1. Track Reconstruction in the STAR TPC with a Cellular Automaton Based Approach
Y. Fisyak1, I. Kisel2, I. Kulakov2, J. Lauret1, M. Zyzak2
1BNL, Brookhaven, USA; 2GSI, Darmstadt, Germany
CHEP-2010
Taipei, October 18-22, 2010

2. STAR Experiment at BNL Collider experiment at RHIC, BNL
Up to 200 AGeV Au-Au collisions
Main detector – TPC
Increased RIHC luminosity
New HFT detector (2014)
Upgrade the reconstruction algorithms for:
vectorization
multi-threading
many-core systems
19 October 2010, CHEP-2010
CA in STAR

3. Vector classes enable easy vectorization of complex algorithms
Vector Classes (Vc)
Vector classes overload scalar C operators with SIMD/SIMT extensions
Scalar
SIMD
c = a+b vc = _mm_add_ps(va,vb)
Vector classes:
provide full functionality for all platforms
support the conditional operators
phi(phi<0)+=360;
Vc increase the speed by the factor:
SSE2 – SSE x
future CPUs x
MICA/Larrabee x
NVIDIA Fermi research
Vector classes enable easy vectorization of complex algorithms
M. Kretz, Uni-Frankfurt/FIAS
19 October 2010, CHEP-2010
CA in STAR

4. Cellular Automaton (CA) Track Finder Algorithm in STAR TPC
Reconstruction of track segments in each TPC sector:
Find and link neighbors hits
Clean links
Create segments by fitting chains and adding outer hits
Refit tracks and add inner hits
Selection of tracks
Merge sector tracks into TPC global tracks. a) b) c) d) e)
Based on the ALICE HLT TPC CA Track Finder
19 October 2010, CHEP-2010
CA in STAR

5. CA Tracker
Front view
Side view
19 October 2010, CHEP-2010
CA in STAR

6. CA Tracker: p-p Event with 19 Tracks
Front view
Side view
19 October 2010, CHEP-2010
CA in STAR

7. CA Tracker: Au-Au Event with 1446 Tracks
Front view
Side view
19 October 2010, CHEP-2010
CA in STAR

8. Charged Tracks Reconstruction Procedure (CA+Sti)
Reconstruction of track segments in each TPC sector:
Find and link neighbors hits
Clean links
Create segments by fitting chains and adding outer hits
Refit tracks and add inner hits
Selection of tracks
Merge sector tracks into TPC global tracks.
Refit the global tracks with the STAR baseline reconstruction (Sti) with accounting other detectors and all dead materials.
Run Sti with hits left. This recovers ~2% tracks.
Fit primary vertex/vertices.
Fit primary tracks.
19 October 2010, CHEP-2010
CA in STAR

9. Comparison Baseline Reconstruction (Sti) with CA Tracking (CA+Sti)
Real Au-Au 200 GeV/n data.
(fitted within 0.2–2.1 GeV)
Global tracks
Primary tracks
Sti
CA+Sti
Mult < 200
90.3%
97.7%
97.3%
99.3%
200 < Mult < 400
90.2%
97.5%
97.0%
99.1%
400 < Mult < 600
86.9%
96.6%
96.0%
98.9%
Mult > 600
84.4%
96.2%
95.4%
All
88.1%
97.1%
96.4%
CPU time per event
Sti tracker
CA+Sti tracker
Sti tracker
CA+Sti tracker
global CA tracker
sector CA tracker
Efficiency for global tracks has been increased on 7-9%
CA+Sti is ~50% faster than Sti alone
CA track seed finder takes ~10% of the all event reconstruction time
19 October 2010, CHEP-2010
CA in STAR

10. Conclusions
The TPC Cellular Automaton (CA) track finder works within the STAR reconstruction framework as seed finder.
Efficiencies with the CA track finder (CA+Sti) are higher by ~3% for primary tracks and ~9% for global ones, respectively, than for Sti alone.
The efficiency with the CA track finder (CA+Sti) is stable with respect to higher track multiplicities.
Reconstructed track parameters are practically the same (CA+Sti tracks contain slightly more hits than Sti ones).
CA+Sti is ~50% faster than Sti alone. For very high occupancy events CA+Sti speeds up reconstruction by factor of ~10.
The TPC CA track finder takes only ~10% of the whole event reconstruction time.
The TPC CA track finder has been moved to the official repository.
The test production with high statistics is planned for a more accurate tracking comparison and physics evaluation in order to make CA+Sti as baseline tracker and to use it for this year (Run X) data production.
There are possibilities for further speedups of the TPC CA track finder algorithm.
TPC CA track finder will be evaluated in order to use it in the STAR High Level Trigger.
19 October 2010, CHEP-2010
CA in STAR