Discussion points for taking zero field data to use in TPC alignment

• Collider energy-ramp affected by STAR magnetic field
  • Want the collider to ramp up with the STAR magnet on, then ramp down the STAR magnet before running?
• Beam time is precious during the low energy operations
  • If we want beam, it will be easier to ask for during full energy (AuAu200)
  • Can also run after the collider turns off in the summer with cosmics
    • TPC,TOF,Trigger,DAQ would need to stay on, with people sitting gas shifts and acquiring the data (perhaps for a day or two after RHIC shutdown), but costs won't be bad because the magnet is the major expense
• Electric field distortions are still present
  • In particular, the SpaceCharge will still be there, and GridLeak can still break and distort tracks
    • Want low luminosity? For GridLeak, very low
  • Jamie D. noted that z-dependent signed DCAs were observed in straight-up tracks using zero field data a decade ago by Bum Choi (when collision vertices were widely spread and there were good statistics for straight-up tracks at small drift distances)
  • Distortion corrections (field cage shorts, field cage shifts, padrow 13, SpaceCharge, GridLeak), as currently implemented, expect a strong magnetic field principally along the z axis, and would need to be notably modified for zero field
    • GridLeak sheet of charge will also diffuse significantly (all the more reason to use very low luminosity)
• Diffusion is much worse
  • Clusters with long drift will be very broad, perhaps enough to cause fitting problems, and occupancy overlap
  • ...But short drift length tracks have a larger dip angle, so their clusters are still long in the drift direction (though certainly better in transverse)
• Tracking code (and vertex finder?) already set up to handle zero field (treats tracks as very high pT)
• No reconstruction of track charge sign, pT, PID (TOF still available if helpful?)
• No guarantee that alignment without magnetic field is the same as with it (though it should be, and this should not be a reason to pass up on this)

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 -Gene