How can we study the magnetic distortion effect? Magnetic Distortion Measurement System of the LHCb RICH2 Detector B. Storaci on behalf of the LHCb RICH Collaboration CERN European Organization for Nuclear Research and Università degli Studi di Milano Bicocca LHCb Detector The LHCb experiment at the CERN LHC collider is optimized for the study of CP violation and rare B-decays. Two Ring Imaging Cherenkov detectors provide particle identification in the momentum range 1-100 GeV/c The stringent requirements on the photon position measurements are met by Hybrid Photon Detector, HPD  A photoelectron is accelerated through a voltage of 20kV onto an encapsulated anode. The presence of a magnetic field will degrade the HPD spatial resolution: LHC: pp collision @ 14 TeV bunch crossing 40 MHz luminosity ~2×1032 cm-2s-1 LHCb: dedicated B physics experiment B hadrons mostly produced at small polar angle single arm forward spectrometer (10-300 mrad acceptance) particle ID provided by RICH detectors covering the momentum range 1-100 GeV/c RICH1 RICH2 Cherenkov radiators: silica aerogel: 1-10 GeV/c C4F10: 10-60 GeV/c CF4: 40-100 GeV/c A correction procedure has been adopted! How can we study the magnetic distortion effect? Beamer and optical system 5.5m long pipe to simulate the length of the RICH2 A completely dressed HPD column with two PM tubes for the trigger signal and alignment procedure Computers for each ECS/DCS/DAQ subsystem A well defined pattern is projected using a LED beamer. The system is aligned using 3 PM tubes. Background is reduced by triggering the data on the external signal coming from the PM tubes. It will be possible to compensate the magnetic distortion effect by means of the comparison of the pattern without magnetic field with the corresponding positions at different magnetic field values. We are taking measurements during the LHCb magnet ramp up to record the displacement of the point-positions at different values of the magnetic field. We set up a laboratory with a complete acquisition system like the one used in the experiment to optimize the data analysis process The ultimate goal is to achieve such an accuracy in the correction procedure so that the residual uncertainty due to magnetic distortions is negligible in comparison to the effective pixel size 2.5 x 2.5 mm2 Magnetic distortion analysis software 1st Goal: some HPDs can present ion-feedback contributions. For an efficient pattern reconstruction it is necessary to maximize the signal to noise ratio. 2nd Goal: developing a maximum local search algorithm which is robust against uniform background 3rd Goal: developing an advanced clusterization procedure to determine the region where the centre of gravity procedure is applied. Analyzing only data with more then 4 hits per events and with a maximum cluster size less then 3 we reduce the incidence of ion-feedback events by 93% and we have a signal to noise ratio greater than 30. The maximum is accepted only if it is greater than the local statistical median value (computed in a square of 3 x 3 cells) and the total statistical median, computed over the entire anode. The centre of gravity analysis method gives a point position reconstruction uncertainty during the magnetic distortion analysis to be less than 0.93 x 0.63 mm2 PATTERN RECOGNITION Accumulation picture after a run with a pattern of 16 points (B=0) Results obtained after the centre of gravity procedure Poster presented at the 10th ICATPP (Como, 8-112 October 2007)