1. DCH missing turn analysis
Gordon Lim, UCI

2. Missing turn positrons
Positrons can make more than one turn in the DC: n-turn positron
Reconstruction of n-turn positrons:
Hit reconstruction
Cluster reconstruction
Pattern recognition (using clusters) --> track candidates (1 turn each)
Track fitting (using hits) --> fitted tracks (1 turn each)
Track merging --> fitted track (n turns)
However, a n-turn positron can be reconstructed as a n’-turn positron if there is a failure in any of these steps: Missing turn positron
Missing turn positrons are a source of background because by definition they can not be signal positrons
Identifying missing turn positrons can be used to reduce background
Refitting missing turn positrons that originate from the target will increase our signal sensitivity

3. How to identify missing turn positrons:
Consider all hits except those used in the track

4. How to identify missing turn positrons:…
Select potential missing turn hits: hits in the same DC modules as the track ± one module and with zhit > zvertex - 2 cm (θtrack> 90°) or zhit < zvertex + 2 cm (θtrack< 90°)

5. How to identify missing turn positrons:…
Propagate state vector at vertex backward to POCA of each hit

6. How to identify missing turn positrons:…
Select hits that have Δr < Δrmax (1.2 cm) and |Δz| < |Δz|max (7.5 cm) with respect to their corresponding propagated state vector positions.
Calculate μ|Δz| and σΔz

7. How to identify missing turn positrons:…
Resolve L/R ambiguities and refine the XY positions of all missing turn hits using a circle fit, and calculate the time and energy of the missing turn (in a similar way as is done in the TrackFinder2 task)

8. Missing turn positron identification criteria:
Maximum Δr (1.2 cm) && |Δz| (7.5 cm) for each hit and its corresponding propagated state vector
Maximum μ|Δz| (2.5 cm) && maximum σΔz (2.5 cm)
Minimum # of adjacent chambers (3)
Minimum # of chambers (4)
Minimum # of hits (6)
Maximum ΔE and Δt between missing turn and track (not used yet) …

9. The positron is clearly due to downstream muon decay
Why is the turn missing?
Consider all track-candidate hits
In this example
the track candidate
of the missing turn
was not identified
The positron is clearly due to downstream muon decay

10. Missing turn examples without matching track candidates due to downstream muon decay

11. Missing turn examples without matching track candidates due to upstream muon decay

12. Missing turn examples without matching track candidates due to muon decay in the target

13. Missing turn examples without matching track candidates due to muon decay in the target

14. Missing turn examples with matching track candidates due to muon decay in the target

15. Missing turn examples with matching track candidates due to muon decay in the target

16. Missing turn examples with matching track candidates due to 3-turn positron from target

17. Missing turn examples with matching track candidates due to 4-turn positron from target

18. Wrongly identified missing turns?

19. Origins of missing turns
US/DS muon decays never have a track candidate
Two turns that cross the same DCH-cell(s) close in z. This can confuse the hit reconstruction due to overlapping charge
One of the two or both hits end(s) up at the wrong z-coordinate
One of the hits is not reconstructed at all
This can confuse the cluster reconstruction (i.e the cluster splitting)
Trackfinder algorithm is vulnerable because it starts its search for track seeds using hits which it assumes are at maximum radius of the track. This is exactly the region where a double turn positron crosses itself
Without a track candidate, the fitter is not invoked
New Kalman filter task fails to merge track fits
Merging procedure is not optimal (it attempts to merge any two track fits, as soon as it finds a match it merges the two track fits and replaces the original two track fits with the merged fit, and so on)
Merging criteria too strict?

20. Data analysis Data sample: 100 official open runs from 2009
Standard positron selection using only new Kalman filter using MEGPhysicsSelection::PositronSelection()
Total # of events with a selected track = 7194
# of single turn tracks = 6680 (92.8%)
# of double turn tracks = 514 (7.2%)
Total # of events with a missing turn = 448 (6.2%)
Analysis of identified missing turn events “by eye”:
Muon decay from target without track candidate: 30-35%
Muon decay from target with track candidate: 25-30%
US/DS muon decay (without track candidate): 25-30%
Wrongly identified missing turns: 5-10%

21. Conclusion
Developed first version of positron missing turn identification algorithm
Currently implemented as a stand-alone task to be used after the track fitter
The other option is to improve the cluster reconstruction, the track finder and the merging procedure in the track fitter
To be done:
Improve and tune missing turn identification criteria. In principle this can be tested using double turn events
Refit track and missing turn together
Usage:
Reject background events (US/DS muon decay)
Improve signal sensitivity (by using the refitted n-turn positron in the analysis)