The MECO Experiment to Search for NeN with 1017 Sensitivity

1
The MECO Experiment to Search for ?-N?e-N with
10-17 Sensitivity
W. MolzonUniversity of California, Irvine May
14, 2004 LFV Workshop at 2004 RHIC-AGS Users
Meeting

• The basis for experimental design
• Implementation
• Prospects

2
Muon to Electron COnversion (MECO) Experiment

• Boston University
• J. Miller, B. L. Roberts
• Brookhaven National Laboratory
• K. Brown, M. Brennan, G. Greene,L. Jia, W.
  Marciano, W. Morse, P. Pile, Y. Semertzidis, P.
  Yamin
• University of California, Irvine
• C. Chen, M. Hebert, W. Molzon, J. Popp, V.
  Tumakov
• University of Houston
• Y. Cui, E. V. Hungerford, N. Klantarians, K. A.
  Lan
• 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 Syracuse University R. Holmes, P.
Souder College of William and Mary M. Eckhause,
J. Kane, R. Welsh
3
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.
• Experimental signature is an electron with
  Ee105.1 MeV emerging from stopping target, with
  no incoming particle near in time.
• 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.

4
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
5
Why ?-N?e-N Conversion Experiment?

• Rate for t LFV processes might be significantly
  higher, but reaching equivalent sensitivity in
  popular models is very difficult, requiring new
  accelerator and very large improvement in
  experimental techniques significant progress is
  unlikely to be made in next decade
• Improvements in kaon processes appear very
  difficult, and rates are not higher in most model
  predictions.
• m?eg decay is more sensitive at same branching
  fraction for the most popular extensions to the
  Standard Model, but is less sensitive for other
  modes and appears to be limited by background
  considerations at 100-1000 times larger branching
  fraction than could be achieved in next
  generation conversion exp.
• Best prospect for discovering LFV in charged
  sector (funding and other non-technical
  considerations aside) appears to be in ?-N?e-N
  experiment.
• Sufficient proton intensity to make the necessary
  muon beam is available at an operating
  accelerator, the BNL Alternating Gradient
  Synchrotron
• Essentially existing beam and detector
  technologies will meet the needs of the
  experiment
• A technically driven schedule could have the
  experiment built in 4 years after conceptual
  design is complete

6
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.
7
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
  (Al, Ti, Ca)
• 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
8
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
  shieldingor very small duty cycle

9
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. Determine background
  levels from the data.

?
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
10
Features of the MECO Experiment

• 1000 fold increase in muon intensity (technique
  of MELC proposal)
• Graded solenoidal field to maximize pion capture
• Produce few x 10-3 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 (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
• Stopping target in graded solenoid field
• Improved acceptance
• Improved rate handling
• Improved background rejection
• Very high resolution spectrometer in uniform
  magnetic field

11
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)
Collimators
12
Pulsed Proton Beam from AGS for MECO
Promptbackgrounds

• 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 slightly higher
  than 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.

Proton pulse
Quiet detection time
13
Removing Out-of-Bucket Protons in the AGS

• Extinction measurements
• Initial test at 24 GeV with one RF bucket filled
  yielded lt10-6 extinction between bucketsand 10-3
  in unfilled buckets
• A second test at 7.4 GeV with a single filled
  bucket found lt10-7 extinction

• Improvements in extinction in the AGS
• 40 kHz AC dipole used to destabilize all orbits
• Fast kicker magnets to cancel effect of AC magnet
  for particles in buckets (field shown inverted).
• Some early tests done
• Changes needed
• Modifications to kicker for continuous operation
• Controls modifications

magnetic kick
14
Radio Frequency Modulated Magnet to Improve
Extinction

• Proton beamline is designed for achromatic
  transport of 8 GeV protons to muon production
  target
• Includes a radio frequency modulated magnet plus
  Lambertson septum magnets to separate filled
  buckets from other particles in beam
• Pre-conceptual design of RFMM completed
• 5 m long stripline magnet with ferrite return
  yoke
• Provides 2.1 mrad separation between
  filled/unfilled buckets ( uniform 75 Gauss
  field, 5 m long)
• Resonantly driven at Q of 100 for efficient
  operation

Unfilled bucket
Unfilled buckets
Filled bucket
Filled bucket
-2.500 0.000 2.500

I------------------I 0.8 I
11 I 0.6 I
13DJTSXED I 0.4 I 8X
I 0.2 I 1
I 0.0 I 16 I -0.2 I
4 I -0.4 I
I -0.6 I
LF1 I -0.8 I 27HNKODB6
I -1.0 I 11 1
I -1.2 I I -1.4 I
21574761 I -1.6 I
7WV8 I -1.8 I
I -2.0 I 24
I -2.2 I 5 I -2.4 I
33 I -2.6 I
O I -2.8 I 1LK2
I -3.0 I 1123 2 2
I -1.4 I I
I-----------------I

15
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)
Collimators
16
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 and completed

1 T
2 T
1 T

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

17
Comments on Technical Aspects of the Magnets

• Field configuration is based on a variety of
  considerations
• Gives high muon flux
• Has good muon transport properties and good
  suppression of unwanted particles
• Has sufficiently good uniformity in detector
  region to make analysis easy
• Studied with a combination of analytical and
  Monte Carlo techniques
• Magnet technology is chosen to be very
  conservative
• Operating parameters (fraction of critical
  current, temperature margin) are very
  conservative.
• Magnets can be built using existing industrial
  techniques at reasonable cost.
• Implementation has low risk of failure in the
  anticipated radiation environment.
• No excessively tight tolerances on materials or
  workmanship are required to meet magnetic
  specifications.

18
Muon 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
19
Production Target for Large Muon Yield

• Production target region designed for high yield
  of low energy muons
• High Z target material
• Little extraneous material in bore to absorb p/m
• Diameter 0.6 - 0.8 mm, length 160 mm
• 5 kW of deposited energy
• Water cooling in 0.3 mm cylindrical
  shellsurrounding target
• Simulated with 2D and 3D thermal and turbulent
  fluid flow finite element analysis
• Target temperature well below 100? C
• Pressure drop is acceptable ( 10 Atm)
• Prototype built, tested for pressure and flow
  

Fully developed turbulent flow in 300 mm water
channel
Preliminary cooling tests using induction heating
completed
20
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
21
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.
3-15 MeV
Detection Time
Relative particle flux
Relative particle rate in mbunch
22
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 downstream of target to reduce
  rates
• Polyethylene with lithium/boron to absorb
  neutrons
• Thin absorber to absorb protons

1T
Electron Calorimeter
1T
Tracking Detector
2T
Stopping Target 17 layers of 0.2 mm Al
23
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 at high rates 500
  kHz rates in individual detector elements
• Energy resolution dominated by multiple
  scattering
• Implemented in straw tube detectors
• Vanes and octants, each nearly axial, and each
  with 3 close-packed layers of straws
• 2800 detectors, 2.6-3.0 m long, 5 mm diameter,
  0.025 mm wall thickness issues with
  straightness, wire supports, low mass end
  manifolds, mounting system
• r-f position resolution of 0.2 mm from drift time
• axial resolution of 1.5 mm from induced charge on
  cathode pads requires resistive straws,
  typically carbon loaded polyester film
• High resistivity to maximize induced signal
• Low resistivity to carry cathode current in high
  rates
• Alternate implementation in straw tubes
  perpendicular to magnet axis has comparable
  performance

24
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.
• Geometrical acceptance 50 (60?-120?)
• See A maximum likelihood method for particle
  momentum determination, T.J. Liu and W. R.
  Molzon, NIM A.
• Alternate transverse geometry has similarly good
  tracking performance with sophisticated fitting.

?
FWHM 900 keV
25
Tracking Detector Rates vs. Time
Rate MHz
Rate kHz
Full time between proton pulses
Detection time interval
25 20 15 10 5 0
800 700 600 500 400 300 200 100 0
m-capture protons
beam electrons
m- decay in flight
0 400 800
1200 700 900
1100 1300
time with respect to proton pulse ns

• Very high rate from beam electrons at short times
  potential problems with chamber operation
• Protons from m capture are very heavily ionizing
  potential problems with noise, crosstalk

26
Proton Contribution to Tracker Rates
105 103 10

• Protons are very heavily ionizing up to 50
  times minimum ionizing
• Protons below 18 MeV absorbed in target
• Rate hitting tracker reduced by absorbers
  further optimization possible
• Studies of response of chambers to low energy
  protons planned at TUNL at Duke University

all generated protons
100 10 1
protons hitting tracker
0 20 40 60
80 100
Proton energy MeV
27
Straw Construction Possibilities

• Requirements on mechanical strength
• Resistivity 1-10 M?/square
• Thickness 25 mm
• Yield Stress gt 10(MPa)
• Young's modulus gt 1.0(GPa) by assuming 1
  swelling maximum
• Radiation Dose gt 100200(Gy)
• Possible construction methods
• Spiral wound carbon loaded kapton (now 50 mm, 25
  mm possible)
• Technology of spiral wound straws is well known
• Arbitrary lengths are possible
• Making them thin is difficult, especially carbon
  loaded
• Polyimide(PI) (25 mm)
• Outstanding mechanical strength
• Good radiation tolerance
• Not-easy to process hard to make long tube
• PEEK(PolyEther-Ether-Ketone) (30 mm)
• Good mechanical strength and radiation tolerance
• Thermoplasticextrusion -gt potential to make long
  tube
• Straightness??

vacuum
1 atm
Swelling lt 1
PEEK tubes
28
Prototype Resistive Straw Tracking Chamber (Osaka
University)

• Prototype resistive straw tracking chamber tests
• Seamless and spiral wound straws tested
• Three layers (outer two resistive)
• Axial position measured with pads, interpolating
  using charge measurements
• Tested in KEK test beam
• Axial position resolution 400-800 mm (MECO
  requirement 1500 mm)

29
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 lt5 to confirm
  electron momentum measurement
• Original design consists of 20003 cm x 3 cm x
  12 cm PbWO4 or BGO crystals with APD readout
• Small arrays studied for light yield, APD
  evaluation, electronics development

30
Energy Load in Electron Calorimeter
E/cell/mbunch MeV
70
0
r
z
E/cell/100ns keV/100ns
45
0
r
z
31
Crystal, APDs and Setup Schematic View

• 3 ? 3 ? 14 cm3 PbWO4 crystals
• Large area (13mm x 13mm) APD from RMD Inc.
• Hamamatsu (5mm x 5mm) APD used by CMS
• Crystal / APD combinations were tested using
  cosmic rays. The crystal, APD, and preamplifier
  are cooled, increasing the crystal light yield
  and decreasing dramatically the APD dark current.

Crystal / APD Test Arrangement
32
Calorimetric Electron Detector

• Indications are that PbWO4 will meet MECO
  resolution requirements, demonstrating 20-30
  photo e-/MeV (as compared with CMS 5 pe/MeV)
• We need to verify the system performance via beam
  tests of an 8?8 crystal array
• It appears that we can make use of fewer (larger)
  crystals allowing reductions in APD, and
  associated HV and readout channel counts (1152
  crystals vs. 2000 originally)

Estimated
33
Expected Signal and Background in MECO Experiment
Background calculated for 107 s running time at
intensity giving 5 signal event for Rme 10-16.

• Sources of background will be determined directly
  from data.

5 signal events with0.5 background events in
107 s running if Rme 10-16