1. February 2001B Factory Detectors1 B-Factory Detectors 9 th Vienna Conference on Instrumentation Daniel Marlow Princeton University February 23, 2001

2. February 2001B Factory Detectors2 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.

3. February 2001B Factory Detectors3 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.

4. February 2001B Factory Detectors4 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.

5. February 2001B Factory Detectors5 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

6. February 2001B Factory Detectors6 The Measurement tag CP Need to: Measure momenta ID leptons & K’s Measure vertices CP eigenstate

7. February 2001B Factory Detectors7 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.

8. February 2001B Factory Detectors8 A Specific Reaction (Case Study) Momentum Measurement Hadron Identification Lepton Identification tag CP eigenstate

9. February 2001B Factory Detectors9 Asymmetric Colliders SLAC: PEP II 9.0 x 3.1 GeV/c Lumi: Design Achieved KEK: KEK- 8.0 x 3.5 GeV/c Lumi: Design Achieved 12 fb -1 25 fb -1

10. February 2001B Factory Detectors10

11. February 2001B Factory Detectors11 Detector Drawing B=1.5 T

12. February 2001B Factory Detectors12 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:C 2 H 6 ), detector-mounted readout electronics, and shaped endplates.

13. February 2001B Factory Detectors13 Wire-Stringing Robot (BaBar) Drift Chamber Mechanics Curved Endplate (Belle)

14. February 2001B Factory Detectors14 Belle Drift Chamber Side View The chambers are not huge by HEP standards.

15. February 2001B Factory Detectors15 Drift Chamber Performance BaBar Position Resolution Cosmic Rays

16. February 2001B Factory Detectors16 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

17. February 2001B Factory Detectors17 Mass Reconstruction Belle

18. February 2001B Factory Detectors18 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.

19. February 2001B Factory Detectors19 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

20. February 2001B Factory Detectors20 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 ~10 4 crystals.

21. February 2001B Factory Detectors21 CsI EM Calorimeter Performance Belle

22. February 2001B Factory Detectors22 Electron Identification 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. BaBar

23. February 2001B Factory Detectors23 Electron Identification A number of quantities in addition to E/p are combined to form an electron-ID likelihood function. Belle

24. February 2001B Factory Detectors24 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.

25. February 2001B Factory Detectors25 Instrumented Flux Return BaBar

26. February 2001B Factory Detectors26 RPC Principles of Operation 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. Spacers Signal pickup (x) India Ink Glass plates 8 kV Signal pickup (y) India Ink +++++++++++++++ _ _ _ _ _ _ _ _ _ _ _ +++ +++++ _ _ _ _ _ _ _ The discharged area recharges slowly through the high-resistivity glass plates (Belle) or linseed-oil coated bakelite (BaBar). Before After

27. February 2001B Factory Detectors27 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.

28. February 2001B Factory Detectors28 Exclusive Mode Reconstruction The muon systems also are used to reconstruct events by detecting the direction of the K L. This mode complements the mode, but has more background. K L hits KLKL

29. February 2001B Factory Detectors29 Particle Identification Hadron Identification tag CP eigenstate Thus far we have seen the tools needed to detect the B 0 that decays to a CP eigenstate. The analysis also requires that we know the flavor of the decaying B 0, which is done by observing the way in which the other B 0 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.

30. February 2001B Factory Detectors30 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.

31. February 2001B Factory Detectors31 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.

32. February 2001B Factory Detectors32 Belle: Aerogel Barrel Module

33. February 2001B Factory Detectors33 The DIRC BaBar

34. February 2001B Factory Detectors34 The DIRC

35. February 2001B Factory Detectors35 A DIRC Event

36. February 2001B Factory Detectors36 PID Bake Off BaBar Belle Looks like BaBar does better.

37. February 2001B Factory Detectors37 The Measurement tag CP eigenstate X 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.

38. February 2001B Factory Detectors38 Silicon Vertex Detector Belle

39. February 2001B Factory Detectors39 Silicon Vertex Tracker BaBar

40. February 2001B Factory Detectors40 SVT Performance Position resolution vs. angle. BaBar

41. February 2001B Factory Detectors41 The Results Belle Babar

42. February 2001B Factory Detectors42 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.