After some recent confusion about the STAR coordinate system given the upward shift of the STAR detector for Run 14, and with regard to some ongoing work that will affect many subsystems, I'd like to post some notes about the STAR coordinate system here.

1. Since 2006, the TPC group has used alignment parameters that place the TPC ~0.7 cm vertically offset. Before that time, the offsets used were closer to ~0.2 cm. The existing notes from the 2006 alignment are that it was done from a survey, but the survey data used for that has not been located and the person who worked on it left STAR some time ago.
    
2. Several subsystems are mounted on the TPC structure, including TOF, the inner tracking systems, and the beampipe. Therefore, the offset used by the TPC is reflected in the reconstructed positions of all of these other systems as well. This was notably observed by the TOF group a few years ago when they did their initial alignment.
    
3. Part of the TPC group's recent R&D efforts on alignment (towards resolving h^-/h⁺) is to look at the TPC's global alignment with respect to the magnet. Whatever changes the TPC group makes to its alignment, it will impact all the other subsystems' alignment if they are not using the TPC global position as their reference when it is deployed, because TPC tracks will get shifted in the STAR coordinate system.
   
   
   • We will need to make sure that the expected shift is well-advertised and everyone has some idea what to expect when they re-do their alignment. This point is relevant to all datasets for which the TPC alignment gets changed if we are to produce or re-produce such data.
      
   • The TPC group has been unable to project a timeline for delivery of a new alignment. As such, subsystems should be proceeding with their alignment as if the current TPC geometry is not going to change any time soon, but with the awareness that a change is to come eventually.
      
4. Even if a subsystem uses the TPC global position as their reference, there is still the possibility to observe some effect of a TPC global position shift. This is because there are TPC distortion corrections which depend on the TPC position within the STAR magnet. This particular effect is expected to be very small (well below 1 mm level). Subsystems like EMC and TOF (even FTPC) would likely be blind to such a shift, but the HFT would see it.
    
5. Because the recent physical shift to the entire STAR Detector involved all subsystems, nothing changed relative to anything else inside STAR that was mounted to STAR. This includes the beampipe, but does not include the beams themselves. Thus, we expected to see, and have seen, a downward shift of the beam in the STAR coordinate system this year.
    
6. There a some relationship between this topic and the topic of alignment in simulations (recently discussed at an S&C meeting), particularly with respect to susbsytems using the TPC reference system as a volume to which their systems are (mis)aligned.
    

 

-Gene