Title: Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation
1Physics 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
2Outline
- 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
3What 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
?
4Sensitivity 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
5Supersymmetry 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
6Rates 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
7What 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
8Possible 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)
2s limit
Carvalho, Ellis, Gomez, Lola (since superseded)
9Current Limits on Muon Number Violating Processes
Mass limit
?G0
?G1
10History 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
12Coherent 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))
13The 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
15What 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.
16Potential 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
17Other 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
18Features 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)
19Pulsed 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
20Producing 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
21The 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
22MIT Plasma Science and Fusion Center Conceptual
Design of MECO Magnet System
5 T
2.5 T
1 T
2 T
1 T
23Muons 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
24Production 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
25Muon 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
26Sign 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
27Muon 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
28Stopping 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
29Magnetic 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
30Spectrometer 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
?
31Tracking 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
32Scintillating 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
33Expected Sensitivity of the MECO Experiment
34Expected 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.
35Current 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
36PSI-MEG m?eg Experiment
Search for m?e g with sensitivity of 1 event
for B(m?e g) 10-14
37Principal 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)
38Overview 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
39Calculation 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
40Producing 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
41Muon Beam Transport System
42The PSI-MEG Apparatus
4353 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
44Photon 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
45Small 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
46Small 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
47Large 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
48Positron Detection and Energy Measurement
COBRA spectrometer
- Thin superconducting magnet with gradient
magnetic field - Drift chamber for positron tracking
- Scintillation counters for timing measurement
49Principle 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
50PSI-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
51Drift 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
52Positron 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
53Status 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
54Comments 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.
55Excerpts 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.
56MECO at Brookhaven National Laboratory