RF Beam Bunch Selection

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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 typically arrive every 4.008 ns (249.5 MHz), although sometimes every 2.004 ns (499 MHz) (e.g. the Fall 2014 commissioning run). Thus, the time of the event-causing RF bunch is +/- N*4.008 ns (or +/- N*2.004 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.
  • New DChargedTrackHypothesis and DNeutralParticleHypothesis objects are created with new PID FOMs are computed using this new RF time. It is these objects that are ultimately saved to the pre-kinfit DParticleCombo 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. All DBeamPhoton's within this time window originated from the same RF bunch, and thus each could have caused the event. This can multiply the number of particle combinations by a large factor.

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)
  4. DBeamPhoton, "TAGGEDMCGEN": The reconstructed photon matching tag "MCGEN."
  • RF time (Stored in REST):
  1. DRFTime: The actual time is +/- 4.008*N ns this value (or +/- 2.004*N for Fall 2014 run). For MC data, the simulated "measured" value is ~0.
  2. DRFTime, "TRUTH": The true start time of the event (propagated to the target center) (same as DBeamPhoton, "MCGEN")
  • RF Bunch:
  1. DEventRFBunch: The reconstruction software's best guess for the RF bunch (time) that started the event, independent of the reaction being studied.
  2. DEventRFBunch, "Calibrations": Used for performing timing alignment. Uses wire-based tracks and only the start counter times.
  3. DEventRFBunch, "Thrown": The thrown RF bunch.

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 > +/-
  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, vote with DNeutralShower's.
  5. 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 / 4.008 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-based RF bunch selection works

  • For each combo, PID cuts are placed on the particles to select the possible RF bunches.
  • Then, the intersection between all of these RF bunches is found for the combo.
  • If more than one RF bunch is possible, the delta-t chisq is used to select the final bunch.