1. JT: 1 The Berkeley Lab STAR TPC Distortions in the Transverse Plane: An Update Jim Thomas

2. JT: 2 The Berkeley Lab STAR A Mnemonic for Listing the Potential Distortions ZCal Barrel EM Calorimeter Endcap Calorimeter Magnet Coils TPC Endcap & MWPC ZCal FTPCs Vertex Position Detectors Central Trigger Barrel or TOF Time Projection Chamber Silicon Vertex Tracker RICH Central MembraneEndwheel and PadplaneOuter Field Cage Inner Field Cage

3. JT: 3 The Berkeley Lab STAR The List of Distortions in the Transverse Plane The list can be enumerated by surfaces: Outer field cage corrections Inner field cage corrections Central membrane corrections End-wheel and pad-plane corrections Pad Row 13 corrections and other local electrostatic defects Rotation and miss-alignment of sectors with respect to their ideal locations Rotation of either TPC end-wheel with respect to its ideal location and by volume: Space Charge corrections due to charge in the volume of the TPC Magnetic field corrections due to B fields in the volume of the TPC Twist of the TPC with respect to the magnetic field axis and/or the measured map General coordinate transformations A few additional items are listed for completeness. (These items affect the drift of the electrons in the Z direction but do not strongly affect the distortions in the transverse plane.) Gas composition and variations in the drift velocity Barometric pressure changes and variations in the drift velocity Pressure variations as a function of height in the TPC Temperature gradients in the TPC

4. JT: 4 The Berkeley Lab STAR We think of our events like this … Data Taken June 25, 2000.

5. JT: 5 The Berkeley Lab STAR Most of our events look more like this …

6. JT: 6 The Berkeley Lab STAR SpaceCharge from (all) events cause distortion Z / 5 cm Radius / 5 cm Z Radius Distortion

7. JT: 7 The Berkeley Lab STAR Model for the distortions Old Model –Beam gas events leave a uniform deposition of charge in the TPC –The charge from the collisions is not significant New Model –Beam gas events leave a 1/R 2 distribution of charge in the TPC –The charge from the collisions is not significant in the 2001 data, but will be in the future (?)

8. JT: 8 The Berkeley Lab STAR Uniform.vs. 1/R 2 Space Charge Distribution Radial  Distortions Z Z Radius Distortion

9. JT: 9 The Berkeley Lab STAR Two sources of SpaceCharge Beam gas and other up stream events –not synchronous with our trigger –Scales with beam intensity (not Luminosity) –1/R 2 distribution of charge (?) The collisions at STAR –synchronous with our trigger –Scales with Luminosity –A/R + B/R 2 distribution of charge (?) In the future, the average Luminosity will go up a factor of 40 but the beam intensity will only go up a factor of 2 to 4 –We have to prepare for a significant increase in space charge due to the collisions in the detector. We have to be able to distinguish the two sources of distortion

10. JT: 10 The Berkeley Lab STAR A Wide Range of Charge Distributions 1/R 3 1/R 2 Wieman’s HiJet 1/RLinear 2:1Linear These (and other) Distributions are Available in StMagUtilities Radius Voltage

11. JT: 11 The Berkeley Lab STAR 2 Equations, 2 Unknowns We can simultaneously fit the DCAs and match the steering at the RICH due to the beam gas induced space charge –Choose the right charge distribution 1/R, 1/R 2, HiJet, etc. –Choose the RICH scaler normalization constant with the DCA held constant

12. JT: 12 The Berkeley Lab STAR These Items on the List Are Ready to Go The list can be enumerated by surfaces: Outer field cage corrections Inner field cage corrections Central membrane corrections End-wheel and pad-plane corrections Pad Row 13 corrections and other local electrostatic defects Rotation and miss-alignment of sectors with respect to their ideal locations Rotation of either TPC end-wheel with respect to its ideal location and by volume: Space Charge corrections due to charge in the volume of the TPC Magnetic field corrections due to B fields in the volume of the TPC Twist of the TPC with respect to the magnetic field axis and/or the measured map General coordinate transformations A few additional items are listed for completeness. (And these I don’t know about.) Gas composition and variations in the drift velocity Barometric pressure changes and variations in the drift velocity Pressure variations as a function of height in the TPC Temperature gradients in the TPC

13. JT: 13 The Berkeley Lab STAR Conclusions Tools are available to calculate all known distortions –in the transverse plane –gain, t0, and drift velocity corrections have not been discussed SpaceCharge corrections are significant –especially at 40x We need to track the beam current and/or the distribution of charge in the TPC for untriggered events. We will need scalers and diagnostics for each source of charge –Monitor L –Monitor beam current The RICH Mult Scaler is gone and we need a replacement Recent progress with the laser cluster finder means we might be able to use this data …

14. JT: 14 The Berkeley Lab STAR DCA, Steering at the RICH, and  p The RICH Scaler has an arbitrary normilization The DCA’s have been tuned to be the same in both cases  p is different by a factor of 2 Steering at the Rich changes sign under these conditions