# Presentation: 683968

## Tracking performance in LHCb

- Tracking Performance

- Jeroen van Tilburg
- NIKHEF
- On behalf of the LHCb collaboration

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

- Overview:
-  LHCb setup
-  Velo/ST/OT performance
-  Track finding
-  Track fitting

## LHCb setup

- VELO

## 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.

## Silicon Tracker

_**The Silicon Tracker**_:
- Two parts:
- 1. IT (Inner Tracker)
- 2. TT (Trigger Tracker)

## 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%

## Outer Tracker

- _OT double layer cross section_

- 5mm straws

- _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.

- Short prototype module

- pitch 5.25 mm

- Track

*e*-

*e*-

*e*-

*e*-

*e*-

## Outer Tracker

- _Resolution:_
- Tails due to low momentum secondaries:

- Average occupancy in OT ~ 4 %
- (hottest region ~ 7 %)

- p > 2 GeV

- Cross shape determined by restricting the OT occupancy:

- Core σ = 200 μm

## Track finding

- _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).

## Track finding

- _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).

- Zoom of OT station (hits in red)

## Track finding algorithms

- Velo tracks

- _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%.

## Track finding algorithms

- Forward tracks

- Velo tracks

- _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.

## Track finding algorithms

- Velo tracks

- Forward tracks

- Seed tracks

- _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.

## Track finding algorithms

- Velo tracks

- Forward tracks

- Seed tracks

- Matched tracks

- _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.

## Track finding algorithms

- Velo tracks

- Forward tracks

- Seed tracks

- Matched tracks

- VTT tracks

- _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%.

## Track finding algorithms

- Velo tracks

- Forward tracks

- Seed tracks

- Matched tracks

- VTT tracks

- T  TT tracks

- _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%.

## 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

## Event display

- Event withaverage occupancy:

- Red: measurements (hits)

- Average efficiency = 92 %
- Efficiency for B daughters ~ 95%

- Blue: _reconstructed_ tracks

## Tracking performance

- Long tracks

- Long tracks

- 95%

- 8%

- Long tracks

- 8%

- Average efficiency = 92 %
- Efficiency for p>5GeV > 95%

- Total ghost rate = 16%
- Ghost rate pT>0.5 GeV ~ 8%.
- Large event to event fluctuations.

- Efficiency vs p

- Ghost rate vs pcut

- Ghost rate vs pTcut

## Robustness tests

- Tracking is also robust against:
-  Lower hit efficiencies,
-  Decreased hit resolutions,
-  More noise hits.

- Tracking is robust against number of interactions

- _Note_:
-  Luminosity adjusted to have the maximum number of single interactions.
-  Pile-up veto trigger (L0) rejects multiple interaction events.

## Track fit

- _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.

- The tracks are fitted using the Kalman Filter.

- The Kalman Fit:
- The prediction step.
- The filter step. Adds measurements one-by-one.
- The smoother step.

- direction of the filter

- track prediction

- filtered track

## Track fit resolution

- LHCb provides an excellent momentum estimate at the vertex.

- Δp/p

- _Momentum resolution_
- core σ = 0.35%
- 2nd σ = 1.0% (fraction 0.1)

- Note: Fitted with single Gaussian in each bin.

- percent

## Mass and vertex resolutions

-  4-prong *prompt* decay  all long tracks.
-  Total tracking efficiency 84.6±0.5 % (= 95.6 % per track)
-  Good resolutions:

- Good tracking performance essential input for physics analysis:

- _Decay channel_:
- Bs  Ds-(KKπ) π+

## Ks reconstruction

- _Example: B ____ __J/__ψ__ K__s_
-  Combine oppositely charged TTT tracks.
-  pT > 250 MeV.
-  Common vertex.
-  Tracking efficiency for Ks: 54% (74% per track).

- Where does the Ks decay?
- 	~25 % in Velo		 Long tracks
- 	~50 % between Velo and TT	 TTT tracks
- 	~25 % after TT		 Lost

- _Reconstruction of K__s__(____ __π__+__π__-__) challenging_:
-  Long decay lengths (~ 1 m); many decay outside Velo.
-  Tracks don’t point to interaction point.
-  Leave less hits in detector.

- π+π- mass of selected B  J/ψ Ks

- σ=10.9±0.6 MeV/c2

## 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.

## Slide 25

## BACKUP SLIDES

## Event display

- Event with average occupancy:

- Red: measurements (hits)
- Blue: _reconstructed_ tracks

- Average efficiency = 90.6 %
- Efficiency for B daughters ~ 95%

## Tracking performance

- Mean momentum 13 GeV
- On average 37 measurements (Velo, IT, OT)

- Long tracks:

## 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.

## LHCb classic setup

- LHCb light setup

- VELO