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

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

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

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

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

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

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

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

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

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

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

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

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

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

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