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					                 B-Factory Detectors
                9th Vienna Conference on
                     Instrumentation

                       Daniel Marlow
                    Princeton University
                     February 23, 2001
February 2001            B Factory Detectors   1
                            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.

February 2001                  B Factory Detectors            2
                             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.




February 2001               B Factory Detectors                  3
                           Physics Goal
                       
The principal
physics goal of the
                                      , 
B Factories is to            
                           V                        Vtd
                                         2
test the KM picture         ub
of CP violation by         A  3
                                                    A3
measuring the
angle  1 (aka  )
of the unitarity             3                    1
triangle.                                                             
                  2
                                          Vcb             1,0
                                          A3
  February 2001              B Factory Detectors                  4
                             Indirect CP Violation
                                          Thus for a decay B  f
                                                            0
                         0
                     B                    where f is a CP eigenstate, we
B0                                 f      have two “indistinguishable”
                                          decay paths
This leads to a time-                          ( B 0  f )  ( B 0  f )
dependent CP                      ACP ( t ) 
asymmetry
                                               ( B 0  f )  ( B 0  f )
                                             sin(md t ) sin 21
                                  where
                                                      VtbVtd 
                                                         *
                                         1    arg *   
                                                     VcbVcd 
     February 2001                 B Factory Detectors                 5
                     The Measurement
                                           
                              tag
                                                    D0
                     0
                                                         Need to:
e      e       B
                                                         •Measure momenta
             0
                  z                                     •ID leptons & K’s
         B
                 J /                                    •Measure vertices
                          
         0
                                       z  ct
     K
                       CP
CP eigenstate
 February 2001                B Factory Detectors                            6
                             The Measurement
The time-dependent
asymmetry appears
mainly as a mean shift
in the t distribution          dN
between events tagged
            0                   dt
as      B       decays and
events tagged as        B0
decays.


                                                       t (ps)

     February 2001               B Factory Detectors             7
        A Specific Reaction (Case Study)
        
     e e B B         0   0                     CP eigenstate

                                    J /  KS
                tag   K X         
                                                                    


                                                      
                                                l l
   Hadron
   Identification                                               Momentum
                                  Lepton
                                  Identification                Measurement


February 2001                 B Factory Detectors                           8
                                                 
                   Asymmetric e e Colliders

                                                          25 fb-1

       12 fb-1

                                         ee




KEK: KEK- 8.0 x 3.5 GeV/c                        SLAC: PEP II 9.0 x 3.1 GeV/c
                          -2 1
                                                 Lumi: Design 3  1033 cm-2s1
                     34
Lumi: Design 10 cm s
                     33 -2 1
    Achieved 2.4  10 cm s                              Achieved    3.1  1033 cm-2s1
   February 2001                  B Factory Detectors                          9
February 2001   B Factory Detectors   10
                 B=1.5 T
                Detector Drawing




February 2001       B Factory Detectors   11
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.


February 2001           B Factory Detectors              12
 Wire-Stringing
                                 Drift Chamber
 Robot (BaBar)
                                  Mechanics




                            Curved Endplate (Belle)
February 2001     B Factory Detectors                 13
           Belle Drift Chamber Side View




     The chambers are not huge by HEP standards.
February 2001         B Factory Detectors          14
                                             Drift Chamber
                                              Performance

                                               Cosmic Rays




 BaBar Position Resolution



February 2001          B Factory Detectors                   15
                   dE/dx Particle Identification
A side benefit of the drift
chambers is the ionization
measurement, which is
sensitive to velocity and
therefore mass.
                                                    tag
     1  dE                                        region
            1  log p 
dE
    2
dx   dx 




   February 2001              B Factory Detectors            16
                  K S    
                    0
                                                   Belle
                   eff  4.6 MeV/ c 2
    Mass
Reconstruction




                                 M     (GeV/c2 )
  February 2001     B Factory Detectors                    17
      Mass
  Reconstruction
B0  J /  KS
             
           
               

       l l
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.

  February 2001               B Factory Detectors   18
                     Photon Detection

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

B0  J /  KS
            
           
              0 0

      l l

   February 2001              B Factory Detectors           19
                              Photon Detection
                        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
February 2001     B Factory Detectors                20
           CsI EM Calorimeter Performance
                                                  Belle

                                                 Reconstruc
                                                              tion
  E  50 MeV




         0 Reconstruc
                     tion

February 2001           B Factory Detectors                    21
       Electron
     Identification
                                                  BaBar
The ability to discriminate
between leptons and pions is                      e e  e e
crucial to the physics analysis,
for example in identifying
                      
          J / e e
decays.
The most powerful separation
techniques involve comparison
between the measurements in
the drift chamber and the
calorimeter, for example E/p.
  February 2001             B Factory Detectors                   22
     Electron
   Identification                                   Belle

A number of quantities in
addition to E/p are
                                                  e e  eee e
combined to form an
electron-ID likelihood
function.
                                                    K S    




    February 2001           B Factory Detectors                       23
   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.

 B   J / K 
       K 
 plus tag - side 
     February 2001       B gaps are instrumented with RPCs. 24
                       The Factory Detectors
                Instrumented Flux Return




February 2001           B Factory Detectors
                                              BaBar   25
                   RPC Principles of Operation
Signal pickup (x)                                                  India Ink


Glass plates                                                8 kV


Signal pickup (y)                                                   India Ink
                                Spacers
                           A passing charged particle induces an avalanche,
   +++++++++++++++         which develops into a spark. The discharge is
   ___________             quenched when all of the locally (A  0.1 cm2 )
                           available charge is consumed.
 Before
                                                      +++            +++++
     The discharged area recharges slowly through     ___            ____
     the high-resistivity glass plates (Belle) or
     linseed-oil coated bakelite (BaBar).                After
   February 2001                B Factory Detectors                       26
    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.


   February 2001             B Factory Detectors   27
                   Exclusive Mode Reconstruction


The muon systems also are
used to reconstruct
  B 0  J / K L
events by detecting the
direction of the KL. This
                                                   KL
mode complements the
  B 0  J / K S
mode, but has more
background.        KL hits

   February 2001             B Factory Detectors        28
   
e e B B               0   0
                                   CP eigenstate                    Particle
           tag                  J /  KS                        Identification
                                  
Hadron                    K X               Thus far we have seen the tools needed
Identification                               to detect the B0 that decays to a CP
                               
                                             eigenstate. The analysis also requires
                           W                 that we know the flavor of the
                                             decaying B0, which is done by
   b                                        observing the way in which the other
                                   W         B0 decays. One way of doing this is
                                             to measure the sign of charged kaons.
             c                               Measuring the sign is easy, but
                                             distinguishing kaons from the ~10X
                                             more copious pions requires special
       u           s                         detectors.
                           
   February 2001
                       K                   B Factory Detectors                29
                  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.
February 2001                B Factory Detectors                30
                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.
February 2001           B Factory Detectors                31
                                                Belle: Aerogel




          Barrel Module




February 2001             B Factory Detectors               32
The DIRC




February 2001   B Factory Detectors
                                      BaBar   33
The DIRC




February 2001   B Factory Detectors   34
A DIRC Event                 e e  
                                               




           t  300 ns                         t  8 ns
 February 2001          B Factory Detectors              35
                        PID Bake Off




                BaBar                           Belle
  Looks like BaBar does better.
February 2001             B Factory Detectors           36
                                       K
                                 tag            X            The Measurement
                             0
e       e              B

                 0
                          z
             B
                         J /
     K   0                       
CP                       
eigenstate

     The characteristic decay
     times are ~1 ps, which is too
     short to measure by direct
     timing. Instead, we measure                            See also this
     the decay distances                                    morning’s talk by S.
     (z=~200 m) using a                                   Stanic.
     silicon vertex detector.
         February 2001                      B Factory Detectors                    37
Silicon Vertex
   Detector


  Belle




   February 2001   B Factory Detectors   38
                Silicon Vertex
                                                BaBar
February 2001
                    Tracker
                          B Factory Detectors           39
                SVT Performance


                                           Position
                                           resolution vs.
                                           angle.




                                          BaBar



February 2001       B Factory Detectors                40
                The Results




    sin 21  0.58  0.34  .10        Belle
    sin 21  0.34  0.20  .05        Babar
February 2001    B Factory Detectors           41
                    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            42

				
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