L3 Tracking FAQ

This document is intended to provide the end user of the tracking algorithm, such as a physicist running D0TrigSim, with a guide to jumping the temporary technical hurdles required for running correctly in various environments. These hurdles arise from issues in handling the geometry and calibration for clustering the data from the Central Fiber Tracker and Silicon Microstrip Tracker.

The need for this document underscores the importance of a long-term solution which would require no knowledge in the consumer.

Please send me corrections/suggestions/clarifications...

August 01, 2002

This page contains the following useful items:

Overview of tracking algorithm

The L3 Global Tracking algorithm is described in detail in D0Note 3808. Please see that document for a thorough understanding of the tracking and its performance in MC events. A note describing its performance on real data is in preparation. There are two central features of the tracking algorithm: The tracking begins with track candidates formed with pairs of clusters in the outer two layers of the CFT. These candidates are propagated toward the center of the detector, through each axial layer of the CFT and the SMT barrels. At each layer, the track is allowed to pick up hits within a cone (the size of which is determined by the PT threshold) or to miss hits in that layer.

Axial tracks are kept if they have either

For those axial tracks with a PT larger than the threshold, a histogramming algorithm is used to search for stereo information in both the CFT and SMT. If no stereo information is found, the track is kept as an axial-only track.

Common problems

If you have problems with the tracks at level 3, please run the reference macro and compare the plots to those below. You may be suffering from one of these easy-to-fix problems:
Problem Most of my tracks have only 8 hits
Cause: The geometry or calibration files are incorrect or missing. Make sure you've carefully followed the cookbook for data or MC

Problem: My track phi distribution has big holes!
Cause:You may be using a MC setup on real data. Specifically, make sure that the REALDATA parameter is set to TRUE for data. Otherwise, it may represent detector problems.Check your CFT configuration for data, or try a different run.

Problem: I see lots of axial only tracks: spikes in Z or tan(lambda)
Cause: If you've ensure that the system is configured correctly (check the distribution of the number of hits on the track, and see above if it peaks at 8 hits. Otherwise, this may be the desired performance -- many axial tracks are found below the threshold and no stereo tracking is performed. Check the correlation between the pt and the stereo information

Problem: My tracks don't match my electrons/muons/etc
Cause: If you're using the MC geometry on a data file or vice versa, then you'll see this problem. To confirm it, you will see very few SMT hits because the CFT is rotated with respect to the SMT and the rest of the geometry.

Problem: My efficiency is very low!
Cause: If you've ensured that the system is configured correctly (check the distribution of the number of hits on the track, and see above if it peaks at 8 hits), then make sure that you're calculating your efficiency in the CFT fiducial region. Remember that unlike offline tracking, online tracking is only performed in the central region, to an eta of roughly 1.1.

Problem: My DCA resolution is very large
Cause: The beam spot is shifted with respect to the center of the detector, so that the tracks will typically not come from (0,0). Unlike the offline tracking, the track parameters are always calculated with respect to the detector (0,0), so a plot of track DCA versus track angle will show a sine wave (see the reference plots). If you fit this sine wave and calculate the DCA relative to the beam spot, you should be able to get a DCA resolution of ~65 microns for tracks above 1 GeV.

Problem: My Pt resolution is poor
Cause: The Pt resolution is greatly enhanced by the addition of SMT information. Make sure that you have correctly configured the SMT so that the most likely number of hits on a track is 10 or 11 rather than 8 (see above).

Cookbook for running over MC

Cookbook for running over real data

To run over real data, you must do the following:

Macro and reference plots

A simple ROOT macro to produce simple diagnotic L3 tracking plots.

Reference plots for data: run 160588

Note some important items:

Reference plots for MC: j/psi

From Abid Patwa and Avdhesh Chandra
Note some important items: