global tracking for cbm andrey lebedev 1,2 ivan kisel 1 gennady ososkov 2 1 gsi helmholtzzentrum...

Global Tracking for CBM

Andrey Lebedev 1,2

Ivan Kisel 1

Gennady Ososkov 2

1GSI Helmholtzzentrum für Schwerionenforschung GmbH , Darmstadt, Germany2Laboratory of Information Technologies, Joint Institute for Nuclear Research, Dubna, Russia

1st CBM Russia MeetingMay 18-22, 2009JINR, Dubna, Russia

2 A.Lebedev, I.Kisel, G.Ososkov, Global Tracking for CBM

Outline• Global tracking

– Geometries– Requirements – Tracking– Results

• Speed up of the tracking software


CBM setups

• “Muon” setup– STS+MUCH+(TRD)+(TOF)

• “Electron” setup– STS+(RICH)+(TRD)+(TOF)+(ECAL)


Segmented MUCH• 6 absorbers

– 225 cm of iron• 2 stations in first 5 gaps• 3 stations in the last gap

for trigger• 2 sides in each station

Back sideFront side

Optionally: STRAW TUBES in the last stations (V.Peshehonov et al)

V.NikulinE.KryshenM.Ryzhinskiy

Front side Back sideFront side Back sideFront side


Segmented TRD– Segmented geometry– 2 cm thick support frames– More dead zones

• Leads to detector inefficiency


Global tracking procedure• CbmLitFindGlobalTracks

– Tracking + Merging– Automatic configuration, depending on

the setup• STS+(TRD)+(TOF)• STS+MUCH+(TRD)+(TOF)

STS

MUCHTRD TOF

L1 STStracking

LIT tracking

Hit-to-track merging


Tracking• Initial seeds are tracks reconstructed

in STS (L1 tracking)• Tracking is based on

– Track following– Kalman Filter– Validation gate– Different hit-to-track association

techniques


Tracking methods• Branching

– Branch is created for each hit in the validation gate.

• Nearest Neighbor– The closest by a Euclidean statistical distance hit

from a validation gate is assigned to track.

• Weighting– Doesn’t allow track splitting, it collects all the hits in

the validation gate.


Simulation parameters• Events

– Muons: 1000 UrQMD at 25 AGeV + 10 mu in each event

– Electrons: 500 UrQMD at 25 AGeV + 10 electrons in each event

• Efficiency (CbmLitReconstructionQa)– Global tracking:

• STS – 4 MC STS points• STS+TRD(MUCH) – 4 MC STS and 8(12) MC TRD(MUCH)

points• STS+TRD(MUCH)+TOF - 4 MC STS and 8(12) MC

TRD(MUCH) points + TOF point– Local tracking and merging:

• TRD(MUCH) – STS as 100% + 8(12) MC points in TRD(MUCH)

• TOF merging – STS+TRD(MUCH) as 100% + TOF point


Muons

muonsall

STS STS+MUCH STS+MUCH+TOF MUCH TOF

all 74.9 90.0 90.7 92.1 98.7

reference 94.8 90.9 91.0 92.7 98.7

muons 98.0 91.4 91.0 93.1 98.7

Efficiency in %


Electrons

electronsall

STS STS+TRD STS+TRD+TOF TRD TOF

all 81.7 85.0 77.6 92.9 91.1

reference 95.5 91.3 83.1 94.9 91.1

electrons 96.6 87.9 74.5 89.3 84.6

Efficiency in %


Speed up [1]: number of branches• Motivation: The main problem with branching

algorithm is that its computational and memory requirements can grow with time and saturate computing system.

• Solution: Only a limited number of the nearest hits in the validation gate can start a new branch.

Maximum number of hits in the

validation gate30 20 15 10 7 5 4 3 2 1

Tracking efficiency,%

93.1 93.1 93.1 93.1 93.1 93.1 93.1 92.9 92.9 91.8

Time, s/event 1.91 1.63 1.37 1.05 0.8 0.62 0.52 0.44 0.34 0.20

MUCH tracking, UrQMD 25 AGeV Au-Au + 10 muons

4 times faster!

CPU: Intel C2D P8400


Speed up [2]: fast search of hitsMotivation -Hits are sorted by x position

-Binary search is used to find Min and Max index

2 2cov posdX k

pos Maximum measurement error on the station

Fast search of hits

Loop over MIN and MAX

indices

Loop over MIN and MAX

indices

2 times increase in speed!1.05 s/event -> 0.52 s/event(MUCH tracking, UrQMD+10mu)



Speed up [3]: simplification of the geometry• Simplified geometry

– TGeoManager: storage + navigation– Stations are approximated as planes

• Reduction of number of nodes from 800k to 100• 0.52 s/event 0.3 s/event

• Future plans– Custom navigation (assuming detector planes

are perpendicular to z)– Optimization of the step size in the track

propagation



Speed up: plans of further developments

• Magnetic field access (same as for L1)– approximation of the magnetic field

• Parallelism– SIMDization

46% of total track propagation costs is due to access to the magnetic field

Reachable gain of speed:

100? 1000?...


Summary and outlook• Summary:

– The Lit track reconstruction package has been significantly improved

• global tracking procedure implemented• algorithm speed up results in 0.3 s/event for

MUCH tracking• Outlook

– Continue speed up studies– MUCH trigger


Back up


Track propagation• Extrapolation. Two models:

– Straight line in case of absence of magnetic field.– Solution of the equation of motion in a magnetic field with the 4th order Runge-Kutta method.

• Material Effects.– Energy loss (ionization: Bethe-Bloch,

bremsstrahlung: Bethe-Heitler, pair production)– Multiple scattering (Gaussian approximation)

• Navigation.– Based on the ROOT TGeoManager.

The Algorithm:Trajectory is divided into steps. For each step: Straight line approximation for finding

intersections with different materials (geometry navigator)Geometrical extrapolation of the trajectory

Material effects are added at each intersection point

Track propagation components


Weighted track fit• Weight is assigned to each hit• Weight is proportional to the multivariate Gaussian

distribution and includes temperature parameter T• Weighted hit is calculated as a weighted mean of the

hits from the same station• Weighted track is fitted with standard KF• Temperature is gradually decreasing. Simulated

annealing is applied to track fit

1) The process starts at a high temperature, all hits have the same weights.2) Weights are updated at a lower temperature.3) Weights are frozen.

global tracking for cbm andrey lebedev 1,2 ivan kisel 1 gennady ososkov 2 1 gsi helmholtzzentrum...

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