FAQ about Event 503 from Run 1690
1) Is it real data?
Yes, of course. Time-to-distance calibration has been run on this data, and the t0 comes from a scintillator running down the center of the detector. The tracks drawn (3) are found with standard BaBar (C++) tracking code, which works as well without a 1.5T field as it does with one.
2) What are those superlayer segements which don't appear to be on the tracks?
The hits and segments are really on the tracks. They are segments in the stereo superlayer, and are drawn (in z) at the center of the drift chamber. not at the z where the track is closest to the wire. If they were, all hits would line up on the tracks.
3) You've only drawn the hits which were associated with tracks.
No, ALL hits in the event are draw, with 2 caveats. At least half of the Dch is not instrumented with electronics, so no hits will show up there (more on this later). Some wires with TDC hits have been suppressed, but the threshold is low for the TDCs, and the hits not drawn had no legitimate FADC waveform. If we really had faith in our waveform algorithm, we might not even write these hits out, but we're still learning. But from experience with our prototype and this chamber, we are confident enough to leave these hits out of pattern recognition (and evt drawing); note that legitimate hits do not seem to be suppressed by requiring valid FADC waveforms.
4) Why are 3 superlayers missing from the lower track?
As we said, not all of the chamber is instrumented with electronics. The FEAs (front-end-assemblies) match the inner 4 superlayers, the middle 3, and the outer 3 superlayers. This is a region where there were no electronics on superlayers 5-7. Note that BaBar trackfinding still finds the track with a large gap in it. The hits in this region are NOT suppressed by the good waveform requirement. We turned this off (one reason for keeping it in software for now) and went back and redrew the event; this region is still empty.
5) Does the track not going near the origin exit thru the endplate?
No, tracks are draw from their point-of-closest approach to the origin to where they exit (or enter) the Dch. Where the hits stop occuring on the track is where the track enters a region not instrumented with electronics. Since the stereo superlayers (2-3-5-6-8-9) match up pretty well with the axial superlayers, this track is at a z close to the center of the chamber. However, the triggering cosmic has a significant dip angle; the lower track is not near the center of the chamber.
6) Is that one long track going near the origin?
No, it's actually found and fit as 2 separate tracks. They miss by 0.2 mm in the x-y plane and by 4 mm in z. Considering we're missing hits because the chamber isn't fully instrumented, and we have no momentum selection on these cosmics, that's doing pretty good.
7) Did you select this event for any of the above reasons (is it atypical)?
No, it was selected because it had the knock-out in the outer cylinder and looked "interesting." The previous 502 event contain lots of good tracks. This event WASN'T selected because the tracks matched well at the origin, or because there were no extra hits.
For comments, questions or changes to this page contact David MacFarlane