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

COBRA magnetCOBRA magnet

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

Advantage of the gradient Advantage of the gradient fieldfield

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

DCH waveformsDCH waveforms

Full information about charge and time is recorded

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

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