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					Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                       Experiments to measure CP-violation –
                                    B-factories
               Seminar on detectors for Particle and Nuclear Physics Experiments


                                                      Benedict Winter

                                                          Bonn University


                                                        7th June 2011




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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                          Content


          1. Introduction
                    • CP-violation in the B-system
                    • Overview B-factories

          2.    Detector components of BaBar and Belle
          3.    B-reconstruction
          4.    Results and outlook
          5.    Summary




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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                       Symmetries

               • Until 1950’s: C ,P,T seem to be fundamental symmetries
               • Wu (1957):   in weak interaction (1957: NP for Lee and Yang)
                              P
               • After that: CP seems to be a fundamental symmetry:




               • Cronin, Fitch (1964, NP 1980): First proof of ¨ in Kaon-decays
                                                               CP
                                                                ¨
               • Sakharov (1967): ¨ is condition for matter-antimatter asymmetry
                                  CP
                                   ¨



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                          Theoretical description: CKM-matrix
               • CKM-matrix ↔ coupling strength of u-type quarks to d-type
                 quarks in CC weak interactions:

                                                              
                  d       Vud                   Vus      Vub     d
                 s  =  Vcd                   Vcs      Vcb   s 
                  b       Vtd                   Vts      Vtb     b

               • Kobayashi & Maskawa (1973): Get a CP-violating phase e iδ = ρ + iη
                 if one assumes three families → expand Cabibbos theory.
                                                           2
                                                                                                   
                                               1− λ 2
                                                                         λ            Aλ3 (ρ − iη)
                                                                            2                             4
                     VCKM        =                                      1− λ              Aλ2       + O(λ )
                                                                                                  
                                                −λ                        2
                                          Aλ3 (1 − ρ − iη)              −Aλ 2
                                                                                           1


               • Expect large ¨ in some B-decay channels
                              CP
                               ¨


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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                          Theoretical description: CKM-matrix
               • CKM-matrix ↔ coupling strength of u-type quarks to d-type
                 quarks in CC weak interactions:

                                                              
                  d       Vud                   Vus      Vub     d
                 s  =  Vcd                   Vcs      Vcb   s 
                  b       Vtd                   Vts      Vtb     b

               • Kobayashi & Maskawa (1973): Get a CP-violating phase e iδ = ρ + iη
                 if one assumes three families → expand Cabibbos theory.
                                                           2
                                                                                                   
                                               1− λ 2
                                                                         λ            Aλ3 (ρ − iη)
                                                                            2                             4
                     VCKM        =                                      1− λ              Aλ2       + O(λ )
                                                                                                  
                                                −λ                        2
                                          Aλ3 (1 − ρ − iη)              −Aλ 2
                                                                                           1


               • Expect large ¨ in some B-decay channels
                              CP
                               ¨


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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                           CKM-matrix: Unitarity
       • Unitarity of CKM-matrix ↔ Conservation of quark number in 3 families

  Unitarity triangle


                                                                                 • If CP was conserved the
                                                                                     triangle would be flat (η = 0)
                                                                                 • If the triangle is not closed:
                                                                                     NP (FCNC or, quark family. . . )




       • ¨ in ”golden decay” immediately gives β ≡ Φ1
         CP
          ¨

  Goal of B-factory experiments
  Precise test of KM-mechanism in B-system and determination of SM parameters

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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                           CKM-matrix: Unitarity
       • Unitarity of CKM-matrix ↔ Conservation of quark number in 3 families

  Unitarity triangle


                                                                                 • If CP was conserved the
                                                                                     triangle would be flat (η = 0)
                                                                                 • If the triangle is not closed:
                                                                                     NP (FCNC or, quark family. . . )




       • ¨ in ”golden decay” immediately gives β ≡ Φ1
         CP
          ¨

  Goal of B-factory experiments
  Precise test of KM-mechanism in B-system and determination of SM parameters

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Introduction    SVT (BaBar)    CDC (Belle)   ACC und TOF (Belle)    DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                          B-mesons
        Lightest mesons with a b-quark:
                                    ¯
                              B + = bu             B − = b¯
                                                          u                     ¯
                                                                          B 0 = bd                ¯     ¯
                                                                                                 B 0 = bd
        Properties:
                               m ≈ 5280 MeV                        τ ≈ 1.5 ps ⇒ cτ ≈ 500 µm

        Production of B-mesons – Y (4S) resonance
                                          ¯
               • Y (4S) is the lightest b b state with m > 2mB
                                     ¯
               • ≈ 100% decay into B B-pairs:




               • mY (4S) = 10.6 GeV ⇒ B-mesons almost at rest in CMS


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                                            Mixing and CP-violation
                                ¯
        Diagram: Mixing of B 0 B 0 -pairs:




                                          ¯
               • Before decay: Coherent B B-state ⇒ simultaneous mixing.
                 After the decay of one B the remaining oscillates independently.
               • For common final states there are two decay paths:




               • Interference can lead to time dependent CP-asymmetry.
               • If f is a CP-eigenstate the decay amplitude is the same for both
                 paths

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                                                             CP
                                           ”Golden mode” for ¨¨
                                                              ¨

                                                             ¯
    • Time dependent CP-Asymmetry in the ”golden mode” B 0 /B 0 → J/ψK 0 :

                                                     ¯
                                Γ(B 0 → J/ψKS ) − Γ(B 0 → J/ψKS )
        AJ/ψKS (∆t) =                                ¯            = sin (∆m∆t) sin (2β)
                                Γ(B 0 → J/ψKS ) + Γ(B 0 → J/ψKS )
    Measurement



                                                                                • Flavor Tagging: Charge of
                                                                                                          ¯
                                                                                     W identifies one B or B
                                                                                • B-Reconstruction:
                                                                                     Verification of CP-eigenstate
                                                                                • Determine ∆t via ∆z




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                 Example for another B-factory measurement


    • Until now: ¨ in interference. Also possible: ”direct” ¨ in decay:
                 CP
                  ¨                                         CP
                                                             ¨



                                  ¯
           Γ(B 0 → K + π − ) = Γ(B 0 → K − π + )
           But Γ(B 0 → K + π − ) = 1.9 · 10−5

         ⇒ Need efficient ID of K ± and π ±




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                        Requirements on B-factory experiments



        Requirements
               • High vertex resolution
               • Good momentum resolution
               • Efficient ID of e, µ, K , π
               • Large acceptance despite boost
               • Low amount of ”dead” material




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                                                     Accelerators
                     PEP II (SLAC)                                              KEKB (KEK, Tsukuba)




                                                                         BaBar                            Belle
                 Ee − ; Ee + / GeV                                     9.0 / 3.1                       8.0 / 3.5
                         βγ                                               0.56                           0.425
           Design luminosity / cm−2 s−1                                 3 · 1033                        1 · 1034
          Reached luminosity / cm−2 s−1                                1.2 · 1034                      1.7 · 1034
        Integrated luminosity on Y (4S)/fb                          433 (1999-2008)                 711 (1999-2010)
                           ¯
                   # B/B-pairs                                           470 M                           780 M
                       ¯
                  B/B-pair rate                                          13 Hz                           19 Hz
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                                                    Belle-detector



                                                                                                    • Asymmetric design
                                                                                                    • |B| = 1.5 T
                                                                                                    • ≈ 130 k channels
                                                                                                    • Length ≈ 8 m
                                                                                                    • Height ≈ 10 m




        Mounted on rails for quick servicing and repair


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                                              Belle event display




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                                                  BaBar-detector



                                                                                                    • Asymmetric design
                                                                                                    • |B| = 1.5 T
                                                                                                    • ≈ 230 k channels
                                                                                                    • Length ≈ 8 m
                                                                                                    • Height ≈ 7.5 m




        Unique Cherenkov detector ”DIRC” for particle ID


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                                             BaBar event display




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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                          SVT: Silicon Vertex Tracker (BaBar)




        Purpose:
          • Innermost component providing tracking of charged particles
               • SVT is only detector for charged particles with pt < 100 MeV/c




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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                             Double sided silicon strip detectors


                                                                                   Get 2D-information:
                                                                                      1. Low n-doped bulk (300 µm)
                                                                                         is fully depleted
                                                                                      2. Charged particle creates
                                                                                         e − -hole pairs
                                                                                      3. e − → n-strips
                                                                                         holes → p-strips
                                                                                      4. Amplification and readout
                                                                                       • Strip pitch 50 µm




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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                              Setup




         • Number of modules in each layer:
               6, 6, 6, 15, 18
         • Third layer has a diameter of 10.8 cm
         • Fifth layer has a diameter of ≈ 25 cm



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                                                      Performance
                                             Polar angle dependence of σz




               • Multiple Coulomb scattering constraints resolution for p < 1 GeV/c
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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                              Belle event display




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Introduction    SVT (BaBar)    CDC (Belle)   ACC und TOF (Belle)    DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                              CDC: Central Drift Chamber (Belle)




    Purpose:                                                 Setup:
         • Measure momenta and                                     • 50 layers, 18 stereo layers (24 − 42 ◦ )
               trajectories                                        • 8400 drift cells
         • dE /dx-measurement                                      • Gas: 50% He, 50% Ethan
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                                                     Performance
  Momentum resolution (cosmic rays)                                   Energy loss in the CDC (collision data)




Method:                                                               Conclusion:
    • Compare momenta measured in                                        • ID with dE /dx is effective for
         the upper and the lower half                                       p      0.6 GeV
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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




               ACC: Aerogel Cherenkov counter system (Belle)




        Purpose:
               • K ± /π ± separation above 1.2 GeV/c
        Principle:
           1. If β > 1/n Cherenkov light is emitted. n is polar angle dependent.
           2. The photons are detected by PMT

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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




               ACC: Aerogel Cherenkov counter system (Belle)




        Purpose:
               • K ± /π ± separation above 1.2 GeV/c
        Principle:
           1. If β > 1/n Cherenkov light is emitted. n is polar angle dependent.
           2. The photons are detected by PMT

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                                    TOF: Time Of Flight (Belle)




        Purpose:
               • K ± /π ± separation below 1.2 GeV/c
        Setup:
               • Flight path: 1.2 m to IP; time resolution: 100 ps
               • 64 modules with 2 TOF counters and 1 TSC each
               • Fast organic scintillators and special PMT (B-field)


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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                      Performance

               K ± /π ± separation (Belle)
                                                                                 K ± /π ± separation (BaBar)




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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                             BaBar event display




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     DIRC: Detector of Internally Reflected Cherenkov light
                           (BaBar)




        Purpose
               • Efficient π ± K ± separation for 0.7 GeV/c < p < 4.2 GeV/c




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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                          Principle


                                                                                     1. Production of Cherenkov
                                                                                                       1         1
                                                                                        light if β > nquartz = 1.473 .
                                                                                                                       1
                                                                                        Opening angle: cos θC = βn
                                                                                     2. Total reflections inside the
                                                                                        quartz bar
                                                                                     3. Wedge enlarges angle
                                                                                     4. Propagation in pure water
                                                                                     5. Detection by PMT



               • Only one detection region in backward end


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                                                             Setup


                                                                            • 12 bar boxes with each 12
                                                                              quartz bars
                                                                            • 12 sectors with 896 PMT each
                                                                            • PMT have 29 mm diameter




     Reasons to use quartz                                             Reasons to use pure water
          • Large n = 1.473                                                • High transmission (98 %/m
          • Good optical finish on the                                           if 325 nm < λ < 642 nm)
               surfaces possible                                           • n = 1.346 close to quartz
          • Long attenuation length                                        • Relatively cheap
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                                                          Time cut
           + −                + −
        e e → µ µ event
       PMT signals within ±300 ns of                                     PMT signals within ±8 ns of the
       the trigger window:                                               expected arrival time:




               • Arrival time and measured θC depend on the particle trajectory
               • Uncertainty on track direction dominates ∆θC for low momenta

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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)    ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                     Performance
        Cherenkov angles for kinematically identified Pions / Kaons:




                      K /π separation:


                                                                                • Data from D -decays that are
                                                                                  followed by D 0 → K − π +



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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                              Belle event display




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                    ECL: Electromagnetic Calorimeter (Belle)


 Purpose:
      • 4-momenta of π 0 → measure
          50 MeV < Eγ < 4 GeV
      • 4-momenta of e ±


 CsI(Tl)-crystals and silicon
 photodiode readout:
      • Good energy resolution
          (Photon output: 5000 γ/MeV)                                   Crystals:
      • But: slow (∼ 1 µs) → pile up                                        • Length: 30.0 cm = 16.2 X0
                                                                                              ˆ
      • Photodiodes work well in B-field                                     • Diameter: ∼ 5.5 cm (front face)



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                                           Setup and performance



                                                                         Invariant π 0 -mass (collision data)




    • Crystals almost show to IP
        ∼ 1.5 ◦ tilted to prevent gaps

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Introduction    SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                 KLM: KL and Muon detection system (Belle)


   Purpose:
        • Muon- and KL ID

   Principle:
        • Detector- and iron layers are
               alternated
        • Measure ionization of gas
               by charged particles
        • Measure shape of clusters
        • No energy information can
               be obtained




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                                                      Performance
        Momentum dependence of                                              Momentum dependence of
      muon ID efficiency (cosmic rays)                                       muon fake rate (KS → π + π − )




               • Only muons with p > 600 MeV reach the KLM (B-field)
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                                                 B-reconstruction




        Test: Look weather
                                                            √        2
                                                               s
                                           mES =                         −        pi2 = mB
                                                              2
        Note: ES=energy substitute (BaBar), bc=beam constraint (Belle)
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                                Comparison of Belle and BaBar



               • Higher boost of BaBar → better ∆t resolution.
               • 5 SVT-layers and larger radii of BaBar → better low energy ID
               • CDC of Belle is larger → momentum resolution
               • K /π-separation of DIRC (BaBar) superior in efficiency and miss ID
               • Belle has larger ELM-radius → better angular resolution




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                                                           Results
 Time dependent CP-asymmetry of the golden mode



      • After removing the exponential
          decay dependence one gets:

     AJ/ψKS (∆t) = sin (∆m∆t) sin (2β)
 sin(2β) = 0.673 ± 0.023 (PDG 2010)




 Confirmation of the KM-mechanism of CP-violation
 2008: Nobel Price for Kobayashi and Maskawa

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Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                                           Results
 Time dependent CP-asymmetry of the golden mode



      • After removing the exponential
          decay dependence one gets:

     AJ/ψKS (∆t) = sin (∆m∆t) sin (2β)
 sin(2β) = 0.673 ± 0.023 (PDG 2010)




 Confirmation of the KM-mechanism of CP-violation
 2008: Nobel Price for Kobayashi and Maskawa

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                                                          Outlook




        There are new ”Super-B-factories” planned:
          • SuperKEKB with design luminosity 8 · 1035 cm−2 s−1
          • SuperB at Frascati with design luminosity 1036 cm−2 s−1
        B physics is also done at hadron colliders where also BS are produced:
          • LHCB at Cern (pp-collider)
          • CDF and D0 at Tevatron (p¯ -collider)
                                        p
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                                                         Summary



        • B-factories precisely measure ¨ in many decay channels
                                          CP
                                           ¨
            • Measure time dependent ACP with asymmetric designs (”golden mode”)
            • Highest luminosities so far → large statistics
            • High acceptance needed to reconstruct B-decays
            • Relatively low energy → ”dead material” is an issue
            • Unique DIRC provides efficient particle ID for p ∼ few GeV
        • The experiments are also sensitive for SM parameters and tests for NP




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                                  Thank you for your attention




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                                                        Literature
                ”The BaBar detector”, The BaBar collaboration (2001)

                ”The Belle detector”, National Instruments and Methods in Physics research
                A479 (2002)

                http://www.kek.jp/

                http://www.slac.stanford.edu/

                http://ppd.fnal.gov/

                Particle Physics Booklet, Particle Data Group, 2010

                http://www.hep.princeton.edu/marlow/talks/vienna/vienna.pdf

                http://cerncourier.com/

                http://www.hll.mpg.de/

                Lecture on particle physics I (WS2010/11) and II (SS2011), Bonn University

                http://www.wikipedia.org/ Press, second edition, 2007

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                  Contributions to likelihood functions (BaBar)
                                                               Muons:
               Variable        Description
               Ecand           Energy released in the EMC
               NL              Number of IF hit layers in a cluster
               λmeas           Measured number of interaction lengths traversed
               ∆λ              Difference between the expected (for muons) and measured number
                               of interaction lengths traversed
               χ2 fit           χ2 /dof of IFR hit strips in a polynomial fit of the cluster
               χ2 mat          χ2 /dof of the IFR hit strips in the track extrapolation
               Tc              Track continuity
               m and σm        Average multiplicity of hit strips per layer and its std. deviation

                                                                   Pions:
               Variable          Description
               dE /dx            Energy loss in the SVT and DCH
               θC                Cerenkov angle in the DRC
               Nγ                Number of photons in DRC
               TrkQual           Track quality
               iselectron        Whether the track passes the LH electron selector
               ismuon            Whether the track passes the MicroVeryTight muon selector
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                     Backup: Constraints on unitarity triangle




                                                                 45
Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                    Backup: Determination of CKM-elements




                                                                 46
Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




                                           Backup ACC: (Belle)
Purpose:
     • K ± /π ± separation above 1.2 GeV/c
Principle:
    1. If a particle has β > 1/n Cherenkov
       light is emitted in the Aerogel.
    2. The photons are detected by PMT




                                                                                              • 960 modules
                                                                                              • n polar angle
                                                                                                 dependent
                                                                                              • Special PMT for
                                                                                                 B = 1.5 T

                                                                 47
Introduction   SVT (BaBar)   CDC (Belle)   ACC und TOF (Belle)   DIRC (BaBar)   ECL (Belle)   KLM (Belle)   B-reco   Results   Summary




           Backup: Shape of time dependent asymmetry plot




                                                                 48

				
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posted:10/18/2012
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