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Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation

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Title: Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation


1
Physics with Very Intense Muon BeamsSearch for
Lepton Flavor Violation
W. MolzonUniversity of California, Irvine April
8, 2002
  • 20th ICFA Advanced Beam Dynamics WorkshopHigh
    Intensity High Brightness Hadron Beams
  • Fermilab

2
Outline
  • Motivation for improved lepton flavor violation
    searches
  • Current experimental status
  • Prospects for improved searches
  • PSI-MEG ??e? experiment
  • MECO ?-N? e-N experiment
  • Comments on possible improvements beyond
    currently approved experiments

3
What Will Observation of ?-N ? e-N or ? ? e g
Teach Us?
  • Discovery of ?-N ? e-N or a similar charged
    lepton flavor violating (LFV) process will be
    unambiguous evidence for physics beyond the
    Standard Model.
  • For non-degenerate neutrino masses, n
    oscillations can occur. Discovery of neutrino
    oscillations required changing the Standard Model
    to include massive ?.
  • Charged LFV processes occur through intermediate
    states with n mixing. Small n mass differences
    and mixing angles ? expected rate is well below
    what is experimentally accessible.
  • Charged LFV processes occur in nearly all
    scenarios for physics beyond the SM, in many
    scenarios at a level that MECO or PSIMEG will
    detect.
  • Effective mass reach of sensitivesearches is
    enormous, well beyondthat accessible with direct
    searches.

One example of new physics, with leptoquarks
lmd
led
?
4
Sensitivity to Different Muon Conversion
Mechanisms
Supersymmetry
Compositeness
Predictions at 10-15
Second Higgs doublet
Heavy Neutrinos
Heavy Z, Anomalous Z coupling
Leptoquarks
After W. Marciano
5
Supersymmetry Predictions for LFV Processes
  • From Hall and Barbieri
  • Large t quark Yukawa couplingsimply observable
    levels of LFV insupersymmetric grand unified
    models
  • Extent of lepton flavor violation in grand
    unified supersymmetry related to quark mixing
  • Original ideas extended by Hisano, et al.

Current MEGA bound
Current SINDRUM2 bound
B(? ? e g)
R?e
PSI-MEG single event sensitivity
MECO single event sensitivity
100 200
300 100
200 300
6
Rates for LFV Processes Linked to n Oscillations
MEGA Bound
MSW large angle
PSI-MEG goal
MSW small angle
Possible solutions to solar n oscillations
MECO goal
Just so
From the model of J. Hisano and D. Nomura, Phys.
Rev. D59 (1999) SU(5) grand unified model with
heavy, right-handed neutrinos
7
What Does Muon g-2 Say About ?-N ? e-N and ? ? e
g ?
  • Same SUSY loop diagrams contribute to both.
  • ?-N ? e-N also requires LFV in SUSY sector
  • g-2 sets scale for chirality-changing loop
    contributions are effective L-
    and R-handed couplings for loop contributions.
  • Reported 2.6s discrepancy with theory raised
    possibility that supersymmetric contributions
    might be large.
  • ? ? e g (?-N ? e-N) also have loop contributions,
    with couplings defined similarly.
  • Going beyond this requires an assumption about
    the LFV mechanism.
  • ?-N ? e-N may also have contributions from
    chirality-conserving diagrams that do not
    contribute to g-2

8
Possible Constraints on m0 and m1/2 From g-2
  • With 2.6s discrepancy in g-2
  • m gt 0 is preferred
  • With one choice for LFV texture, very large ?-N
    ? e-N rates would be expected
  • Taking the difference as a measurement, limits on
    allowed m0 , m1/2 space can be derived
  • With correction to error in calculation of
    hadronic correction, discrepancy is 1.6s and
    allowed region is very large
  • Uncertainty in higher order corrections may not
    allow confrontation between experiment and
    Standard Model (Wise and Ramsey-Musolf)
  • 10-14
  • 10-17

2s limit
Carvalho, Ellis, Gomez, Lola (since superseded)
9
Current Limits on Muon Number Violating Processes
Mass limit
?G0
?G1
10
History of Lepton Flavor Violation Searches
1
?- N ? e-N ? ? e? ? ? e e e-
10-2
10-4
10-6
Branching Fraction Upper Limit
10-8
MEGA
10-10
SINDRUM2
10-12
K0?? ?e- K?? ? ?e-
PSI-MEG goal
10-14
MECO goal
10-16
1940 1950 1960 1970 1980
1990 2000 2010
11
?-N ? e-N vs. m?e g as Probes of LFV
  • ?-N ? e-N is more sensitive for essentially all
    processes not mediated by photon
  • ?-N ? e-N is more sensitive than is ??e g to
    chirality conserving processes
  • ??e g is more sensitive for processes mediated by
    photons
  • B(??e ?) ? 300 ? Rme for these processes
  • The motivation is sufficiently strong that both
    experiments should be done
  • Relative rates for ??e g and ?-N ? e-N would give
    information on underlying mechanism
  • A significant rate for ??e g with polarized muons
    could give additional information on mechanism
  • Experimental considerations are different
  • ??e g is rate-limited due to accidental physics
    backgrounds physics (correctly measured e and g)
    at about 10-14 level
  • ?-N ? e-N is rate-limited only in the sense that
    high detector rates will contaminate events and
    cause measurement errors

12
Coherent Conversion of Muon to Electrons
(?-N?e-N) ?
  • Muons stop in matter and form a muonic atom.
  • They cascade down to the 1S state in less than
    10-16 s.
  • They coherently interact with a nucleus (leaving
    the nucleus in its ground state) and convert to
    an electron, without emitting neutrinos ? Ee
    Mm - ENR - EB.
  • 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.

R?e ? ?(?-N?e-N) / ?(?-N???N(Z-1))
13
The First ?-N ? e-N Experiment Steinberger and
Wolf
  • 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
9
Conversion e- reach this counter
14
MECO Collaboration
  • Boston University
  • J. Miller, B. L. Roberts, O. Rind
  • Brookhaven National Laboratory
  • K. Brown, M. Brennan, 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, B.
    W. Mayes, L. S. Pinsky, J. Wilson
  • University of Massachusetts, Amherst
  • K. Kumar

Institute for Nuclear Research, Moscow V. M.
Lobashev, V. Matushka, A. N.
Toropin 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
15
What Drives the Design of the MECO Experiment?
Considerations of potential sources of fake
backgrounds specify much of the design of the
beam and experimental apparatus.
Cosmic raybackground
Prompt background
SINDRUM2 currently has thebest limit on this
process
Expected signal
Muon decay
Experimental signature is105 MeV e- originating
in a thin stopping target.
16
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 Eelt 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
17
Other Potential Sources of Backgrounds
  • Muon decay in flight e- scattering in
    stopping target
  • Beam e- scattering in stopping target
  • Limits allowed electron flux in beam
  • Antiproton induced e-
  • Annihilation in stopping target or beamline
  • Requires thin absorber to stop antiprotons in
    transport line
  • Cosmic ray induced e- seen in earlier
    experiments
  • Primarily muon decay and interactions
  • Scales with running time, not beam luminosity
  • Requires the addition of active and passive
    shielding

18
Features of the MECO Experiment
  • 1000 fold increase in muon intensity using an
    idea from MELC at MMF
  • High Z target for improved pion production
  • 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 suppressing
    high momentum negatives and all positives and
    neutrals (new for MECO)
  • Pulsed beam to eliminate prompt backgrounds
    following PSI method (A. Badertscher, et al.
    1981)
  • 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 for improved
    acceptance, rate handling, background rejection
    following MELC concept
  • Spectrometer with nearly axial components and
    very high resolution (new for MECO)

19
Pulsed Proton Beam from AGS for MECO
Promptbackgrounds
Proton pulse
  • Machine will operate at 8 GeV with 4?1013 protons
    per second 50 kW beam power.
  • Cycle time of 1.0 s with 50 duty factor
  • Revolution time 2.7 ms with 6 RF buckets in
    which protons can be trapped and accelerated
  • Fill 2 RF buckets for 1.35 ms pulse spacing
  • 2 ?1013 protons / RF bucket - twice current bunch
    intensity
  • 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.

Detection time
20
Producing Pulsed beam with 10-9 Extinction
  • Extinction Measurements
  • Initial test at 24 GeV with one RF bucket yielded
    lt10-6 extinction between buckets and 10-3 in
    unfilled buckets
  • A second test at 7.4 GeV with a single filled
    bucket found lt10-7 extinction
  • Multiple means of improving extinction
  • AGS internal cleanup with 40 kHz AC dipole and
    fast kicker magnets (field shown inverted).
  • External cleanup and extinction monitoring with
    RF modulatedmagnet and septum magnets

21
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
22
MIT Plasma Science and Fusion Center Conceptual
Design of MECO Magnet System
5 T
2.5 T
1 T
2 T
1 T
23
Muons Production and Capture in Graded Magnetic
Field
  • Pions produced in a target located in an axially
    graded magnetic field
  • 50 kW beam incident on W target
  • Charged particles are trapped in 5 2.5 T,
    axial magnetic field
  • Pions and muons moving away from the experiment
    are reflected
  • Superconducting magnet is protected byCu and W
    heat and radiation shield

150 W load on cold mass15 ?W/g in
superconductor20 Mrad integrated dose
Superconducting coil
mW/gm in coil
2.5T
5T
Azimuthal position
Productiontarget
Heat Shield
Axial position
24
Production Target for Large Muon Yield
  • Production target region designed for high yield
    of low energy muons
  • High Z target material
  • No extraneous material in bore to absorb p/m
  • Cylinder with diameter 0.8 mm, length 160 mm
  • 5 kW of deposited energy
  • Two target configurations being considered
  • Radiation cooled tungsten
  • Requires high emissivity coating
  • Segmented into 40 disks
  • Maximum temperature is 2100 K
  • Well below melting point, low evaporation
  • Thermally induced stresses near yield stress
  • Requires a uniform proton beam profile on the
    target
  • Water cooling in narrow channels in target
  • Required flow rate is reasonable
  • Temperature rise is below 100 K

Temperature distribution in target segment
25
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
26
Sign and Momentum Selection in the Curved
Transport Solenoid
Transport in a torus results in charge and
momentum selection positive particles and low
momentum particles absorbed in collimators.
Detection Time
Relative particle flux
Relative particle rate in mbunch
27
Muon Beam Studies
  • Muon flux estimated with Monte Carlo calculation
    including models of p- production and simulation
    of decays, interactions and magnetic transport.
  • Estimates scaled to measured pion production on
    similar targets at similar energy
  • Expected yield is about 0.0025 ?- stops per
    proton

Stopping Flux
Total flux at stopping target
Relative yield
Data Model
0 50
100
0 0.5
1.0
Muon Momentum MeV/c
Pion Kinetic Energy GeV
28
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
29
Magnetic Spectrometer to Measure Electron Momentum
  • 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

30
Spectrometer Performance Calculations
  • Performance calculated using Monte Carlo
    simulation of all physical effects
  • Resolution dominated by multiple scattering in
    tracker and energy loss in target
  • Resolution function of spectrometer convolved
    with theoretical calculation of muon decay in
    orbit to get expected background.

FWHM 850 keV
?
31
Tracking Performance Potentially Compromised by
High Rates
  • There are about 1011 muon decays and muon
    captures per second
  • electrons from ?- decay mostly trapped at small
    radius in the detectorsolenoid and dont reach
    detectors
  • 1011 neutrons and 1011 photons from nuclear
    de-excitation following muon capture can
    interact in detectors
  • 1010 protons per second produced from muon
    capture
  • potentially serious background from low energy
    electrons contaminated with noise signals
    producing fake signal events

real e- from ?- decay
fake e- trajectory
32
Scintillating Crystal Absorption Calorimeter
  • Provides prompt signal proportional to electron
    energy for use in online event selection
  • Provides position measurement to confirm electron
    trajectory
  • Provides energy measurement to 5 to confirm
    electron momentum measurement
  • Consists of 2000 3 cm x 3 cm x 12 cm (PbWO4 or
    BGO) crystals

33
Expected Sensitivity of the MECO Experiment

34
Expected Background in MECO Experiment
Background calculated for 107 s running time at
intensity yielding 1 signal event for Rme 2 ?
10-17.
  • Sources of background will be determined directly
    from data.

35
Current Status of MECO Approval, Review, Funding,
Schedule
  • Scientific approval status
  • 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 review status
  • Positively reviewed by many NSF and Laboratory
    appointed panels
  • Some pieces (primarily magnet system) positively
    reviewed by external expert committees appointed
    by MECO leadership
  • Funding status
  • Currently operating on 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 MRE funding
  • Construction schedule
  • Construction schedule driven by superconducting
    solenoids estimate from the MIT Conceptual
    Design Study is 41 months from signing of
    contract for engineering design and construction
    until magnets are installed and tested

More information at http//meco.ps.uci.edu
36
PSI-MEG m?eg Experiment
Search for m?e g with sensitivity of 1 event
for B(m?e g) 10-14
37
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)

38
Overview of the PSI-MEG Experiment
  • Getting desired sensitivity is a tradeoff between
    increased rate (and background) and increased
    acceptance (typically giving lower
    resolution)PSI-MEG has optimized these
    tradeoffs with following parameters
  • Rm 1 x 108 Hz
  • T 2.2 x 107sec
  • Solid angle W/4p 0.09
  • Acceptances eg 0.7, ee 0.95 esel 0.8
  • Reducing accidental background to less than 1
    event requires resolution functions that are
    Gaussian with widths given below

One event for B(m?eg) 0.94 x 10-14
39
Calculation of m ? eg Accidental Backgrounds
Signal for B(m?eg) 10-13
  • Backgrounds calculatedwith Gaussian
    resolutionfunctions with conservatively chosen
    widths.
  • 0.5 background events expected

40
Producing the Required m Beam
m from p decay at rest
Fluxes of p and m at pE5
  • Paul Scherrer Institute590 MeV proton cyclotron
  • Operating current 1.8 mA (Max gt 2.0 mA) 1.2 MW
  • DC muon beam rate above 108 m/s at pE5 beam line

41
Muon Beam Transport System
42
The PSI-MEG Apparatus
43
53 MeV Photon Detection
  • Detector requirements
  • Energy resolution 0.6 0.9 (sRMS)
  • Timing resolution 65 psec (sRMS)
  • Position resolutions 1.0 mm (sRMS)
  • Ability to operate in high rate environment

?
Liquid xenon scintillation detector
  • Detector design
  • Active volume of LXe 600 liter
  • Scintillation light collected by 800 PMTs
    immersed in LXe
  • Effective photo-cathode coverage 35
  • Detector outside magnet

44
Photon Detector Based on Liquid Xenon Scintillator
  • High light yield (75 of NaI(Tl))
  • Fast signals to avoid accidental pileup
  • Spatially uniform response - no need for
    segmentation

45
Small Prototype of Liquid Xenon Calorimeter
  • 32 x PMTs
  • Active Xe volume
  • 116 x 116 x 174 mm3 (2.3 liter)
  • Energy, position and timing resolutionfor g up
    to 2 MeV

46
Small Prototype Test Results
  • Tests done
  • At low rate
  • With small detector
  • At low energy

Simple extrapolation to 53 MeV implies
senergy 0.8 (0.6-0.9) sposition a few mm
st 50 psec
47
Large Prototype Photon Detector Tests
  • Purpose
  • Measure resolutions with high energy g
  • Check of cryogenics and other detector components
  • Measure absorption length
  • Construction
  • 228 PMTs, 69 liter LXe

1 m
  • Results
  • Cryogenic performance is good
  • First test with 40 MeV g beam completed
  • Analysis in progress

48
Positron Detection and Energy Measurement
COBRA spectrometer
  • Thin superconducting magnet with gradient
    magnetic field
  • Drift chamber for positron tracking
  • Scintillation counters for timing measurement

49
Principle of the COBRA Spectrometer
  • Bending radius independent of emission angles

Gradient field
Uniform field
  • Low energy positrons quickly swept out

Uniform field
Gradient field
e from m?eg
50
PSI-MUEG Superconducting Spectrometer Magnet
  • Bc 1.26T, Bz1.25m0.49T, operating current
    359A
  • Five coils with three different diameters to
    realize gradient field
  • Compensation coils to suppress the residual
    field around the LXe detector
  • High-strength aluminum-stabilized superconductor
    ? very thin coil (3 g/cm2)
  • Design completed, conductor produced

51
Drift Chambers for Positron Energy Measurement
  • 17 chamber sectors alignedradially with
    10intervals
  • Two staggered layersof drift cells
  • He-C2H6 chamber gas to reduce multiple
    scattering
  • Vernier cathode pad pattern to determine
    z-position
  • Prototypes tested

52
Positron Timing Counter
  • Two orthogonal layers of scintillator bars
  • Outer timing measurement
  • Inner additional trigger information
  • Goal stime 50 psec CR test of prototype
    gives about 60 psec

53
Status and Schedule for PSI-MEG Experiment
  • Preparations of all parts of the experiment are
    going well
  • Next major milestone is test of large LXe
    calorimeter this year
  • Expect to begin taking data late 2004
  • More information at

http//meg.psi.ch
http//meg.icepp.s.u-tokyo.ac.jp
54
Comments on Improvements Beyond MECO and PSI-MEG
  • Improvements to m ? eg
  • Improved resolutions will be very difficult
  • PSI-MEG uses DC beam (no further improvements in
    duty cycle)
  • Kuno and Okada have suggested using polarized
    muons to reduce backgrounds very high
    polarization needed
  • Factor of a few in acceptance may be possible
  • Muon flux is probably not an issue
  • Improvements to ?-N?e-N
  • Pulsed beam will probably remain a requirement
  • Higher muon flux will be needed
  • Detector rates (instantaneous intensity) per muon
    capture must be improved
  • Some ideas from JHF/PRISM to make very clean,
    nearly monoenergetic beam
  • Possibility of shielding detectors from neutrons
    and photons discussed
  • Improved electron energy resolution will be
    needed
  • Current round of experiments will be very helpful
    in understanding where improvements are needed
    and can be made.

55
Excerpts from Letter from NSF AD Eisenstein to
John Sculli
Letter from Robert Eisenstein, NSF Assistant
Director for Mathematical and Physical Sciences
to John Sculli, PI for RSVP, sent January 29.
First, the initial proposal was peer reviewed
and found to have outstanding prospects for major
advances in our understanding of some of the most
important questions in fundamental particle
physics. Second, RSVP MECOKOPIO was
critically reviewed internally at NSF by senior
management from all of the NSF Directorates,
resulting in broad, strong support for going
forward. Third, RSVP was selected by the NSF
Director as a project whose scientific goals and
readiness warranted consideration by the National
Science Board. Fourth, the National Science Board
reviewed the case for RSVP and approved it for
inclusion in the FY2002 or later budget. This
is the current status of RSVP in the funding
process. It represents a strong showing by RSVP
in an extremely competitive process between
different fields of science, and the rate of
advancement is faster than any other major
construction project that I can remember. I
can say that RSVP is now the highest priority
construction project from the Directorate of
Mathematical and Physical Sciences The
Division of Physics is continuing to fund RD to
advance the project design and planning.
56
MECO at Brookhaven National Laboratory
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