics
Wire-Stringing Robot (BaBar)
Drift Chamber Mechanics
Curved Endplate (Belle)
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B Factory Detectors

14. Belle Drift Chamber Side View
The chambers are not huge by HEP standards.
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B Factory Detectors

15. Drift Chamber Performance
Cosmic Rays
BaBar Position Resolution
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B Factory Detectors

16. dE/dx Particle Identification
A side benefit of the drift chambers is the ionization measurement, which is sensitive to velocity and therefore mass.
tag region
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B Factory Detectors

17. Belle
Mass Reconstruction
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B Factory Detectors

18. Mass Reconstruction
By reconstructing the invariant mass and the CM momentum of the decaying B meson, it is possible to eliminate almost all of the background for the gold-plated CP mode.
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B Factory Detectors

19. Photon Detection
Since roughly half of the energy in B decays comes in the form of neutrals, it is important to have precision photon detection.
Belle
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B Factory Detectors

20. Photon Detection BaBar
Photons are detected by using CsI(Tl) crystals read by silicon photodiodes. Nearly 4 coverage is attained with a “fly’s eye array comprising ~104 crystals.
BaBar
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B Factory Detectors

21. CsI EM Calorimeter Performance
Belle
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B Factory Detectors

22. Electron Identification
BaBar
The ability to discriminate between leptons and pions is crucial to the physics analysis, for example in identifying
decays.
The most powerful separation techniques involve comparison between the measurements in the drift chamber and the calorimeter, for example E/p.
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B Factory Detectors

23. Electron Identification
Belle
A number of quantities in addition to E/p are combined to form an electron-ID likelihood function.
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B Factory Detectors

24. Muon Identification
Muons are important for the same reason. They are identified by the way in which they penetrate the iron return yoke of the magnet.
The gaps are instrumented with RPCs.
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B Factory Detectors

25. Instrumented Flux Return
BaBar
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26. RPC Principles of Operation
Signal pickup (x)
India Ink
Glass plates
8 kV
Signal pickup (y)
Spacers
_ _ _ _ _ _ _ _ _ _ _
A passing charged particle induces an avalanche, which develops into a spark. The discharge is quenched when all of the locally ( ) available charge is consumed.
Before
_ _ _ _ _ _ _
The discharged area recharges slowly through the high-resistivity glass plates (Belle) or linseed-oil coated bakelite (BaBar).
After
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B Factory Detectors

27. The RPC Experience Is Not All Positive
If high dark currents develop, RPCs can suffer a serious efficiency loss due to high IR voltage drops in the resistive plates. This problem developed in the early days of Belle (fortunately, we were able to solve it). BaBar also has serious problems along these lines.
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B Factory Detectors

28. Exclusive Mode Reconstruction
The muon systems also are used to reconstruct
events by detecting the direction of the KL. This mode complements the
mode, but has more background. KL
KL hits
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B Factory Detectors

29. Particle Identification
CP eigenstate
Particle Identification
tag
Hadron Identification
Thus far we have seen the tools needed to detect the B0 that decays to a CP eigenstate. The analysis also requires that we know the flavor of the decaying B0, which is done by observing the way in which the other B0 decays. One way of doing this is to measure the sign of charged kaons. Measuring the sign is easy, but distinguishing kaons from the ~10X more copious pions requires special detectors.
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B Factory Detectors

30. Particle Identification
It is in this area that Belle and BaBar differ the most. Belle chose to adopt and refine existing techniques, whereas BaBar employed a newly invented device called the DIRC. In both cases the idea is to measure the particle’s velocity.
Belle uses time of flight (ToF) plus a system of aerogel Cerenkov counters (ACC).
BaBar’s DIRC measures the emission angle of Cerenkov light in precisely machined and polished quartz bars.
Both groups complement this information with dE/dx measurements mentioned earlier.
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B Factory Detectors

31. Belle: ToF & Aerogel
The ToF has a resolution of =100 ps, which provides /K separation up to about 1.2 GeV/c, at which point the ACC array kicks in. The index of refraction of the counters varies as a function of angle, reflecting the boosted CM.
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B Factory Detectors

32. Belle: Aerogel
Barrel Module
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B Factory Detectors

33. The DIRC
BaBar
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34. The DIRC
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35. A DIRC Event
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36. PID Bake Off BaBar Belle Looks like BaBar does better. February 2001
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37. The Measurement tag X CP eigenstate
The characteristic decay times are ~1 ps, which is too short to measure by direct timing. Instead, we measure the decay distances (z=~200 m) using a silicon vertex detector.
See also this morning’s talk by S. Stanic.
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B Factory Detectors

38. Silicon Vertex Detector
Belle
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39. Silicon Vertex Tracker
BaBar
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40. SVT Performance BaBar Position resolution vs. angle. February 2001
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41. The Results
Belle
Babar
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42. Conclusion
The B-Factory detectors have taken good advantage of advances in detector technology and are on the verge of making the first direct observations of CP violation outside of the neutral K system. It is sometimes said that the technology needed for these detectors is “easy,” but that is only because we are standing on the shoulders of giants.
February 2001
B Factory Detectors