MEG positron spectrometer Oleg Kiselev, PSI on behalf of MEG collaboration.
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Transcript of MEG positron spectrometer Oleg Kiselev, PSI on behalf of MEG collaboration.
MEG positron MEG positron spectrometerspectrometer
Oleg Kiselev, PSIOleg Kiselev, PSI
on behalf of MEG on behalf of MEG collaborationcollaboration
Motivations of the Motivations of the experimentexperiment
+ → 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
Signal and backgroundSignal and background
Signal
e+ +
Background
+ → e+
e+ +
+ → e+e+e+ →
e
e+ +
= 180E = Ee = 52.8 MeVT = Te
Key features – intense DC muon beam; precise gamma energy measurement;precise positron energy measurement;precise time measurement
MEG setupMEG setup108 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
Spectrometer - Spectrometer - requirementsrequirements
Very high counting rate – up to 10Very high counting rate – up to 1088 stopped muonsstopped muons
Good momentum (Good momentum (0.4%) 0.4%) position position ( ( 300 300 m for r, z) & time m for r, z) & time resolution (resolution (50 ps)50 ps)
Multiple scattering is a limiting factor Multiple scattering is a limiting factor & & -background should be -background should be suppressed suppressed low mass system low mass system
Layout of DCsLayout of DCs
Low mass – the most hard requirement → He-filled spectrometer, He-based gas mixture, no strong frames
Opened-frame structure!
DC structureDC structure
Two independent layers for resolving left-right ambiguityDrift field 4 kV/cm, drift velocity 4 cm/sec
anode readout
Resolution 1 cm via charge division
0.3 cm via ratio of signals fromtwo strips
Gas regulationGas regulation
Due to the 12 m foils and opened-frame structure a pressure regulationneeds to be extremely precise
dP 1 Pa,P 0.1 Pa!
Timing counterTiming counter
Parameters:2-layer structure –
outer thick scintillation barsPMT readout for timinginner scintillation fibersAPD readout for z-trigger
Requirement of the experiment – 40 ps ()
One of the best results!
MEG electronicsMEG electronics Key feature - waveform digitizing of all Key feature - waveform digitizing of all
signals signals best pile-up rejection possibility best pile-up rejection possibility Use of DRS2 and DRS3 FADC chips – 12 bit, Use of DRS2 and DRS3 FADC chips – 12 bit,
1.5 GHz for calorimeter, 500 MHz for DCHs1.5 GHz for calorimeter, 500 MHz for DCHs Customized trigger system – FPGA perform a Customized trigger system – FPGA perform a
fast energy, time and position reconstruction; fast energy, time and position reconstruction; set of trigger criteria is programmed set of trigger criteria is programmed
Slow control with a connection to the MIDAS Slow control with a connection to the MIDAS DAQ DAQ logging of all important parameters logging of all important parameters
Very high demands for processing powerVery high data rate
DRS4 – improved design, up to 5 GHz!
Status of experimentStatus of experiment
All components of MEG setup are All components of MEG setup are operational and tested during a operational and tested during a commissioning run in December commissioning run in December 20072007
Unique feathers of the positron Unique feathers of the positron spectrometer should allow to reach spectrometer should allow to reach the goal of the experiment the goal of the experiment
Start of data taking – July 2008Start of data taking – July 2008