Searching for Muon to Electron Conversion Below the 1016 Level

1
Searching for Muon to Electron Conversion Below
the 10-16 Level

• Michael Hebert
• UC Irvine
• PANIC02
• Osaka, Sept. 29 Oct. 4, 2002

2
MECO Collaboration
Eleven Institutions worldwide at present.
Substantial growth expected following
formal project start

• Institute for Nuclear Research, Moscow
• V. M. Lobashev, V. Matushka,
• New York University
• R. M. Djilkibaev, A. Mincer,
  P. Nemethy, J. Sculli, A.N. Toropin
• Osaka University
• M. Aoki, Y. Kuno, A. Sato
• University of Pennsylvania
• W. Wales
• Syracuse University
• R. Holmes, P. Souder
• College of William and Mary
• M. Eckhause, J. Kane, R. Welsh

• Boston University
• J. Miller, B. L. Roberts, O. Rind
• Brookhaven National Laboratory
• K. Brown, M. Brennan, G. Greene,
• L. Jia, W. Marciano, W. Morse,
  Y. Semertzidis, P. Yamin
• University of California, Irvine
• M. Hebert, T. J. Liu, W. Molzon, J.
  Popp, V. Tumakov
• University of Houston
• E. V. Hungerford, K. A. Lan, L.
  S. Pinsky, J. Wilson
• University of Massachusetts, Amherst
• K. Kumar

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Charged Lepton Flavor Violation

• Lepton Flavor is NOT conserved in the neutral
  sector as shown by recent neutrino mixing results
• Lepton Flavor Violation (LFV) in the charged
  sector should occur via n mixing, but far
  below the experimentally accessible range,
  meaning that any observed signal necessarily
  requires new physics
  
• Fortunately a wide variety of proposed extensions
  to the Standard Model predict observable LFV
  processes in charged lepton sector

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One Example

• SU(5) SUSY- GUT predicts a signal only a few
  orders of magnitude below the current
  experimental limit
• SO(10) prediction is enhanced by

Courtesy Hisano
MECO single event sensitivity
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History of Charged LFV Searches
1

• Goal A four order of magnitude leap in
  sensitivity to the 2 ? 10 17 level for a
  single event
• Effective mass reach is enormous, e.g. for
  leptoquark exchange

?-N ? e- N ? ? e ? ? ? e e e- K0??
? e- K?? ??e-
10-4
10-8
Sensitivity to Lepton Flavor Violation
10-12
MECO Goal
10-16
1940 1950 1960 1970 1980 1990
2000 2010
Year
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Muon to Electron Conversion

• Low energy muons are stopped in Ti foils, forming
  muonic atoms
• Three possible fates for the muon
• Nuclear capture
• Three body decay in orbit
• Coherent LFV decay
• Signal is a single mono-energetic electron
• Single particle signal avoids accidental
  backgrounds at high rate
• Rate is normalized to the kinematically similar
  weak capture process

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MECO Features

• 1000fold increase in muon beam intensity (from
  MELC at MMF)
• High Z target for improved pion production
• Axially-graded 5 T solenoidal field to maximize
  pion capture
• Muon transport in a curved solenoid
  suppressing
  neutrals, positives,
  and high
  momentum negatives
  (new for
  MECO)
• Pulsed beam to eliminate prompt
  backgrounds
  (A. Baertscher et al.)
• Beam pulse duration ltlt muon lifetime
• Pulse separation muon lifetime
• Extinction between pulses lt 10-9
• High rate capability and improved acceptance
  electron detectors
• Axially-graded 2 T solenoidal field for improved
  acceptance (from MELC)
• Spectrometer with axial components and good
  resolution (new for MECO)

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Potential Backgrounds

• Muon Decay in Orbit
• The dominant background
• Steeply falling spectrum near endpoint, e.g.
• Sets required energy resolution
• Nbkgd 0.25 for Rme 2 ? 10-17 ? DEe 900 keV
  (FWHM)
• Radiative Muon Capture also eliminated by
  energy resolution
• Radiative Pion Capture requires beam extinction
  lt 10-9
• Muon decay in flight e- scattering
  negligible with pulsed beam
• Beam e- scattering in stopping target
  eliminated by pulsed beam
• Antiproton induced e- requires thin stopping
  window
• Cosmic ray induced e- requires active and
  passive shielding

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The MECO Apparatus
Superconducting Detector Solenoid contains
conversion electron detectors
Superconducting Production Solenoid captures muons
Superconducting Transport Solenoid selects low
momentum m-
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Production Region

• Axially graded 5 T solenoid captures pions and
  muons, transporting them toward the stopping
  target
• Cu and W heat and radiation shield protects
  superconducting coils from effects of 50kW
  primary proton beam

Superconducting coils
2.5 T
Proton Beam
Production Target
Heat Radiation Shield
5 T
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Transport Solenoid

• Curved solenoid eliminates
  line-of-sight transport of photons
  and neutrons
• Curvature drift and collimators sign and momentum
  select beam
• dB/ds lt 0 in the straight sections to avoid long
  transit time trajectories

2.1 T
Collimators
2.5 T
Curvature Drift
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Detector Region

• Axially-graded field near stopping target to
  increase acceptance and reduce cosmic ray
  background
• Uniform field in spectrometer region to simplify
  momentum analysis
• Electron detectors downstream of target to reduce
  rates from g and neutrons

Electron Calorimeter
Straw Tracking Detector
Stopping Target Foils
1 T
1 T
2 T
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Electron Spectrometer Performance
Side View
Axial View
Background with Detector Response
Conversion electron produced in the stopping
target, detected in the Tracker, and triggered in
the Calorimeter
900 keV FWMH

• 900 keV resolution dominated by
• Energy loss in muon stopping target (640 keV
  FWHM)
• Tracker intrinsic resolution (350
  keV FWHM)

Full GEANT Simulation Signal
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MECO Sensitivity Background

• Expected Sensitivity

• Expected Background

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Current Status

• Scientific approval
• Approved by BNL and by the NSF through level of
  the Director
• Approved (with KOPIO) by the NSB as an MREFC
  Project (RSVP)
• Endorsed by the recent HEPAP Subpanel on
  long-range planning
• Technical and management reviews
• Positively reviewed by many NSF and Laboratory
  appointed panels
• Magnet system design positively reviewed by
  external expert committees appointed by MECO
  leadership
• Funding
• Currently operating on 2.1M RD funds from the
  NSF
• Project start awaits Congressional action RSVP
  (MECO KOPIO) is not in the FY03 budget
  efforts in Congress to improve NSF MREFC funding
• Construction schedule
• Construction schedule driven by superconducting
  solenoids estimate from the Conceptual Design
  Study is 41 months from signing of contract until
  magnets are installed and operational

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Outlook

• The physics potential for MECO is extremely
  robust. Numerous extensions of the Standard
  Model predict an observable m?e signal if we are
  able to achieve the predicted four order of
  magnitude increase in sensitivity
• We expect to make this leap forward using several
  advances in the muon conversion state of the art
• 1000fold increase in rate of the m- stops
• Muon beam line that minimizes contamination while
  maximizing yield
• Improved detector acceptance, high rate
  capability, and good resolution
• We expect to move into the detailed design and
  construction phase very soon, meaning now is the
  perfect time for people to get involved!

More information visit http//meco.ps.uci.edu