1. C.Cheshkov 15/09/2005 Weekly Offline Meeting
Alignment in TPC
C.Cheshkov
15/09/2005
Weekly Offline Meeting

2. Contents Introduction of misalignments in TPC simulation:
Retrieving from TGeo
Misalignment in Hits->(S)Digits
TPC tracking using alignment data
Discussion on possible approaches in the tracking
Alignment in TPC
15/09/2005

3. Simulation of misalignment
In TPC we consider `alignable` only the 72 readout chambers
3 translations + 3 rotation angles per chamber = 6*72 float numbers
During the initialization of AliTPCParam:
Retrieve the position of the chamber volumes from TGeo (‘TIRC’ volumes)
Translate them to the local coordinate system of the TPC sectors
Store the differences with respect to the default positions as members of AliTPCParam
Alignment in TPC
15/09/2005

4. Sector coordinate system
Origin and Z axis coincide with the origin and Z axis of the global coordinate system
In XY plane it is rotated by
 = 10˚+ i x 20˚
where i is the sector index x y
Alignment in TPC
15/09/2005

5. Simulation of misalignment
Transport MC is used to produce hits along the track trajectories
During the digitization of the hits, the ionization electrons are transported ‘by hand’ to the surface of the readout chambers (AliTPC::TransportElectron())
After that we read the alignment consts from AliTPCParam and use them to put the electrons into the local coordinate system of the readout chambers
The rest of the digitization is not affected
Alignment in TPC
15/09/2005

6. Chamber coordinate system
‘Ideal’ geometry
Hit
Chamber and Sector
coord systems coincides y x
Misaligned geometry
Chamber coord system
is translated and
rotated with respect to
Sector one
Alignment in TPC
15/09/2005

7. Example of misaligned TPC
Inner chambers rotated by +10mrad
Outer chambers rotated by -10mrad
Alignment in TPC
15/09/2005

8. Tracking using alignment data
The basic idea:
Do not touch cluster positions
Do the tracking always locally in the chamber coordinate system
While propagating a track in between sectors, translate and rotate the track from the current chamber coordinate system to the next chamber coordinate system
No need to modify the propagation and update track methods
Track translation and rotation just few times per tracking pass
Alignment in TPC
15/09/2005

9. Tracking using alignment data
The approach was implemented for translation and rotation of the readout chambers in XY plane
New AliTPCtrack::Translate method was added
Already available AliTPCtrack::Rotate was used
Inside the tracker code the calls to PropagateTo and Rotate methods of AliTPCtrack were replaced by new methods of the tracker
Alignment in TPC
15/09/2005

10. Tracking using alignment data
New PropagateSeed and RotateSeed method of the tracker:
Check if the track is to enter in a new sector
If yes then:
Take from AliTPCParam the position and angles of the current and the next readout chamber
Transform the track into the global coordinate system
Rotate the track (in RotateSeed method)
Transform the track into the next chamber coordinate system
Propagate the track (in PropagateSeed method)
Alignment in TPC
15/09/2005

11. Tracking using alignment data
The chamber index is identified by:
index of the last pad row on which track is propagated
azimuthal angle of sector coord system (10˚ + i x 20˚)
z position of the track
At entrance/exit of PropagateBack and RefitInward tracker methods, the tracks are transformed to Chamber/Sector coordinate system, respectively.
The ESD tracks are always stored in the Sector coordinate system (‘global’)
Alignment in TPC
15/09/2005

12. Tracking using alignment data
The method was tested with several misalignment configs showing no caveats
The efficiency and resolution is restored back to the ‘ideal geometry’ one
The methods to transform between Chamber<->Sector systems are called about 3.5/track (no effect on the timing)
Alignment in TPC
15/09/2005

13. Discussion on tracking approach
The presented method has two main problems:
Misalignment rotations in YZ and XZ planes violates our helix track model
Effective correction on the track curvature and z position during the propagation?
Space charge effects
The alternative is to move the cluster positions:
Mislalignment rotations in XY plane violates our propagation approach
Propagate the track to the ‘ideal’ reference plane X, find the nearest cluster and propagate the track to the actual X position of the cluster
The third choice is to ‘mix’ the above two methods
Alignment in TPC
15/09/2005

14. Discussion on tracking approach
Shift
Rotation
XYZ XY XZ YZ
Track translation & rotation OK
Cluster position translation & rotation
Alignment in TPC
15/09/2005

15. Conclusions and Outlook
The code for the simulation of the misalignment is almost ready (need just to add the part which reads the chamber positions from TGeo)
The tracking code which uses track rotation and translation in XY is ready to be committed
The two possible approaches in the tracking should be further discussed and evaluated
Alignment in TPC
15/09/2005