Status of the MEG m?eg Experiment at PSI

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Status of the MEG m?eg Experiment at PSI
W. MolzonUniversity of California, Irvine August
28, 2006 NuFact 2006

• Goals of the experiment
• Overview of technique
• Status of detector and software
• Schedule

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The MEG Collaboration
University of Tokyo Y. Hisamatsu, T. Iwamoto,
T. Mashimo, S. Mihara, T. Mori, Y. Morita, H.
Natori, H. Nishiguchi, Y. Nishimura, W. Ootani,
K. Ozone, R. Sawada, Y. Uchiyama, S.
Yamashita KEK T. Haruyama, K. Kasami, A. Maki,
Y. Makida, A. Yamamoto, K. Yoshimura Waseda
University K. Deguchi, T. Doke, J. Kikuchi, S.
Suzuki, K. Terasawa INFN, Pisa A. Baldini, C.
Bemporad, F. Cei, L.del Frate, L. Galli, G.
Gallucci, M. Grassi, F. Morsani, D. Nicolò, A.
Papa, R. Pazzi, F. Raffaelli, F. Sergiampietri,
G. Signorelli INFN and University of Genova S.
Cuneo, D. Bondi, S. Dussoni, F. Gatti, S.
Minutoli, P. Musico, P. Ottonello, R. Valle INFN
and University of Pavia O.Barnaba, G. Boca, P.
W. Cattaneo, G. Cecchet, A. De Bari, P. Liguori,
G. Musitelli, R. Nardò, M. Rossella,
A.Vicini INFN and University of Roma I A.
Barchiesi, D. Zanello INFN and University of
Lecce M. Panareo Paul Scherrer Institute J.
Egger, M. Hildebrandt, P.-R. Kettle, S. Ritt, M.
Schneebeli BINP, Novosibirsk L. M. Barkov, A.
A. Grebenuk, D. N. Grigoriev, B. I. Khazin, N.
M. Ryskulov JINR, Dubna A. Korenchenko, N.
Kravchuk, A. Moiseenko, D. Mzavia University of
California, Irvine P. Huwe, W. Molzon, H.
Topchyan, V. Tumakov, F. Xiao, S. Yamada
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Principal Features of m ? eg Experiment

• Stop m in thin target
• Measure energies of e (Ee) and g (Eg)
• Measure angle between e and g (??)
• Measure time between e and g (?t)

Kinematics
qeg 180
g
e
m
Ee 52.8 MeV
Eg 52.8 MeV
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Experimental Requirements

• Muon beam with stopping rate times acceptance of
  order 106-7
• Acceptance limited by photon coverage (cost for
  calorimetric detectors, conversion probability
  for measuring photons by converting to ee-)
• Rate limited by accidental backgrounds or
  instrumental effects at high rates
• Thin stopping target needed for limiting
  annihilation in flight, scattering in target
• Typically surface muon beam used pm peaked near
  29 MeV
• Detectors capable of measuring Eg, Ee, Dt, Dq to
  sufficient precision without long tails in
  resolution function
• Photon energy, time, position with single large
  liquid xenon calorimeter
• High, uniform light yield energy resolution
  1.5 sRMS at 53 MeV
• Good timing tdecay 45 ns
• Good position resolution 4 mm
• Large contiguous volume care needed in pileup
• Positron momentum, angle from magnetic
  spectrometer
• Resolution dominated by multiple scattering
  premium on low mass 0.4 sRMS
• Geometry optimized for resolution, rate, trigger
  considerations
• Positron time from scintillation counters
• High light yield, fast, thick scintillators 45
  ps sRMS
• Correction of time to that at vertex requires
  care (energy loss, scattering)

m
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The MEG Experiment
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Complete GEANT3 Description of Detector
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(No Transcript)
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MEG at the Paul Scherrer Institut

• Approved at Paul Scherrer Institut, Switzerland
  in 1999
• Start physics run in 2007
• Initial goal of 10-13, eventually 10-14

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PSI Proton Cyclotron
590 MeV gt 1.8mA gt 150days/year CW Beam
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Beamline Status

• All elements installed
• Commissioning begun
• Some minor problems with alignment
• Yield slightly low with non-standard
  targetexpected stop rate 108, required 4x107
• Beam profile as expected

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COBRA Magnet

• COBRA magnet is built and run to full field
• Mapping completed small asymmetries being
  studied
• Initial problems with quench detection system
  fixed

Signal radius independent of angle
Sweep out Michel positrons
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Stopping Target

• Optimized to stop m beam with high efficiency,
  minimize backgrounds
• Low Z material to reduce positron scattering
  exiting target
• Thin target with well-measured position to enable
  decay position to be measured to high precision
  from positron trajectory intercept at target
• Photon direction from position in LXe
  calorimeter, vertex position
• Positron direction from measured trajectory
  extrapolated to target position
• Target position known to 0.5 mm
• Required to remotely move target to allow
  insertion of auxiliary targets for calibration
  schemes.
• Implementation with 150-175 mm polyethylene film
  on Rohacell frame
• In situ alignment from data by imaging small
  holes in target with Michel decays

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The MEG Detector
COBRA Magnet
Timing Counter
Compensation Coil
Drift Chamber
Liquid Xenon Calorimeter
Surface Muon Beam
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Drift Chamber

• Conventional drift chamber (wires,cathode foils,
  field wires)
• Resolution dominated by scattering with 300 mm
  drift resolution
• Axial coordinate from charge division, vernier
  pads on foils
• Thin foils enclose gas and make cathode ? precise
  pressure control
• Sophisticated, low mass carbon fiber frames to
  reduce interactions
• Helium based gas for reduced multiple scattering
• Precise pressure control to maintain
  anode-cathode spacing
• Readout with 500 MHz waveform digitizer

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Drift Chamber Analysis

• Momentum determination using Kalman filter
• Currently without front-end of analysis
  (time-to-distance, pattern recognition)

Resolution for decay point photon direction sRMS
0.1 cm(other effects more important)
Momentum resolutionDepends on turn topolgy sRMS
0.25 for multi-turn
Angular resolution for dfegsRMS 9.4 mrad
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Timing Counters

• Provide time of electron with a precision below
  100 ps
• Need precise position information for propagation
  delay correction (30ps 1 cm)
• Requires large signal for good timing resolution
• Implementation
• Large (4x4 cm2) bars in axial direction to give
  large signal
• Read out with magnetic-field-insensitive
  phototubes
• Axial coordinate given by 5x5 mm2 scintillating
  fibers with APD readout

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Timing Counter Status
Completed f-measuring module
Z-measuring optical fibers
Tracking contribution to timing error sRMS 35
ps, tails from scattering, E-loss
Intrinsic resolution from test beam sRMS 30 ps
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LXe Scintillation g-ray Detector

• 800 liter Liquid Xenon scintillator
• high light yield 75 NaI(Tl)
• fast response 45ns decay time
• good uniformity
• 850 f2 PMTs
• Facing inwards like Super-Kamiokande
• Directly immersed in LXe
• RD with prototype
• Performance
• LXe purification
• Operation _at_ 165K

g

• Prototype tested using laser back-scattered beam,
  neutral pions from p- capture at rest
• 70 l cubical volume, 228 phototubes

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CEX p0 Beam Test _at_ PSI

• Monochromatic g
• 55 83 MeVp- (at rest) p ? p0 np0 (28MeV)
  ? g g (back-to-back, 54.9 MeV lt Eg lt 82.9MeV)
• 129 MeVp- p ? n g
• Neutron response

Energy (MeV)
sRMS 1.5
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Opening angle (deg)
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Expected Calorimeter Performance

• Tied to large prototype performance tests
• Simulation of performance with GEANT and analysis
  algorithms under development
• Current status of one such algorithm energy
  from weighted sum of photube signals corrected
  for converstion depth sRMS1.2

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Calibration and Monitoring Techniques

• Variety of techniques planned for continuous
  monitoring of calorimeter performance
• Permanently installed alpha sources (few MeV) for
  gain, quantum efficiency monitor
• Photons from neutral pions from p- capture at
  rest sets absolute energy scale
• Liquid hydrogen target installed periodically but
  infrequently
• Beam retuned for negative pions
• Second photon detected with movable NaI detector
  opposite LXe cryostat
• Photons from pLi ? Be g
• Resonant at Ep 440 keV
• Narrow photon line at 17.6 MeV
• Use Cockroft-Walton accelerator operated in situ
• Allows daily monitor of resolution, gain

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DAQ / Waveform Analysis

• Data acquisition based on waveform digitizer no
  ADC, TDC
• 4 GHz maximum sampling rate (2 GHz for LXe,
  timing counters) 0.5 GHz for drift chambers
• 12 bit ADC
• 3000 channels
• 32 channels on VME board

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Pileup Rejection with Waveform Analysis

• Dt 50 ns Dt 15 ns

Peak finding
Differentiation

• Additional techniques for close overlaps
• Waveform fitting (chi-square)
• Combination of spatial and temporal fitting

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Expected Background Sensitivity

• Expected detector resolution
• DEe 0.8 (FWHM)
• DEg 4.5 (FWHM)
• Dqeg 18mrad (FWHM)
• Dteg 141ps (FWHM)
• Expected BKG rate _at_ initial phase
• Accidental background below 10-13
• Radiative m decay down to lt10-14
• Single event sensitivity
• B(m?eg) 10-13
• Eventually down to 10-14 with improved PMTs, etc.

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Schedule
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Summary

• MEG experiment is well into construction phase
• Beam line installed, commissioning going well, to
  be completed in autumn
• Detector systems construction advancing well
• half drift chamber and full timing counters
  ready for autumn engineering run
• Liquid xenon calorimeter delayed by cryostat
  production in industry, scheduled for
  installation and commissioning by end of year
• Engineering run during autumn, further tests and
  commissioning during beam-off period
  January-March
• Anticipate fast startup with data taking in April
  2007
• Increasing effort now going into developing
  analysis software
• Algorithms for low-level analysis each of
  detector systems being developed
• High level analysis (e.g. pileup rejection,
  timing corrections, etc.) begun
• Goal is to be able to analyze data soon after
  collection
• Anticipate single event sensitivity around 10-13
  with two year run, a significant result (e.g.
  with respect to MEGA limit of 1.2x10-11) on
  shorter timescale

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• END