1. Åsmund Skjæveland Magnetic Distortion of HPD Images

2. The LHCb experiment

3. Observe CP violation in decays of heavy hadrons produced in p-p collisions Observe CP violation in decays of heavy hadrons produced in p-p collisions –Need accurate particle identification: Tracking stations track particles with high precision Tracking stations track particles with high precision Magnet deflects track ➨ momentum and charge Magnet deflects track ➨ momentum and charge RICH measure velocity ➨ combine with momentum to find mass RICH measure velocity ➨ combine with momentum to find mass

4. The RICH detectors in LHCb

5. RICH detectors Measures velocities via Čerenkov radiation Measures velocities via Čerenkov radiation –Uses Hybrid Photon Detectors ➨ high resolution –Combine images from many HPDs to find Čerenkov circles

6. RICH detectors Measures velocities via Cherenkov radiation Measures velocities via Cherenkov radiation –Uses Hybrid Photon Detectors ➨ high resolution –Combine images from many HPDs to find Cherenkov circles

7. Hybrid Photon Detector 37.5 mm radius 37.5 mm radius 110 mm long 110 mm long Quartz optical window Quartz optical window Resolution 256x32 pixels Resolution 256x32 pixels Photoelectron optics ➨ sensitive to magnetic fields Photoelectron optics ➨ sensitive to magnetic fields

8. Pixel Hybrid Photon Detector

9. Problem RICH1 is close to spectrometer magnet RICH1 is close to spectrometer magnet HPDs will be measurably affected by small magnetic fields ➨ image is distorted HPDs will be measurably affected by small magnetic fields ➨ image is distorted –Reconstruction of circle requires undisturbed images: Photon hits must be measured very accurately –If perturbation is large, data will be pushed off image chip ➨ uncorrectable data loss

10. Magnetic Shielding RICHes enclosed in iron box—internal field reduced to max. 25 G (RICH1) and 10 G (RICH2) RICHes enclosed in iron box—internal field reduced to max. 25 G (RICH1) and 10 G (RICH2) MuMetal shield around each HPD further reduces magnetic field MuMetal shield around each HPD further reduces magnetic field MuMetal-shield

11. Experiment setup Projector Filter HPD Coils

12. Setup Standard projector connected to laptop Standard projector connected to laptop Dense optical filters Dense optical filters HPD tube HPD tube Read-out electronics Read-out electronics

13. Calibration Projected image much larger than HPD Projected image much larger than HPD Find HPD by sweeping horizontal and vertical lines across the screen Find HPD by sweeping horizontal and vertical lines across the screen Fine-tune with line patterns Fine-tune with line patterns

14. Calibration

15. Calibration

16. Calibration

17. Data taking Illuminate HPD with a defined pattern Illuminate HPD with a defined pattern Vary magnetic field Vary magnetic field Record HPD image Record HPD image

18. Observations for axial fields 0 G 30 G 20 G 50 G

19. Observations Small but clear background Small but clear background –thermal electrons –ion feedback Image is rotated Image is rotated Image is enlarged Image is enlarged Image center is slightly displaced Image center is slightly displaced

20. Conclusion Entire image on chip: No data loss Entire image on chip: No data loss –Reconstruction of undisturbed image probably possible

21. To Be Done Make and test other test patterns Make and test other test patterns Measure other field configurations Measure other field configurations Make and test image correction algorithm Make and test image correction algorithm Publish Publish Have a beer Have a beer