Tracking performance in LHCb

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					              Tracking performance in LHCb
          IV INTERNATIONAL SYMPOSIUM
         ON LHC PHYSICS AND DETECTORS
                 FermiLab, Chicago
                   May 1-3 2003


                               Tracking Performance
                                Jeroen van Tilburg
                                      NIKHEF
                        On behalf of the LHCb collaboration
     Overview:
     • LHCb setup
     • Velo/ST/OT performance
     • Track finding
     • Track fitting

3 May 2003,                         Tracking Performance in LHCb,   1
LHC2003 Symposium, FermiLab               Jeroen van Tilburg
21 stations
R and φ sensors
                              LHCb setup




            VELO

       ~1.41.2 m2
                                        ~65 m2




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                                  Vertex Locator
                        Vertex Locator:
                        • 21 stations.
                        • R/φ sensors (single sided, 45º sectors).
                        • Pitch ranges from 37 μm to 103 μm.
                        • 220 μm thin silicon.
                        • Sensitive area starts at only 8 mm
                        from beam axis.




                                         32  m
                      IP      17  m 
                                           pT

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                              Silicon Tracker
                                                                 T1T2T3
            The Silicon Tracker:
            Two parts:
            1. IT (Inner Tracker)                                          OT
            2. TT (Trigger Tracker)                                   IT


                                                  TT

                                             Trigger Tracker:
                                             • 4 layers arranged in 2 stations.
                                             • 30 cm split.
                                             • 400/500 μm thin silicon.
                                             • Stereo views: 0°,+5°,-5°,0°.
                                             • 198 μm readout pitch.
                                             • 170k readout channels.
                                             • Used in L1 trigger.
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LHC2003 Symposium, FermiLab            Jeroen van Tilburg
                              Inner Tracker
      Inner Tracker
      • 3 stations (T1-T3) with 4
      layers each (0°,+5°,-5°,0°).
      • 320 μm thin silicon.
      • 198 μm readout pitch.
      • 130 k readout channels.

                                        After clustering:

                                        ~65%                    ~30%




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                                Outer Tracker
      Outer Tracker
      • 3 stations (T1-T3) with 4 double layers (0°,+5°,-5°,0°).
      • 5 mm straws.
      • Fast drift gas (Ar(75)/CF4(15)/CO2(10)) → Signal collection < 50 ns.
      • 25 ns beam crossing → spillover from previous and next spills.
      • Straws are 4.7 m with readout on top and below (long modules).
      • 50k readout channels.

OT double layer cross section                                           Short prototype module
                                                  Track
5mm straws
                                    e
                                    -   e
                                e       -
                                -

 pitch 5.25 mm
                                         e
                                         -
                                        e
                                        -

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  LHC2003 Symposium, FermiLab                     Jeroen van Tilburg
                                 Outer Tracker
                              Average occupancy in
                              OT ~ 4 %
                              (hottest region ~ 7 %)            Cross shape determined by
                                                                restricting the OT occupancy:




Resolution:                   Core σ = 200 μm
Tails due to
low momentum                         p > 2 GeV
secondaries:




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LHC2003 Symposium, FermiLab                 Jeroen van Tilburg
                               Track finding

                                                  Geant3 LHCb event display


Track finding challenges in LHCb
• High density of hits and tracks.
• Track pattern recognition must be
fast (in trigger and offline).
• High track efficiency important
(especially for many-prong decays).




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                               Track finding

                                               Zoom of OT station (hits in red)


Track finding challenges in LHCb
• High density of hits and tracks.
• Track pattern recognition must be
fast (in trigger and offline).
• High track efficiency important
(especially for many-prong decays).




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                      Track finding algorithms
Velo tracks:
• Find straight line segments in Velo.
• Start search for triplets in R-z projection.
• Then add φ hits and extend track to other sensors.
• Important for finding primary vertex.
• Efficiency ~ 97%, ghost rate < 5%.



Velo tracks




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                      Track finding algorithms
Forward tracks:
• Starts with Velo track and find continuations in TT and T1-T3.
• Uses optical model.
• Accurate measurement of momentum.
• Long track: important for most physics studies. B decay products.
• Efficiency ~ 90%.
• Afterwards the used hits are discarded for use in remaining algorithms.

Velo tracks
Forward tracks




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LHC2003 Symposium, FermiLab          Jeroen van Tilburg
                      Track finding algorithms
Seed tracks:
• Stand-alone track finding in stations T1-T3.
• Tracks almost straight lines (parameterized as parabola).
• First search for x-hits then add stereo hits.
• Improves RICH2 performance.




Velo tracks
Forward tracks
Seed tracks




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LHC2003 Symposium, FermiLab          Jeroen van Tilburg
                      Track finding algorithms
Matched tracks:
• Try to match Seed tracks with Velo tracks.
• First, estimate momentum from deflection of Seed track.
• Then extrapolate Seed track to Velo. Match with Velo track.
• Finds remaining long tracks: additional to Forward tracks.
• Adds ~ 2% to efficiency for long tracks.



Velo tracks
Forward tracks
Seed tracks
Matched tracks




3 May 2003,                   Tracking Performance in LHCb,     13
LHC2003 Symposium, FermiLab         Jeroen van Tilburg
                      Track finding algorithms
Velo  TT (VTT):
• Finds tracks without hits after the magnet (momentum too low).
• Start with unused Velo tracks.
• Find a continuation of at least 3 hits in TT.
• Magnetic deflection before TT: moderate momentum estimate Δp/p~20%.
• Improve RICH1 performance, slow pions, kaon tagging. Efficiency ~75%.



Velo tracks
Forward tracks
Seed tracks
Matched tracks
VTT tracks




3 May 2003,                   Tracking Performance in LHCb,               14
LHC2003 Symposium, FermiLab         Jeroen van Tilburg
                      Track finding algorithms
T  TT (or Upstream):
• Find tracks without hits in Velo.
• Start with unused Seed tracks and try to add hits in TT.
• Estimate momentum from deflection Seed track.
• Final momentum estimate Δp/p~0.4%
• Enhance KS finding. Pion efficiency ~74%.



Velo tracks
Forward tracks
Seed tracks
Matched tracks
VTT tracks
T  TT tracks




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LHC2003 Symposium, FermiLab          Jeroen van Tilburg
                      Track finding algorithms
Finally apply clone killing algorithm.
• Select the best candidate among tracks that share many hits.




Velo tracks
Forward tracks
Seed tracks
Matched tracks
VTT tracks
T  TT tracks


 Many track types, many algorithms
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                              Event display
Average number of tracks per event:                   Average efficiency = 92 %
        27 Long (Forward + Matched),                  Efficiency for B daughters ~ 95%
        23 Velo,
        10 with
   Event VTT,
   average TT
        10 T occupancy:
        4 Seed tracks         +
Total   74




            Red: measurements (hits)
            Blue: reconstructed tracks

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LHC2003 Symposium, FermiLab           Jeroen van Tilburg
                          Tracking performance
    Efficiency vs p                   Ghost rate vs pcut                 Ghost rate vs pTcut

                                              Long tracks                     Long tracks
                              95%
                                                                    8%                    8%
          Long tracks




                                                  Total ghost rate = 16%
Average efficiency = 92 %                         Ghost rate pT>0.5 GeV ~ 8%.
Efficiency for p>5GeV > 95%                       Large event to event fluctuations.

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                              Robustness tests
       Tracking is robust against number of interactions




                                Ninteractions                           Ninteractions

Note:                                              Tracking is also robust against:
• Luminosity adjusted to have the                  • Lower hit efficiencies,
maximum number of single interactions.
• Pile-up veto trigger (L0) rejects                • Decreased hit resolutions,
multiple interaction events.                       • More noise hits.
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LHC2003 Symposium, FermiLab             Jeroen van Tilburg
                                  Track fit

          The tracks are fitted using the Kalman Filter.

                                                     The Kalman Fit:
                                                     1. The prediction step.
direction
                                                     2. The filter step. Adds
of the
                                                        measurements one-by-one.
filter                 track prediction
        filtered track                               3. The smoother step.



             The Kalman Fit properties:
             • Adds measurements recursively.
             • Mathematically equivalent to least χ2 method.
             • Needs as input initial track estimate.
             • Multiple scattering and energy loss are naturally included.

3 May 2003,                       Tracking Performance in LHCb,                    20
LHC2003 Symposium, FermiLab             Jeroen van Tilburg
                              Track fit resolution
   LHCb provides an excellent momentum estimate at the vertex.

                   Momentum resolution
                   core σ = 0.35%
                   2nd σ = 1.0% (fraction 0.1)




                                      percent                      Note: Fitted with single
                         Δp/p
                                                                   Gaussian in each bin.
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                   Mass and vertex resolutions
   Good tracking performance essential input for physics analysis:

                                 • 4-prong prompt decay  all long tracks.
Decay channel:                   • Total tracking efficiency 84.6±0.5 %
Bs  Ds-(KKπ) π+                 (= 95.6 % per track)
                                 • Good resolutions:




3 May 2003,                   Tracking Performance in LHCb,         22
LHC2003 Symposium, FermiLab         Jeroen van Tilburg
                              Ks reconstruction
 Reconstruction of Ks( π+π-) challenging:
 • Long decay lengths (~ 1 m); many decay outside Velo.
 • Tracks don’t point to interaction point.
 • Leave less hits in detector.
                              Where does the Ks decay?
                               ~25 % in Velo                         Long tracks
                               ~50 % between Velo and TT             TTT tracks
                               ~25 % after TT                        Lost

                                                 π+π- mass of selected B  J/ψ Ks
    Example: B  J/ψ Ks
                                                                        σ=10.9±0.6 MeV/c2
    • Combine oppositely
    charged TTT tracks.
    • pT > 250 MeV.
    • Common vertex.
    • Tracking efficiency for Ks:
    54% (74% per track).

3 May 2003,                         Tracking Performance in LHCb,                    23
LHC2003 Symposium, FermiLab               Jeroen van Tilburg
                              Conclusions

 • Tracking system provides good spatial and momentum
 resolutions.:
     Vertex IP resolution (17+32/pT) μm,
     Cluster resolutions 45 μm (ST), 200 μm (OT),
     Momentum resolution 0.35%.
 • Many algorithms developed for finding tracks in optimised setup.
     Tracking efficiency ~ 95% for B-daughters,
     For pions from Ks ~ 74%. (~54% both tracks),
     Ghost rate ~ 8% (for pT>0.5GeV).
 • Tracking is robust against worse conditions.
 • Provides excellent input for physics analysis:
     e.g. Bs  Ds-π+: Mass resolution 13 MeV/c2
                        Proper time resolution 42 fs.

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LHC2003 Symposium, FermiLab          Jeroen van Tilburg
3 May 2003,                   Tracking Performance in LHCb,   25
LHC2003 Symposium, FermiLab         Jeroen van Tilburg
      BACKUP SLIDES


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LHC2003 Symposium, FermiLab         Jeroen van Tilburg
                              Event display
Average number of tracks per event:
        27 Long (Forward + Matched),
                                     Average efficiency = 90.6 %
        23 Velo,
                                     Efficiency for B daughters ~ 95%
        10 VTT,
  Event withTT
        10 T 
                 average occupancy:
        4 Seed tracks         +
Total   74


  Red: measurements (hits)
  Blue: reconstructed tracks




3 May 2003,                     Tracking Performance in LHCb,     27
LHC2003 Symposium, FermiLab           Jeroen van Tilburg
                          Tracking performance
          Long tracks:




                  Mean momentum 13 GeV
                  On average 37 measurements (Velo, IT, OT)
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LHC2003 Symposium, FermiLab          Jeroen van Tilburg
                              Efficiency definition

   Efficiencies are normalised to a sample of “reconstructable” particles:
   • in VELO at least 3 r and 3 φ hits,
   • in T stations at least 1 x and 1 stereo hit in each station T1-T3.

   • Long tracks must be reconstructable in VELO and T.
   • VTT tracks must be reconstructable VELO and at least 3 TT hits.
   • TTT tracks must be reconstructable in T and have at least 1 hit in TT.

   Successfully reconstructed track: at least 70% of hits from one MC particle.
   • Long tracks must be successfully reconstructed in VELO and T,
   • VTT tracks must be successfully reconstructed in VELO and have at least
   correct 1 hit in TT,
   • TTT tracks must be successfully reconstructed in T and have at least 1
   correct hit in TT.



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LHC2003 Symposium, FermiLab              Jeroen van Tilburg
                               LHCb light setup
                              LHCb classicsetup




            VELO




3 May 2003,                       Tracking Performance in LHCb,   30
LHC2003 Symposium, FermiLab             Jeroen van Tilburg

				
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