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MEG positron spectrometer

MEG positron spectrometer. Oleg Kiselev, PSI on behalf of MEG collaboration. Motivations of the experiment.  + → e +  decay is a forbidden process in the Standard Model (SM) – conservation of lepton numbers In case of massive neutrinos and mixing – allowed on negligible level

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MEG positron spectrometer

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  1. MEG positron spectrometer Oleg Kiselev, PSI on behalf of MEG collaboration

  2. Motivations of the experiment • +→ e+ decay is a forbidden process in the Standard Model (SM) – conservation of lepton numbers • In case of massive neutrinos and mixing – allowed on negligible level • In all SM extensions the branching ratio is enhanced, predictions are 10-12 – 10-14 (Y. Kuno, Y. Okada, Rev. Mod. Phys. 73 (2001) 151) • Relatively simple process - e+ and should be emitted in the opposite directions with the same energy of 52.8 MeV • The main goal of the experiment is to reach a sensitivity of 10-13 - two orders of magnitude lower than current limit

  3. Signal and background Signal Background  e+ +   e+ +  + → e+ e+e+ →    • = 180 E = Ee = 52.8 MeV T = Te + → e+ e+ +  e Key features – intense DC muon beam; precise gamma energy measurement; precise positron energy measurement; precise time measurement

  4. MEG setup 108 muons/sec Thin CH2 target Liquid Xenon calorimeter (10% acceptance, 800 l, 846 PMTs, t  60 ps, E  1%, high light yield) Scintillation Timing Counter (t  50 ps) COnstant Bending RAdius spectrometer inside superconducting magnet (B = 1.27 T at Z = 0 and decreasing as Z increases, B = 0.49 T at Z =1.25 m) Ultra low positron detection system

  5. COBRA magnet Highly gradient field, 5 superconducting + 2 warm (compensation coils)

  6. Advantage of the gradient field

  7. Spectrometer - requirements • Very high counting rate – up to 108 stopped muons • Good momentum (0.4%)  position ( 300 m for r, z) & time resolution (50 ps) • Multiple scattering is a limiting factor & -background should be suppressed  low mass system

  8. Layout of DCs Low mass – the most hard requirement → He-filled spectrometer, He-based gas mixture, no strong frames Opened-frame structure!

  9. anodereadout DC structure Two independent layers for resolving left-right ambiguity Drift field  4 kV/cm, drift velocity  4 cm/sec Resolution 1 cm via charge division 0.3 cm via ratio of signals from two strips

  10. DCH waveforms Full information about charge and time is recorded

  11. Gas regulation dP  1 Pa, P  0.1 Pa! Due to the 12 m foils and opened-frame structure a pressure regulation needs to be extremely precise

  12. Timing counter Parameters: 2-layer structure – outer thick scintillation bars PMT readout for timing inner scintillation fibers APD readout for z-trigger Requirement of the experiment – 40 ps () One of the best results!

  13. MEG electronics • Key feature - waveform digitizing of all signals  best pile-up rejection possibility • Use of DRS2 and DRS3 FADC chips – 12 bit, 1.5 GHz for calorimeter, 500 MHz for DCHs • Customized trigger system – FPGA perform a fast energy, time and position reconstruction; set of trigger criteria is programmed • Slow control with a connection to the MIDAS DAQ  logging of all important parameters DRS4 – improved design, up to 5 GHz! Very high demands for processing power Very high data rate

  14. Status of experiment • All components of MEG setup are operational and tested during a commissioning run in December 2007 • Unique feathers of the positron spectrometer should allow to reach the goal of the experiment • Start of data taking – July 2008

  15. Paul Scherrer Institute J. Egger, M. Hildenbrandt, P.-R. Kettle, O. Kiselev, S. Ritt, M. Schneebeli

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