Detecting Muon Number Violation with Sensitivity of 1017: Resetting the Standard in Searches for Ext

1
Detecting Muon Number Violation with Sensitivity
of 10-17Resetting the Standardin Searches for
Extremely Rare Processes
California State University Los
AngelesDepartment of Physics and Astronomy
Colloquium
W. Molzon - University of California,
IrvineOctober 16, 2003

• Why are muon conversion experiments interesting?
• Why do we think muon conversion might be
  discovered?
• What are the experimental difficulties?
• What experiments are proposed search for this
  process?
• When can we expect results?

2
Why is Muon to Electron Conversion Interesting?
In 1947, examples of a cosmic ray tracks were
discovered in which a charged particle decayed,
with the daughter particle decaying to an
electron. It was shown that the first particle
was the pion, proposed by Yukawa to be
responsible for strong binding forces between
nucleons, and that the intermediate particle had
all the properties of the electron except that
its mass was 200 times that of the electron. Why
there exists a massive copy of the electron was a
mystery, prompting the question attributed to
Rabi, Who ordered that? In some sense the
question has not been answered. The absence of
radiative decays of the muon indicated that
something prevented these decays. The discovery
of neutrinos and the demonstration that different
neutrinos are associated with electrons and muons
prompted the idea that there is an additive
quantum number associated with each type, or
flavor, of lepton
M conserving - seen m- ? e-?m?e
M violating - not seen m- N? e-N m- ? e-g
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Why Might Muon Number Not Be Conserved?
In our experience, conserved quantities are
associated with symmetries,and interactions are
described by local gauge theories.

• A continuous symmetry implies the existence of a
  conserved quantity translation symmetry ?
  momentum conservation.
• Interactions that are described by local gauge
  theories have a conserved charge and a massless
  gauge boson that mediates the interaction. For
  electromagnetism, electric charge is conserved
  and the photon is the force carrier.
• Attempts to identify muon number as a charge
  associated with a local gauge symmetry dont
  work.
• No force coupled to muon number can be
  identified lepton universality is very well
  tested.
• No massless particle that might be associated
  with such a force is known.
• Perhaps the massless force carrier has developed
  a very heavy mass by some symmetry breaking
  mechanism as do the W and Z bosons what is this
  mass scale?

Unless there is a reason for a quantity to be
conserved, it usually is not. Perhaps muon number
is not conserved and m ?e conversion will be
found. This discovery would have important
implications for understanding fundamental
particles.
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Muon Number in the Standard Model

• Particle physicists have now identified three
  families of both leptons and quarks and have a
  model that describes their known interactions.
• The matter particles are spin ½ particles that
  are weak isospin doublets. Three families of
  fermions are known only three light families are
  allowed.
• Interactions are described by gauge theories, and
  gauge bosons that mediate the forces have been
  discovered and their properties measured.
• The fermion eigenstates that have definite mass
  and lifetime are linear combinations of the
  states that have definite flavor.
• The description of the matter sector is
  incomplete

• It does not explain why there is more than one
  type of lepton and quark.
• It doesnt explain why the families of quarks and
  leptons have different masses or even why they
  have mass.
• The model says nothing about the structure of the
  unitary matrix that quantifies the mixing between
  the mass and flavor eigenstates.

Understanding the relationships among the
families of quarks and leptons is among the most
important issues in particle physics.
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Flavor Mixing in the Standard Model
?
Flavor ChangingNeutral Current
Flavor ChangingCharged Current
Effective Flavor ChangingNeutral Current
liU M Ulj
liU M D?j
liU M D?j
?iD M Ulj
This diagram is not zerosince UD ? I. UD is
thephysical observable.
The sum over neutrino flavors of these diagrams
is not zero only if neutrinos have different
masses andUD ? I
This diagram is zero sinceM is proportional to I
(lepton universality)and UU I.
K- ? ?0 e- ?e mixes first and second
generation quarks
?? ? ?e oscillation mixes first and second
generation leptons
Discovery of neutrino oscillations necessitated
changing the Standard Model to incorporate
neutrino masses and mixing.
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How are Neutrino Flavor-Oscillations and ?-N ?
e-N Related?

• Neutrino flavor-oscillations have now been found.
  This requires non-degenerate neutrino masses and
  non-diagonal mixing matrix.
• Neutrino flavor-oscillations have limited
  implications for flavor mixing (lepton flavor
  violation or LFV) in the charged sector.
• Charged LFV processes will occur through
  intermediate states with n mixing. However,the
  small upper limits on n mass2 differences imply
  that the expected rate is well below what is
  experimentally accessible.
• Because the Standard Model contribution to muon
  conversion processes is so small, their discovery
  will be an unambiguous signal of physics beyond
  the Standard Model.
• In many scenarios for physics beyond the SM
  charged LFV processes might be detectable.
  Effective mass reach of new experiments is
  enormous, well beyond reach of direct searches.

10-31
Ratio of coupling strength to weak coupling
strength
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Sensitivity to Different Muon Conversion
Mechanisms
Supersymmetry
Compositeness
Predictions at 10-15
Second Higgs
After W. Marciano
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Supersymmetry Predictions for m ? e Conversion

• Supersymmetry postulatesexistence of a fermion
  for every boson and a boson for every fermion
• Observable levels of LFV are expected in some
  supersymmetric grand unified models
• Level of LFV related to quark mixing

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Rates for LFV Processes Linked to n Oscillations
Solar neutrino oscillation ne disappearance
From the model of J. Hisano and D. Nomura, Phys.
Rev. D59 (1999) SU(5) grand unified model with
heavy, right-handed neutrinos
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Current Limits on Lepton Flavor Violating
Processes
Branching Fraction Limit Lower limit on
MX 150 TeV/c2 31
TeV/c2 37 TeV/c2
Mass limit assumes electroweak coupling strength
K decays changeboth quark flavorand lepton
flavor.
86 TeV/c2
m decays changeonly lepton flavor.
21 TeV/c2
365 TeV/c2
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History of Lepton Flavor Violation Searches
1
?- N ? e-N ? ? e? ? ? e e e-
10-2
10-4
10-6
10-8
MEGA
10-10
E871
10-12
K0?? ?e- K?? ? ?e-
SINDRUM2
PSI-MEG Goal ?
10-14
10-16
MECO Goal ?
1940 1950 1960 1970
1980 1990 2000 2010
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PSI-MEG m?eg Experiment
Search for m?e g with sensitivity of 1 event
for B(m?e g) 10-14
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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)

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The PSI-MEG Apparatus
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Calculation of m ? eg Accidental Backgrounds
Signal for B(m?eg) 10-13

• Backgrounds calculated with Gaussian resolution
  functions with conservatively chosen widths
• About 0.5 background events expected for a
  sensitivity of 1 event for a branching fraction
  of 10-14

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Muon to Electron COnversion (MECO) Experiment

• Boston University
• J. Miller, B. L. Roberts, O. Rind
• Brookhaven National Laboratory
• K. Brown, M. Brennan, G. GreeneL. 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, B. W. Mayes, L. S.
  Pinsky, J. Wilson
• University of Massachusetts, Amherst
• K. Kumar

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
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What is Coherent Muon-to-Electron Conversion
(?-N?e-N) ?

• Muons are stopped in matter, losing energy by
  ionization, and form a muonic atom.
• They cascade down to the 1S state in less than
  10-16 s.
• They coherently interact with a nucleus and
  convert to an electron without emitting
  neutrinos. The coherence increases the rate by a
  factor of Z with respect to processes like
  inverse ? decay. The conversion electrons have
  energy nearly equal to the muon mass momentum is
  conserved by nuclear recoil with nearly
  negligible recoil energy.
• More often, they are captured on the nucleus
  or decay in the Coulomb bound orbit
  (?? 2.2 ?s in vacuum, 0.9 ?s in
  Al)
• Rate is normalized to the kinematically similar
  weak capture process
• MECO goal is to detect ?-N?e-N if R?e is at least
  2 X 10-17 ,
• with one event providing compelling evidence of a
  discovery.

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How Big a Number is 1017?

• The number of stars within 500 million light
  years of the earth
• The ratio of the surface area of North America
  to that of a postage stamp
• The number of cents in 500 years of the federal
  budget (about 80 years with inflation)

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The First ?-N ? e-N Experiment 1955

• After the discovery of the muon, it was realized
  it could decay into an electron and a photon or
  convert to an electron in the field of a nucleus.
• Without any flavor conservation, the expected
  branching fraction for ??e? is about 10-5.
• Steinberger and Wolf looked for ?-N ? e-N for
  the first time, publishing a null result in
  1955, with a limit R?e lt 2 ? 10-4

Absorbs e- from ?- decay
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Conversion e- reach this counter
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What Drives the Design of the Experiment?
Considerations of potential sources of fake
signals specify much of the design of the beam
and experimental apparatus.
Look at data from the SINDRUM2 experiment, which
currently has thebest limit on this process
Cosmic raybackground
Prompt background
Expected signal
Experimental signature is105 MeV e- originating
ina thin stopping target
Muon decay in orbit
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The Basis of the Experimental Technique

• Produce a very intense beam of low energy muons
  (1000x other beams)
• 1011 muons per second
• 107 seconds
• 10 detection efficiency
• Transport the muons to a thin target and stop
  them
• Reduce flux of unwanted particles
• Minimize thickness of stopping target
• Detect conversion electrons with high efficiency
• Must operate in very high rate environment (1011
  m decays or captures per second)
• Must distinguish conversion electrons from other
  electrons
• Understand and eliminate other potential sources
  of electrons so that a single event is strong
  evidence for a signal.

?
1 detected event if R?e 2 ? 10-17
The conversion electron has fixed energy
Ee m? c2 Ebinding Erecoil
105.6 0.25
0.25 MeV
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Potential Sources of Background

• Muon Decay in Orbit
• Emax Econversion when neutrinos have zero
  energy
• dN/dEe ? (Emax Ee)5
• Sets the scale for energy resolution required
  200 keV
• Radiative Muon Capture ?- N ? ?? N(Z-1) ?
• For Al, Egmax 102.5 MeV/c2, P(Eg gt 100.5
  MeV/c2) 4 ? 10-9
• P(g ? ee-, Ee gt 100.5 MeV/c2) 2.5 ? 10-5
• Restricts choice of stopping targets Mz-1 gt Mz
• Radiative Pion Capture
• Branching fraction 1.2 for Eg gt 105 MeV/c2
• P(g ? ee-, 103.5 lt Ee-lt 100.5 MeV/c2) 3.5 ?
  10-5
• Limits allowed pion contamination in beam during
  detection time

Muon decay in vacuum -- Ee lt m?c2/2
Muon decay in bound orbit -- Ee lt m?c2 -
ENR - EB
Plus many others cosmic rays, anti-proton
interactions
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Features of the MECO Experiment

• 1000 fold increase in muon intensity
• Graded solenoidal field to maximize pion capture
• Produce ?10-2 m-/p at 8 GeV (SINDRUM2 ?10-8,
  MELC ?10-4, Muon Collider ?0.3)
• Muon transport in curved solenoid
• suppress high momentum negatives and all
  positives and neutrals
• Pulsed beam to eliminate prompt backgrounds
• Beam pulse duration ltlt tm
• Pulse separation ? tm
• Large duty cycle (50)
• Extinction between pulses lt 10-9
• Improved Detector Resolution and Rate Capability
• Detector in graded solenoid field
• Improved acceptance
• Improved rate handling
• Improved background rejection
• Very high resolution spectrometer

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MECO at Brookhaven National Laboratory
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Pulsed Proton Beam from AGS for MECO

• Accelerate 4?1013 protons each second to 8 GeV
  50 kW beam power.
• Revolution time in the AGS is 2.7 ms protons
  are accelerated in 2 RF buckets separated by 1.35
  ms.
• Resonant extraction of bunched beam
• To eliminate prompt backgrounds, we require lt
  10-9 protons between bunches for each proton in
  bunch. We call this the beam extinction.

Quiet detection time
Promptbackgrounds
Proton pulse
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The MECO Apparatus
Straw Tracker
Muon Stopping Target
Muon Beam Stop
Superconducting Transport Solenoid
(2.5 T 2.1 T)
Crystal Calorimeter
Superconducting Detector Solenoid (2.0 T
1.0 T)
Superconducting Production Solenoid (5.0
T 2.5 T)
Muon Production Target
Collimators
Proton Beam
Heat Radiation Shield
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MIT Plasma Science and Fusion Center Conceptual
Design of MECO Magnet System
5 T
2.5 T

• Very detailed CDR completed (300 pages)
• Complete 3D drawing package prepared
• TS and SOW for commercial procurement developed
• Industrial studies contracts let

1 T
2 T
1 T

• 150 MJ stored energy
• 5T maximum field
• Uses surplus SSC cable
• Can be built in industry

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Muons Production and Capture in Graded Magnetic
Field

• Pions produced in target in axially graded
  magnetic field
• 50 kW beam incident on W target
• Charged particles are trapped in 5 2.5 T,
  axial magnetic field
• Axially graded field reflects pions and muons
  moving away from the experiment
• Superconducting magnet is protected by Cu and W
  heat and radiation shield

150 W load on cold mass15 ?W/g on
superconductor20 Mrad integrated dose
Superconducting coil
2.5T
5T
Azimuthal position
Productiontarget
Heat Shield
Axial position
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The MECO Apparatus
Straw Tracker
Muon Stopping Target
Muon Beam Stop
Superconducting Transport Solenoid
(2.5 T 2.1 T)
Crystal Calorimeter
Superconducting Detector Solenoid (2.0 T
1.0 T)
Superconducting Production Solenoid (5.0
T 2.5 T)
Muon Production Target
Collimators
Proton Beam
Heat Radiation Shield
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Muon Beam Transport with Curved Solenoid

• Curved sections eliminate line of site transport
  of photons and neutrons.
• Toroidal sections causes particles to drift out
  of planeused to sign and momentum select beam.
• dB/dS lt 0 to avoid reflections

2.5T
2.4T

• Goals
• Transport low energy m-to the detector solenoid
• Minimize transport of positive particles and
  high energy particles
• Minimize transport of neutral particles
• Absorb anti-protons in a thin window
• Minimize long transittime trajectories

2.4T
2.1T
2.1T
2.0T
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Sign and Momentum Selection in the Curved
Transport Solenoid
Transport in a section of a torus results in
particle charge and momentum selection positive
particles and low momentum particles absorbed in
collimators.
Detection Time
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The MECO Apparatus
Straw Tracker
Muon Stopping Target
Muon Beam Stop
Superconducting Transport Solenoid
(2.5 T 2.1 T)
Crystal Calorimeter
Superconducting Detector Solenoid (2.0 T
1.0 T)
Superconducting Production Solenoid (5.0
T 2.5 T)
Muon Production Target
Collimators
Proton Beam
Heat Radiation Shield
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Stopping Target and Experiment in Detector
Solenoid

• Graded field in front section to increase
  acceptance and reduce cosmic ray background
• Uniform field in spectrometer region to minimize
  corrections in momentum analysis
• Tracking detector downstreamof target to reduce
  rates

1T
Electron Calorimeter
1T
Tracking Detector
2T
Stopping Target 17 layers of 0.2 mm Al
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Magnetic Spectrometer for Conversion Electron
Momentum Measurement

• Measures electron momentum with precision of
  about 0.3 (RMS) essential to eliminate muon
  decay in orbit background

Electron starts upstream, reflects in field
gradient

• Must operate in vacuum and in high rate
  environment 500 kHz rates in individual
  detector elements.
• Implemented in straw tube detectors
• 2800 nearly axial detectors, 2.6 m long, 5 mm
  diameter,0.025 mm wall thickness minimum
  material to reduce scattering
• position resolution of 0.2 mm in transverse
  direction, 1.5 mm in axial direction

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Spectrometer Performance Calculations

• Performance calculated using Monte Carlo
  simulation of all physical effects
• Resolution dominated by multiple scattering in
  tracker
• Resolution function of spectrometer convolved
  with theoretical calculation of muon decay in
  orbit to get expected background.
• Prototypes demonstrate required resolution

10
1.0
?
0.1
FWHM 900 keV
0.01
103 104 105
106
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Expected Sensitivity of the MECO Experiment

• MECO expects 5 signal events for 107 s (2800
  hours) running if Rme 10-16

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Expected Background in MECO Experiment

• MECO expects 0.45 background events for 107 s
  with 5 signal events
  for Rme 10-16

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Current Status of MECO Approval, Review and
Funding

• Scientific approval status
• Approved by Brookhaven National Laboratory
• Approved by the National Science Foundation
  through the level of the Director
• Approved by the National Science Board for a
  Major Research Equipment Grant
• Endorsed by the HEPAP Subpanel charged with
  studying the long term scientific goals of the
  particle physics community
• Technical and management review status
• Positively reviewed by multiple NSF and
  Laboratory appointed panels
• Some pieces (primarily magnet system) positively
  reviewed by external expert committees appointed
  by MECO leadership
• Funding status
• MECO is currently operating on three RD grants
  for a total of 2.6M
• A fourth RD grant is pending, awaiting report
  from an NSF panel review.
• We expect significant funding (5M) in FY04,
  currently in the House appropriations bill. We
  expect a formal project start in FY05. We work
  actively with the NSF and with Congress to help
  make that happen.

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Summary and Prospects

• Despite 50 years of experiments, no evidence for
  muon and electron number violation has been found
  in charged lepton interactions.
• Discovery of neutrino oscillations adds
  motivation for improved experiments.
• Theoretical motivation has increased recently,
  particularly in grand unified supersymmetric
  scenarios.
• New ideas for much improved muon beams and
  experiments promise very large improvement in
  sensitivity.

The large expected increase in experimental
sensitivity and the expectations from many models
provide optimism for making a significant
discovery in the next few years.
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Frederick Reines Hall
Home of the Department of Physics Astronomy
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Located 5 miles from the Pacific Ocean
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END