Transcript Mu2e: A Muon to Electron Conversion Experiment at FERMILAB

Mu2e: A Muon to Electron
Conversion Experiment at
FERMILAB
James Miller, Boston University
FNAL, November 10, 2006
The Process: m-->e- Conversion
•
•
•
•
•
•
•
m- are stopped in a target and form hydrogenic atoms (FNAL-Mu2e: muonic aluminum)
Main processes for muon in atomic orbit:: neutrino-less m->e conversion, capture, decay
Measure ratio: (conversion rate) / (ordinary capture rate):
– Rme={Rate(m-+A(N,Z) ->e-+A(N,Z)} / {Rate(m-+A(N,Z) -> m+X}
Lm=+1,Le=0 -> Lm=0,Le=+1
– The electron is mono-energetic, Ee~105 MeV: most background is from muon
decay electrons which are dominantly below 55 MeV
– Lifetime of muonic Al is 0.88 ms
Current limit, from SINDRUM II experiment at PSI: R<6.1x10-13(on Ti)
Mu2e goal at FNAL: R<1x10-16
SINDRUM II was limited by
– Available muon fluxes, <108/s
– Prompt background, mainly from radiative pion capture in the Al target:
p- +A-> X +g,Egup to ~139MeV, g can convert to electron with Ee~105 MeV
Must be highly suppressed: used beam + electron coincidences and degrader
Mu2e: use an intense pulsed beam (ala MECO); no beam counters or degrader
– Inject intense bunches of narrow (<~100-200 ns), low p (<70 MeV/c) muon beams.
Stop them in an aluminum target
– Begin data collection ~700 ns after injection; prompts (mainly pions) have subsided
– Collect data from 700 ns to ~ 1.5 ms (matches muonic Al lifetime of 0.88 ms)
– Inject pulses every ~1.5 ms (total of 1010-1011 muons per second)
– Suppress protons by factor of 109 between injections (‘Extinction’) to avoid prompts
History of Lepton Flavor Violation Searches
1
m- N  e-N
10-2
m+  e+g
m+  e+ e+ e-
10-4
10-6
10-8
10-10
10-12
K0  m+eK+  p+ m+e-
10-14
SINDRUMI
I
(m->e)
FNAL m->e Goal 
10-16
1940
MEG Goal(m->eg) 
1950
1960
1970
1980
1990
2000
2010
SINDRUM II m-A e-A Limit
Experience of SINDRUM II carried over to the design of beam and
SINDRUM II has the
experimental apparatus
Prompt
Backgroun
d
Cosmic Ray
Background
Expected
Signal
Muon Decay
in Orbit
Experimental signature is 104 MeV eoriginating in a thin Ti stopping target
best limit on this process
Rme<6.1x10-13 (in Ti)
Magnet: B=1.2 T,1.3 m dia., 1.5 m long
Candidate Approach for an FNAL-Based
Experiment: MECO Apparatus
1T
1T
2T
Muon
Beam
Superconducting
Solenoids
Calorimeter
Straw Tracker
Stopping
Target Foils
Detector Solenoid
10 m long x 0.95 m rad
Proton Beam
2.5 T
Transport Solenoid
13 m long x 0.25 m rad
5T
Pion Production
Target
Production Solenoid
4m long x 0.75 m rad
Magnetic Spectrometer for Conversion Electron
Momentum Measurement
Sample event- this one first travels upstream, is reflected by B
gradient back toward detector
Electron starts
upstream,
reflects in field
gradient
Shown: Longitudinal straw option
Straws: 2.6 m length  5mm dia., 25 mm wall thickness
to minimize multiple scattering – 2800 total
tracker will intercept between 2 and 3 helical turns
FNAL-Mu2e beam rates and sensitivity
Probability
Rate (proton
flux=1x1013 Hz)
Events (run time=
4x107 seconds, total
p’s =4x1020)
Prob. Muon stopped in
target per proton
0.0025
0.25x1011 Hz
1x1018
Prob. m capture (Al target)
0.60
1.5x1010 Hz
6x1017
Fraction of captures in
detector time window
0.45
(> 700 ns )
0.7x1010 Hz
3x1017
Track fitting and selection
criteria
0.19
1.2x109 Hz
5x1016
Detected events for
Rme=10-16
10-16
1.2x10-7 Hz
5 -> single-event
sensitivity 2x10-17
Estimated background
0.45
Some History: The BNL-Based Experiment
(MECO)
• MECO: one of two experiments on the cancelled NSF- RSVP project
(along with KOPIO,
)
• Cost of MECO by itself was projected at $56 M for the muon
solenoidal beam line magnets + $24 M for the detector system
• Project was not funded mainly because the cost to configure and run
the AGS, plus to build both experiments and then decommission
was deemed to be too large due cost escalations- however MECO
was not the source of most of the cost escalation.
• Advanced conceptual magnet design was produced
• Initial prototypes and concepts of detector systems were produced
• MECO successfully passed through several physics and technical
reviews
Mu2e Personnel
• Significant fraction of ex-MECO collaboration
• Group at FNAL which has spent spare time investigating
how to supply the proton beam and to site at FNAL
• A number of other physicists attended the Sep 15-16
meeting of interested parties at FNAL- total of ~50
attendees
Summary of Beam Requirements
•
•
•
•
•
•
Pulsed ~4-8 GeV proton beam
Width < 100-200 ns (Narrower is better, but not critical)
Repetition Period ~1.5 ms
Proton Intensity > 1013 Hz
Integrated proton flux=4x1020
Beam Extinction <10-9
What Will Observation of m-N  e-N Teach Us?
Discovery of m-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,  oscillations can occur. Discovery of
neutrino oscillations required changing the Standard Model to include massive .
• Charged LFV processes occur through intermediate states
m
with  mixing. Small  mass differences and mixing angles 
expected rate is well below what is experimentally accessible.

W
• Charged LFV processes occur in nearly all scenarios for physics beyond the SM,
in many scenarios at a level that Mu2e will detect.
• Effective mass reach of sensitive
searches is enormous: well beyond
that accessible with direct searches.
R me
Γ(μ-N  e-N)
-16


10
Γ(μ-N  ν μN)
 ML  3000 lmd led TeV/c 2
One example of new
physics, with leptoquarks
lmd
led
e
Sensitivity to Different Muon Conversion
Mechanisms
Supersymmetry
Compositeness
Predictions at 10-15
Λ C = 3000 TeV
Second Higgs
doublet
gHμe =10-4 ×gHμμ
Heavy Neutrinos
2
*
UμN
UeN = 8 ×10-13
Heavy Z’,
Anomalous Z
coupling
Leptoquarks
ML =
MZ = 3000 TeV/c 2
3000 λμd λ ed TeV/c 2
After W. Marciano
B(Z  μe) < 10-17
The MECO Beam and Detector
A number of design parameters are
based on MELC proposal of 1980’s
(T)
B (Tesla) vs. s along beam
5
3
1
m
0
10
20

30
13 m x 0.25 m rad
4m x 0.75 m rad

.
10 m x 0.95 m rad
1
1
1
Outline
• Brief descriptions of the m- -> e- process and the current and
proposed experimental limits
• Preliminary look: Could it be done at FNAL? Start with
MECO-like design? Results of recent meeting at FNAL….
• General design issues of the Mu2e experiment- following
the previously proposed but un-funded BNL-MECO
approach?
Muon to electron conversion
• Measure rate of the lepton flavor violating (LFV) reaction:
neutrinoless muon to electron conversion in the field of a nucleus,
relative to the ordinary muon capture rate on a nucleus.
• Goal: Mu2e (and MECO) Rme< 10-16 on Al which is ~ 4000x better
than the current limit from SINDRUM II: Rme<6.1x10-13 on Ti
Rme is the ratio of rates measured in a muonic atom,
Rme={Rate(m-+A(N,Z) ->e-+A(N,Z)} / {Rate(m-+A(N,Z) -> m+A’(N+1,Z-1)}
Lm=+1,Le=0 -> Lm=0,Le=+1
The conversion electron has a monochromatic energy which is well
above most of the background flux.
Why m-A  e-A ?
Some Options to m-A  e-A :
• t LFV may be significantly stronger in some models, but experimental
challenges are large, solutions are difficult: super B-factory for t->mg?
• Kaon LFV no stronger in most models, experimental improvements are
difficult.
• m->eg decay (MEG) is more sensitive in photon mediated processes by
x200-x400, but is not more sensitive for other types of LFV reactions.
(MEG at10-13 is 3x less sensitive than FNAL m->e proposal even for
photon-mediated LFV, and 1000x less sensitive for other modes). May
be experimentally limited by backgrounds to 10-13 to 10-14.
• What could change before next m-A  e-A?
– MEG (PSI) may see m->eg at 10-13 (or at ‘super MEG’ upgrade limit
of 10-14)
– LHC may discover new particles (e.g. supersymmetry). m-A  e-A
will be needed to help sort things out, e.g. the flavor structure.
Why m-A->e-A at FNAL?
• Tens of man-years are invested in a MECO design which is applicable
to FNAL.
• Physics case was reviewed repeatedly w/excellent outcome
• Well developed conceptual design exists, magnets have preliminary
engineering design, some detector prototype work has been completed
• Technical case reviewed repeatedly w/excellent outcome
• An advanced cost estimate was produced
• The continuing neutrino program at FNAL provides facilities and an
accelerator operation well-matched to m->e experimental needs.
• A working group has been established to understand how the
appropriate proton beam can be supplied at FNAL.
Muonic Atom Formation and Nuclear Capture
A rapid process: low energy m- (KE< 30 MeV) stop in target A(N,Z), undergo
atomic cascade arriving primarily in atomic 1s state
• Bohr radii n/mmZ),EmmZ2/n2 : 200x smaller radius and 200x more binding
energy than atomic electron -> 1s muon is well inside electron orbits ->muon
forms hydrogen-like atom
• Hydrogenic Radial wavefunction: Rnl(r)  rl Z3/2 for small r. Prob. of overlap
between nucleus and muon wavefunction is proportional to r2lZ3, which for small r
and low to medium Z, is large only when l=0.
• Ordinary Muon Capture Rate
m- +A(N,Z) -> m+A’(N’,Z’) + ag + bn +cp: <a>~2, <b>~2, <c>~0.1
Fundamental process: m-+ p -> m + n
Proportional to: (# protons)x(nuclear overlap) ~ Z4 . Capture ~ decay rate for Z=12
• Muon Conversion Rate m- + A(N,Z) -> A(N,Z) + eCoherent process ->proportional to
(# nucleons)2x(nuclear overlap) ~Z5
m-> Rme -> high Z preferred. But…
•
Nucleus
m->e Conversion Rates vs. Z
Plot of
Rme(Z)/Rme(Z=13)
For various photon
couplings
(Rates Normalized
to Z=13, Aluminum)
Kitano, et al.,PRD 66, 096002 (2002)
Aluminum is nominal choice for MECO
Nucleus
Rme(Z) /
Rme(Al)
Bound
lifetime
Atomic Bind.
Energy(1s)
Conversion
Electron Energy
Prob decay
>700 ns
Al(13,27)
1.0
.88 ms
0.47 MeV
104.97 MeV
0.45
Ti(22,~48)
1.7
.328 ms
1.36 MeV
104.18 MeV
0.16
Au(79,~197)
~0.8-1.5
.0726 ms
10.08 MeV
95.56 MeV
negligible
Old BNL- MECO and new FNAL-Mu2e
beam rates and sensitivity
Probability
Rate (proton
Events (run time=
flux=1-2x1013 Hz) 2-4x107 seconds,
total p’s =4x1020)
Prob. Muon stopped in
target per proton
0.0025
0.25-0.5x1011 Hz
1x1018
Prob. m capture (Al target)
0.60
1.5-3x1010 Hz
6x1017
Fraction of captures in
detector time window
0.45
(> 700 ns )
0.7-1.4x1010 Hz
3x1017
Track fitting and selection
criteria
0.19
1.2-2.5x109 Hz
5x1016
Detected events for
Rme=10-16
10-16
1.2-2.5x10-7 Hz
5 -> single-event
sensitivity 2x10-17
Estimated background
0.45
Classes of background
1.
Prompt: due to beam particles which interact almost immediately when
they enter the detector region, producing electrons in the signal region,
100 MeV<E<106 MeV, or low energy background.
Examples:
–
–
–
–
2.
Pions: Radiative pion capture, p-+A(N,Z)->g+X. Very high suppression of
pions is required, since it is a potentially major background.
Beam electrons: incident on the target and scattering into the detector
region. Need to suppress e-with E>100 MeV
In-flight muon decays. Keep pm<75 MeV/c to keep Ee<100 MeV
Antiproton annihilations along beam line or near target- (none in
SINDRUM II, potential problem at 8 GeV at for MECO at AGS and at
FNAL)
Delayed: due to beam particles which take > few hundred nanoseconds
before they produce signals in the detectors.
•
Examples:
–
–
–
3.
Electrons from muon decay in orbit (DIO)
Protons, neutrons, gammas from muon capture
Photons from radiative muon capture
Cosmic Rays- Back/Signal proportional to (run time)/(beam intensity)
Decay of a Muon Bound in Atomic Orbit
(m- + N(A,Z))bound -> N(A,Z) + e- + e + m
•
•
Decay of a muon bound in an
atom is slightly different from
ordinary free muon decay…
Nucleus absorbs momentum > neutrinos can carry zero
momentum, with electron
recoiling off of the nucleus
-> electron can take almost
all of the muon rest energy,
and the endpoint energy is
the same as a potential
conversion electron, but
fortunately the probability is
very low.
(DIO)

Free muon decay,
endpoint =52.8 MeV
Ee(max)=
(mmc2 -NuclearRecoilEnergy
-AtomicBindingEnergy)
For Z=13 (Al), Atomic
BE=0.529 MeV, Recoil
energy=0.208 MeV ->
Ee(max)=104.96 MeV
Muon decay in
Al 1s bound state,
endpoint=104.96 MeV
104.96 MeV

Major Potential Background: Decay of a Muon
Bound in Atomic Orbit (DIO, Continued)
(m- + N(A,Z))bound -> N(A,Z) + e- + e + m
• Rate near the maximum energy
falls very rapidly. Near endpoint:
proportional to (Ee(max)-E)5
• Major potential source of
background- Discriminate against
it with good electron energy
resolution, ~1 MeV FWHM for
Rme~10-16
Endpoint E (Al)=104.96 MeV

Simulation of detected spectrum
• Assumptions
– Rem=1x10-16
– Energy resolution 1 MeV (FWHM)
– Signal region 103.6<E<105.1 gives 0.05 DIO per mA->eA
parametric curve
0.25
0
10-4
Log scale
Linear scale
Acceptance, Back
as Ethresh varied
1
Sources of Background, continued
•
Radiative muon capture in atomic orbit (RMC)- (Regular muon
capture + photon): m- + A(N,Z))bound -> em+ A’(N+1,Z-1) + g
followed by asymmetric photon conversion in matter, gA->e+e-A
– Lower endpoint energy than DIO and mN->eN
– e- Endpoint energy = gEndpoint energy = Endpoint(m->e) - (MA’-MA)c2
For Al, Egmax = 102.5 MeV (Compare 104.96 MeV for mN->eN)
P(Eg> 100.5 MeV) = 4 x 10-9
P(g-> e+e-, Ee>100.5 MeV)=2.5 x 10-5
• Radiative capture of pions in atomic orbit (RPC), B.R. ~ 1.2%
Examplesp- + A(N,Z))bound -> g+ A’*(N+1,Z-1)
p- + A(N,Z))bound -> g+X
followed by asymmetric gA->e+e-A conversion in matter
– Maximum genergy ~ 139 MeV, distribution peaks ~ 110 MeV.
– A potentially serious source of e- background in the 100-106 MeV region
 Pions in the beam line must be greatly suppressed
Backgrounds, continued
• Antiprotons: annihilation on the target or in the beam line can produce
background electrons.
p
+
A
(N
,Z
)
g+X
The g’s, which come from p0’s, radiative p capture, and other mechanisms,
can be very energetic. Pair production, gA-> Ae+e-, can make
electrons 100-106 MeV near the conversion electron energy.
An 8 GeV proton beam (FNAL) is above the antiproton production
threshold, but the production cross section is low. (SINDRUM II and
TRIUMF experiments used 600 MeV proton beam and had no
antiprotons).
 antiprotons in the beam line must be highly suppressed
• In-flight muon decay: Muons with p>75 MeV/c can decay to an electron
with E>100 MeV, and need to be suppressed.
• Electrons: beam electrons with E> 100 MeV, especially those which
scatter from the stopping target, need to be suppressed.
•
•
Go To a Pulsed Muon Beam
In SINDRUM II and TRIUMF m->e experiments
– Continuous beams of muons were used, fluxes up to few x 107 Hz
– Prompt backgrounds (mainly pions) were suppressed by A) vetoing detector
events in close time coincidence with signals in beam counters, and/or B) pions
were suppressed using degrader in the beam line (range pions~1/2 muons).
– Veto method limits the incident muon rate to ~few x 107 Hz. Beam lines at PSI are
limited to ~107-108 Hz. Implies ~109-1010 seconds of beam would be required to
reach the MECO->FNAL goal of Rme< 10-16.
Approach proposed for FNAL: MECO-like approach
– No incident beam counters or pion absorbers.
– Use an intense pulsed muon beam to suppress prompt background
• Stop large flux of muons in a target in a narrow time bunch (< 100 ns): ~1011
Hz muon stopping rate, injection pulse spacing ~1.5 ms, compatible with muon
lifetime of ~0.88 ms in atomic orbit in Al
• Wait 700 ns after injection until prompt background and background from
particles slowly traversing the beam line disappear
• Activate detector system from ~ 700 ns after injection until next injection
• Attenuate incident beam x109 between injection bunches (extinction) to
suppress prompt background (mainly from radiative pion capture)
– Build a detector system with high acceptance and good energy resolution for eoriginating in the stopping target and in the energy range 100-106 MeV; and make
acceptance as low as possible at lower energies where DIO electrons are copious.
– Design a beam line which delivers maximum muon flux but minimal electron
background between 100 and 106 MeV for t >00 ns. Minimize number of
particles at other energies: antiprotons, muons with p>75 MeV/c, pions
Pulsed Proton Beam
• BNL-AGS at reduced energy, 8 GeV,
21013 protons s-1 – 50 kW beam power.
FNAL-Booster operates at 8 GeV.
• BNL-AGS Revolution time = 2.7 ms with
6 RF buckets for protons.
• Match 0.88 ms lifetime of muons in
atomic Al: fill 2 AGS RF buckets -> 1.35
ms pulse spacing. Put one bunch in
FNAL Debuncher -> 1.6 ms pulse
spacing
• Resonant extraction of temporally
narrow (~ 100 ns) bunches
• Collect data >700 ns after injection, after
most prompt particles in beam are gone.
• To eliminate prompt backgrounds, we
require < 10-9 protons between bunches
for each proton in bunch. We call this the
beam extinction.
Proton
pulse
Prompt
background
s
Detection time
8 GeV
Proton sources
Proton Linac (H-)
8 GeV?
H-
t
Mu2e and SNUMI(neutrino) Phase 2
SNUMI 1:
Uses recycler as an 8 GeV pre-stacker.
SNUMI 2:
• Use Accumulator, presently used in the antiproton source, to coalesce 3
booster batches at a time, allowing 18 batches to be loaded as 6 boxcar
batches into the recycler.
• Debuncher ring is not utilized in this scheme, making it available as a
slow spill facility for mu2e: inject any bunches not used by neutrino
program from accumulator into debuncher.
• Use RF to make one narrow bunch in the Debuncher, then slow extract
to Mu2e
PROTON SOURCE RING USAGE
22 cycles = 1467 ms
Booster Batches
NEUTRINO PROGRAM MUONS
4.61012 p/batch
Accumulator
(NuMI +Muons)
Recycler
56 1012 p/sec
(NuMI)
Debuncher
(Muons)
44.61012 p/1467ms = 12.5 1012 p/sec
0.1s
1.367s
31
TECHNICAL ISSUES
1. Booster to Accumulator Transfer Line ( also needed for the
future neutrino program in the proton plan).
2. Radiation limits (same as for NuMI program).
3. Rebunching
4. Slow Extraction from Debuncher
5. Debuncher Beam Dump Location
6. Extinction Factor (10-9 !)
7. Experiment Location
32
Graded Solenoid Field
For adiabatic motion in a straight solenoid with a field gradient,
pt12/B1= pt22/B2 or sin211sin2
•
•
where sin() = pt/p, pt = component of p transverse to B field
When the muon spirals from a low field region, B1, to a high field region, B2
it will be reflected back when sin2(=1, or when sin2(1)=B1/B2.
pt/p decreases as B decreases -> particle movement is enhanced in the
direction of decreasing gradient. Effect is acceleration of particle in the
direction of decreasing field.
– Production Solenoid: Following the MELC scheme: apply a graded field
at the primary production target to collect and accelerate muons to
downstream direction, and reflect a portion of upstream-going muons +
pions back to the downstream direction in order to enhance pion/muon
collection efficiency: going downstream, B goes from 5 T to 2.5 T.
– Detector Solenoid: Use graded field at muon stopping target to reflect
upstream-going electrons produced there to the downstream direction
toward the detectors, to increase acceptance. Going downstream, B
goes from 2T to1T.
– Transfer Solenoid, which connects the Production Solenoid to the
muon stopping target and Detector Solenoid has a small continuous
decline of B moving downstream. (Exception is in curved parts of
solenoids). This prevents local trapping of charged particles, which
could lead to delayed beam particles reaching the stopping target in the
measurement window > 700 ns after injection.
Charged particle motion in a toroid
Drift Property in the curved (toroid) portion of Transport Solenoid
• For a toroid, charged particles spiraling around the B-field lines drift
perpendicular to the toroid bend plane. For R= toroid bending
radius, s =distance of travel along the particle’s central orbit,
ppar=component of p parallel to B, pperp=component of p
perpendicular to B, the vertical displacement is:
• Unwanted positively charged particles and high-energy negatively
charged particles (e.g. E(e-)>100 MeV, p(m-)>75 MeV/c) are
displaced vertically after passing curved solenoid portions in the
Transport Solenoid and are collimated away.
Production Solenoid
• 4 m long x 0.75 radius
• 0.30 radius inside
radiation/heat shield is
available for particle transport Transport solenoid
downstream
12
• 10-20 x 10 protons/s, bunch
spacing ~1.6 ms
• Protons enter at a 10 degree
angle, toward the upstream
direction to reduce background
particle flux into transport line
• Water-cooled platinum or
gold target, 0.4 cm radius x 16
cm long
• B-field tapers going
downstream from 5 T to 2.5 T
to reflect upstream-travelling
low-E pions and muons back
B=2.5 T
downstream toward the
transport solenoid. Particles
are accelerated downstream
by the gradient.

upstream
B=5 T
Transport Solenoid
•Separates detectors from production
target: no straight-line path for neutrals
B=2.5 T
•Selects m- in momentum range <0.08
GeV/c
• Eliminates electrons >100 MeV
13m x 0.25m radius
• Absorbs m+, e+-, p, pbar, pi+Components include:
• Vacuum system
B=2.0 T
• Collimators
• Thin beryllium Pbar
absorbing window
• Neutron absorbers
• Stopping Target:
17x.02 cm Al disks ~8
cm radius, 5 cm
spacing
Detector Solenoid
• 10 m long x 0.95 m radius
• Detector solenoid is evacuated to avoid: scattering of background and signal
particles; and capture of muons in residual gas downstream of stopping target
• B graded from 2 T to 1 T in first 4 m in target region
• Al target is in a graded field in order to
- reflect portion of upstream-going electrons back toward detector
- reduce the transverse momentum of beam electrons with E>100 MeV
to have helix radii< 38 cm so that they do not hit the detectors
• B=1 T, uniform to 0.2% in tracking region, 1.0 % in calorimeter region to
obtain necessary energy resolution
• Thin low-Z shields around the target absorb protons from muon capture
• Central region r<38 cm of detectors is
free of material. Charged particles from
target with pt<55 MeV/c pass without
interacting to downstream beam dump
• Specially enclosed beam dump
minimizes particle albedo
B=1 T
B=2 T
Cross section of longitudinal tracker
Note<0.3of e- from DIO have pt>55 MeV/c
• Geometry: Octagon with eight vanes,
each ~30 cm wide x 2.6 m long
• Straws: 2.6 m length  5mm dia., 25
mm wall thickness to minimize multiple
scattering – 2800 total
pt=105 MeV/c
• Three layers per plane, outer two
resistive, inner conducting
• Pads: 30 cm  5mm wide cathode
strips affixed to outer straws - 16640
total pads
• Position Resolution: 0.2 mm (r,) 
1.5 mm (z) per hit is goal
• Energy loss and straggling in the
target and multiple scattering in the
chambers dominate energy resolution
of 1 MeV FWHM
target
pt=55 MeV/c
pt=91 MeV/c
Alternative: Transverse Tracker
Geometry: 18 Modules of three planes each, 30° rotation between
successive planes
Straws:
70 – 130 cm length  5mm diameter, 15 or 25 mm thickness
12960 total straws
One layer per plane, all
straws are conducting
Baseline: no z-coordinate, charge
division was being considered
136 cm
Position Resolution: 0.2 mm (x,y)
Readout Channels: 13k
L and T tracker performances are
similar in simulations, and more
prototype work is needed to decide
on the best option.
Calorimeter
• Function: provide initial trigger to system (E>75 MeV gives
trigger rate ~1 kHz), and secondary position and energy
information to clean up tracks
• 1024 PbWO4 crystals, 3.75 x 3.75 x 12 cm3 arranged in four
vanes. Density 8.3g/cm3, Rad. Length 0.89 cm,
R(moliere)=2.3 cm, decay time 25 ns
• Each crystal is equipped with two large area Avalanche PhotoDiodes: gives larger light yield and allows rejection of charged
particles traversing photodiode
• Both the front end electronics (amplifier/shapers) and the
crystals themselves are cooled to -240 C to improve PbWO4
light yield and reduce APD dark current.
• Single crystal performance has
been demonstrated with cosmic
rays: 38 p.e./MeV, electronic noise
0.7 MeV, for electrons, ~5-6 MeV
at 100 MeV, position<1.5 cm
Cosmic Ray Veto and Shielding
• Passive shielding: heavy concrete plus 0.5 m
magnet return steel. Latter also shields CRV
scintillator from neutrons coming from stop target.
• Hermetic active veto: Three overlapping layers
of scintillator consisting of 10 cm x 1 cm x 4.7 m
strips
• Goal: Inefficiency of active shielding <10-4
• Cost-efficient solution: MINOS approachextruded rather than cast scintillator, read out with
1.4 mm dia. wavelength-shifting fiber.
• Use multi-anode PMT readout
Backgrounds
(Assumptions: extinction ~ 10-9, energy resolution 1 MeV FWHM, 4x1020 protons)
PRISM=Phase Rotated Intense Slow Muon source, PRIME=PRISM Mu e,
FFAG= Fixed-Field Alternating Gradient synchrotron
Comparison of mu2e and PRISM/PRIME
mu2e
PRISM/PRIME
Rme goal
<10-16
<10-1
Proton beam
8 GeV, 10 x1012 Hz, ~1/1.6
MHz bunch rate
50 GeV(JPARC), 100 x1012 Hz,
~10-100 Hz bunch rate
(JPARC and FFAG cycle limits)
Muon momentum
<77 MeV/c
68 MeV/c +- 3%
-> thin target
Muon stop rates
.25x1011 Hz, 4 year run ?
1011 to 1012 Hz, 5 year run ?
Extinction
<10-9,use internal and
external kickers
<10-10,FFAG suppresses
pions, etc. to high order
Target
Al or perhaps Ti, begin data
at > 700 ns after injection
Ti or higher Z (minimal detector
measurement delay)
Detector
Detector displaced
downstream from stopping
target in a straight solenoid
FWHM<1 MeV
Target and detector separated
by momentum-selecting toroid
and line-of sight shielding
FWHM<0.3 MeV
Comparison withm+->e+g
m-->e-N
+m+->e+g~Rme for nonphotonic processes
+ Far less background at the
signal energy than
m+->e+gand no accidental
coincidences
• Requires special muon source
• MECO: Rme<10-16
• PRISM/PRIME: Rme<10-18
m+->e+g
+m+->e+g00-300x Rme for
photonic processes
(-) Signal: coincidence of back-to-back
e+ and g. Large accidental
background rate:E(e+) = E(g) ~
52.83 MeV, where there is a huge
background of positrons from
ordinary muon decay, m+->e+m,.
(+) Low-energy muon flux, m+ (surface
beam)>m• Requires state-of-the-art detector
• Current limit BR(m+->e+g<1.x10-1
• MEG (PSI) goal: BR<10-13. Longterm with upgrade, BR<10-14
(+) Funded and under construction
STATUS
1.
2.
3.
4.
.
A substantial fraction of the MECO Collaboration is interested in a m-  econversion experiment at Fermilab, if there is a possibility.
A group of Fermilab scientists is also interested, and has been exploring the
beam options.
No show stoppers have been identified, and it seems possible there is an
attractive solution that would enable a m-  e- conversion experiment to
run at Fermilab in parallel with the neutrino program.
A meeting was held at Fermilab on Sept 15-16, 2006: about 50 physicists
attended.
Conclusions at the meeting:
• substantial interest from physics community
• preliminary look says m-  e- is highly adaptable to the present
FNAL accelerator complex
• next steps are under discussion- letter of intent, or some other
approach??? (we would like to get support in the near term for
studies of needed transfer lines, extinction, RF issues,
extraction studies…)
Summary
• The physics potential of m->eN is excellent.
• The beam line magnet systems for MECO received funding priority,
and an advanced design was produced at MIT: a great asset to any
future effort
• The detector systems and DAQ are at the detailed conceptual stageno potential show-stoppers are seen. A very good detector is needed,
but no new inventions are required. Some initial prototype work has
been done for the trackers, calorimeters and cosmic ray veto
counters.
• Groups were awaiting funding to build prototypes when the project
was cancelled- so there is lots of hands-on development work to do.
• A lot of detailed simulation work has been done, but more is needed.
For example with the detailed magnet design, we can study the cost
drivers in detail and perhaps find some savings, or more studies of
backgrounds and shielding, L vs. T trackers, etc.
• The MECO concept is highly viable as a candidate experimental
arrangement, with many man-years of design effort invested, and with
many successful detailed reviews.
• There is the possibility that the design could be modified to improve
performance and/or to reduce cost.
Summary (Continued)
• Participation is open, none of the tasks have been parceled out.
• There is plenty of interesting development work to do.
• You are invited to join in!
Distribution of electron energies from m decay in orbit (DIO)
Free muon decay,
Ee(max)=52.8 MeV

Bound muon decay
A
Aluminum 1s state
l
energy=104.96
u Endpoint
Lifetime=0.88 ms
m
i
n DIO contribution to Rme negligible, need detector electron energy resolution
• To keep
<1 MeV
u (FWHM) for 100-106 MeV electrons, with minimal high-side tails.
• Detector
m acceptance needs to be high for 100-106 MeV electrons, but to control
rates needs to be minimized to avoid copious low energy electrons.
• Backgrounds need to be eliminated between 100-106 MeV
Major Background: Decay of a Muon Bound in Atomic Orbit
•
m-+A,bound ->A,+e-+e + m(DIO)
With nucleus to absorb momentum, neutrinos can carry zero momentum, with
electron recoiling off of the nucleus -> endpoint energy same as a potential
conversion electron
Ee(max)=mmc2-Recoil-AtomicBE
For Z=13 (Al), Atomic BE=0.529 MeV,
Recoil energy=0.208 MeV ->
Ee(max)=104.96 MeV
•
Rate near the maximum energy falls very
rapidly. Near endpoint: proportional to
(Ee(max)-E)5
Most important potential source of
background- Discriminate against it by
measuring electron energy to better
than ~1 MeV FWHM.
• Accept events from 103.6-105.1 MeV
-> 0.05 DIO/(m->eN) if Rme~10-16
Endpoint E (Al)=104.96 MeV

Cosmic Ray Veto and Shield
• Passive shielding: heavy concrete
plus 0.5 m magnet return steel
• Inefficiency of active + passive
shielding <10-4
• Three overlapping layers of
scintillator
Fermilab proton source for a muon
beam line
• Adapt existing facility for μ-e conversion experiment
– Current intensity
– Accumulator to stack protons
– Debuncher for beam formation and extraction
• Protons for muon source – A-D configuration
– extract beam for mu-e conversion
– atom captures muon, muon decays to e without neutrinos
– MECO- or PRISM/PRIME –like experiment
Muon to electron conversion
• Measure rate of the lepton flavor violating (LFV) reaction:
neutrinoless muon to electron conversion in the field of a nucleus,
relative to the ordinary muon capture rate on a nucleus.
• Goal: Rme< 10-16 which is ~ 6000x better than the current limit from
SINDRUM II: Rme<6.1x10-13
Rme is the ratio of rates measured in a muonic atom,
Rme={Rate(m-+A(N,Z) ->e-+A(N,Z)} / {Rate(m-+A(N,Z) -> m+A’(N+1,Z-1)}
Lm=+1,Le=0 -> Lm=0,Le=+1
• In SM, suppressed far below experimental accessibility.
• Experimentally accessible rates are commonly predicted in new
physics models ->excellent process to use in the search for new
physics.
m->e Conversion Rates vs. Z
Plot of
Rme(Z)/Rme(Z=13)
For various photon
couplings
(Rates Normalized
to Z=13, Aluminum)
Kitano, et al.,PRD 66, 096002 (2002)
Aluminum is nominal choice for MECO
Nucleus
Rme(Z) /
Rme(Al)
Bound
lifetime
Atomic Bind.
Energy(1s)
Conversion
Energy
Prob decay
>700 ns
Al(13,27)
1.0
.88 ms
0.47 MeV
104.97 MeV
0.45
Ti(22,~48)
1.7
.328 ms
1.36 MeV
104.18 MeV
0.16
Au(79,~197)
~0.8-1.5
.0726 ms
10.08 MeV
95.56 MeV
negligible