Difference between revisions of "RF Beam Bunch Selection"

Revision as of 12:04, 13 February 2015

Discussion / Summary

• During experimental running, the event start time is not measured. We have the (excellent resolution) time of an RF beam bunch (DRFTime), but we don't know which bunch caused the triggered event. RF bunches from the accelerator arrive every 2.004 ns (499 MHz), except during the Spring 2015 commissioning run, when they arrived every 4.008 ns (249.5 MHz). Thus, the time of the event-causing RF bunch is +/- N*2.004 ns (or +/- N*4.008 ns) from this number (N = some integer).

• To select the event-causing RF bunch (DEventRFBunch), the start-counter hit-times for all tracks with good tracking FOM (within +/- 5 sigma) are used. The tracks "vote" on an RF bunch, and the bunch with the most votes gets selected.

• The track and shower times are then compared to this DEventRFBunch time to calculate the PID FOMs.

• However, some junk and accidental tracks will vote, occasionally throwing off the event start time, and causing good events to fail PID cuts. Therefore, during an analysis, the RF bunch is re-selected separately for each particle combination, using only the tracks and showers in that particle combo (DEventRFBunch, "Combo"). The track/shower times from the BCAL/TOF/FCAL/SC are propagated to the beamline and are averaged to select the correct RF bunch.

• The charged track (DChargedTrackHypothesis, "Combo") and neutral particle (DNeutralParticleHypothesis, "Combo") PID FOMs are recomputed using this new RF time. It is these objects that are ultimately saved to the DParticleCombo "PreKinFit" object.

• To select the possible DBeamPhoton's for the particle combinations, their times are compared to the DEventRFBunch for each combo. The cut used is adjustable through the DReaction, but should be no smaller than +/- 1.002 ns. All DBeamPhoton's within this time window originated from the same RF bunch, and thus each could have caused the event. This multiplies the number of particle combinations.

RF- & Beam-Related Classes

For all of the objects below, the time & position are reported for when the beam (would have) reached the target center:

• Tagger Hits:

1. DTAGMHit: Hit-level tagger microscope objects.
2. DTAGHHit: Hit-level tagger hodoscope (fixed-array) objects.

• Beam photons (Stored in REST):

1. DBeamPhoton: built from DTAGMHit and DTAGHHit objects.
2. DBeamPhoton, "TRUTH": True (unsmeared) photon energy/time data.
3. DBeamPhoton, "MCGEN": The true photon that generated the event (as opposed to the EM background photons)

• RF time (Stored in REST):

1. DRFTime, "TRUTH": The true start time of the event (propagated to the target center) (same as DBeamPhoton, "MCGEN")
2. DRFTime: For MC data, the simulated "measured" value is ~0. The actual time is +/- 2.004*N this value.

• RF Bunch:

1. DEventRFBunch, "Thrown": The thrown RF bunch.
2. DEventRFBunch, "TRUTH": The reconstruction software's best guess for the RF bunch (time) that started the event.
3. DEventRFBunch, "Combo": The reconstruction software's best guess for the RF bunch (time) that started the event for each particle combination.

How (Default Tag) RF bunch selection works for an event WITH RF hits (DRFTime)

• In DEventRFBunch_factory:

1. Select good time-based tracks: DTrackTimeBased with greatest Tracking FOM for track, Tracking FOM > +/- 5σ
2. Of these, use tracks with matching SC hits, if any: Vote
3. If none, use tracks with matching hits in any detector: BCAL/TOF/FCAL: Vote
4. If none, set DEventRFBunch::dTime to NaN

• Voting:

1. Propagate track/shower times to vertex
2. Compare times to possible RF bunches, select the bunch with the most votes
3. If there is a tie: let DBeamPhoton's vote to break tie
   1. If still a tie, and voting with tracks:
      1. Select the bunch with the most total track hits (highest total tracking NDF)
      2. If still a tie: Select the bunch with the lowest total chisq
   2. If still a tie, and voting with neutral showers:
      1. Select the bunch with the highest total shower energy

How (Default Tag) RF bunch selection works for an event WITHOUT RF hits (DRFTime)

• In DEventRFBunch_factory:

1. Use SC hits that have a track match to compute RF time guess, if any
   1. Times could be modulo 2.004 ns still: line them up (after propagating to beamline)
   2. Use RF time guess to vote, just as in found-rf-time case
2. If none: Set RF time as NaN

How (Combo Tag) RF bunch selection works for each combo

• In DEventRFBunch_factory_Combo:

1. For each charged track in the combo, calculate its start time (from BCAL/TOF/FCAL hits), and propagate it to the target center.
2. For each photon shower in the combo, calculate its start time (use DVertex position), and propagate it to the target center.
3. Ignore massive neutral particles since their β is defined by the start time.
4. The above particles vote: the RF bunch time with the most votes wins. The RF bunch times are the DRFTime +/- multiples of 2.004 ns.