22.7. Time Extraction#

22.7.1. Track Time#

The track time is the time at which the track is produced relative to the collision time (given by SVD EventT0). We define the outgoing (ingoing) arm as the part of the helix that exits (enters) the IR. They are identified using the order of the detectors associated to the track hits: for example, the arm is considered outgoing (ingoing) if the hits are found in PXD-SVD-CDC (CDC-SVD-PXD). The arm times are computed with the SVD clusters used in the reconstruction of the arms of the track as the average of the cluster times. If the track has only one arm, the track time is computed as the difference of this arm time and the event time (EventT0, see next section). In case the track has both outgoing and ingoing arms, the one with the smallest time is used.

Note

Tracks can have roughly:

  1. outgoing arm only (98.5%)

  2. ingoing arm only (0.9%)

  3. both outgoing and ingoing arms (0.6%)

22.7.2. EventT0#

The event time (EventT0) is the time of the collision (in ns) with respect to the trigger. There are several estimators in the tracking package:

  1. SVD estimator: the default estimator

  2. CDC Hit Based estimator: used in the CDC pattern recognition step

  3. CDC Chi2-based Full Grid estimator: the most precise estimator, but has a long execution time

SVD Event Time Estimator#

This is a simple estimator that is computed after tracking: the SVD clusters, belonging to the outgoing arm (or to the ingoing arm, if its time is smaller than the outgoing arm time or if it is the only one available), used in the reconstruction of tracks with \(p_T > 250\) MeV/c are selected and then the time of the event is computed as the average of the cluster times. This is the default time of the event computed by tracking.

CDC Hit Based Estimator#

This estimator is run at the beginning of CDC track finding. Its efficiency is very high, i.e. provides an EventT0 for all events provided there are enough CDC hits, but it’s less precise than the other estimators.

CDC Chi2-based Full Grid Estimator#

This is the most precise estimator of the time of the event, and it is used for the time calibration of the other detectors (SVD, ECL, …). It is not run during reconstruction because its execution time is significantly higher than the SVD EventT0 estimator.