1. HLT Kalman Filter Implementation of a Kalman Filter in the ALICE High Level Trigger. Thomas Vik, UiO

2. The ALICE detector

3. Data rates ● Event rates -Central Pb-Pb: < 200 Hz -Min. bias pp: < 1000 Hz ● Event size (zero suppressed) -PbPb: ~75Mbyte -pp: ~2.5Mbyte ● Data rates -PbPb: < 15 Gbyte/sec -pp: ~2.5 Gbyte/sec ● Forseen DAQ bandwidth ~1.2 GByte/sec

4. What is the Kalman Filter ● Iterative data processing algorithm. ● Advantages as a tracker: -Track finding and fitting simultanously -No large matrices needs to be inverted. Number of computations increases linearly with the number of measurements. Therefore fast in situations with multiple scattering and many measurements. -Estimated track parameters follow closely the physical track.

5. Why do HLT need Kalman Filter ● Merge tracks from the different ALICE sub- detectors. ● Get good estimates of track parameters. ● Reduce amount of fake tracks from the HLT tracker.

6. Kalman in the TPC ● Track parameters: (y, z, D, tan, C) ● Three steps: -Make seeds: initialize track parameters. -Propagate to next padrow. Look for clusters in window around predicted position. -Update with hit which gives minimum 2.

7. Track following

8. Kalman in the HLT ● Calculate seeds from track parameters of the HLT fast tracker. ● Use clusters already assigned to tracks. ● Only needs to propagate tracks according to track model, and update parameters with already found clusters. ● Gives a fast global tracker.

9. Status ● Timing: - HLT tracker: setup ~50 ms/sector tracking ~70 ms/sector -HLT Kalman Filter: load tracks ~1000 ms tracking ~1000 ms

10. Example: heavy charm decay ● TRD triggers on possible dielectron candidates. ● Large background pions misidentified as electrons electron from conversions. ● Merge with TPC tracks, add TPC dE/dx PID remove pions misidentified as electrons. ● Merge with ITS tracks, impact parameter cut. ● Background reduction by factor 10.

11. Conclusion ● Merge tracks from the different sub- detectors. ● Need HLT tracking for ITS and TRD. ● Only implemented for the TPC, however some debugging is still needed. Must reproduce offline results.