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 e +  +  Background  + → e +  e +  +   + → e + e + e + →   e e +  +  = 180  E  = E e = 52.8 MeV T  = T e Key features – intense DC muon beam; precise gamma energy measurement; precise positron energy measurement; precise time measurement

4. MEG setup 10 8 muons/sec Thin CH 2 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 10 8 stopped muons Very high counting rate – up to 10 8 stopped muons Good momentum (  0.4%)  position (  300  m for r, z) & time resolution (  50 ps) 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 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. DC structure Two independent layers for resolving left-right ambiguity Drift field  4 kV/cm, drift velocity  4 cm/  sec anode readout 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 Due to the 12  m foils and opened-frame structure a pressure regulation needs to be extremely precise dP  1 Pa,  P  0.1 Pa!

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 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 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 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 Slow control with a connection to the MIDAS DAQ  logging of all important parameters Very high demands for processing power Very high data rate DRS4 – improved design, up to 5 GHz!

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

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