A HighSensitivity Search for Charged Lepton Flavor Violation at Fermilab

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A High-Sensitivity Search for Charged Lepton
Flavor Violation at Fermilab

• Mu2e Experiment
• Craig Dukes
• University of Virginia
• For the Mu2e collaboration
• MANX08 Meeting
• 15 July 2008

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Why Search for Charged Lepton Flavor Violation?

• In Standard Model not there
• Neutrino mass discovery ? CLFV exists, but
  unobservable at 10-52
• Hence, any signal unambiguous evidence of new
  physics
• Exquisite sensitivities can be obtained
  experimentally
• such sensitivities that probe mass scales
  unavailable by direct searches
• sensitivities that should allow favored
  beyond-the-standard-model theories to be tested
• Mu2e proposes to reach a single-event sensitivity
  of Rme 2x10-17

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History of Lepton Flavor Violation Searches
with apologies to Z, t, etc searchers!
Mu2e intends to improve sensitivity by 10,000!
2x10-17 single evt 6x10-17 90 CL
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m-N?e-N and m?eg Complementary
Model independent CLFV Lagrangian
kltlt1 magnetic moment type operator m ? eg rate
300X mN ? eN rate
kgtgt1 four-fermion interaction mN ? eN rate many
orders of magnitude greater than m ? eg rate
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What Sensitivity is Needed?
Randall-Sundrum
Present sensitivity already interesting and
constraining! 10-16 removes many models 10-18
extremely difficult for theorists to deal with
Littlest Higgs
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How to Search for m-N?e-N

• stop muon in atom
• rapidly (10-16s) cascades down to the 1S state
• circles the nucleus for up to 2 ms
• two things most likely happen
• is captured by the nucleus m-NA,Z?nmNA,Z-1
• decays in orbit m-NA,Z?e-nmneNA,Z
• in m-N?e-N the muon coherently interacts with
  nucleus leaving it in ground state
• signature single isolated electron
• Ee mm ENR - Eb 104.97 MeV (Al)

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Three Types of Backgrounds to m-N?e-N
1. Stopped Muon Backgrounds
Muon decay in orbit (DIO) m-NA,Z?e-nmneNA,Z N
ote Ee lt mc2-ENR-Eb not Ee lt ½mc2 ? defeated by
good energy resolution Radiative muon capture
(RMC) m-NA,Z?nmgNA,Z-1, g?ee Note
Egmax(Al) 102.5 MeV ? restricts choice of
stopping targets ? defeated by good energy
resolution mZ-1 gt mZ
E(E-E0)5
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Three Types of Backgrounds (continued)
2. Beam Related Backgrounds
Radiative pion capture (RPC) p-NA,Z?gNA,Z-1 ,
g?ee- Note 1.2 have Eg gt 105 MeV Muon decay
in flight m- ? e-nn Note since Ee lt mmc2/2,
pm gt 77MeV/c Beam electrons scattering in
target Pion decay in flight p- ?
e-ne ?Defeated by interbunch extinction! Antipro
tons annihilating ?Defeated by thin absorber
3. Time Dependent Backgrounds
Cosmic Rays ?Defeated by activepassive shielding
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Mu2e Collaboration
R.M. Carey, K.R. Lynch, J.P. Miller, B.L.
Roberts Boston University W.J. Marciano, Y.
Semertzidis, P. Yamin Brookhaven National
Laboratory Yu.G. Kolomensky University of
California, Berkeley C.M. Ankenbrandt , R.H.
Bernstein, D. Bogert, S.J. Brice, D.R.
Broemmelsiek,D.F. DeJongh, S. Geer, M.A.
Martens, D.V. Neuffer, M. Popovic, E.J. Prebys,
R.E. Ray, H.B. White, K. Yonehara, C.Y.
Yoshikawa Fermi National Accelerator
Laboratory D. Dale, K.J. Keeter, J.L. Popp, E.
Tatar Idaho State University P.T. Debevec, G.
Gollin, D.W. Hertzog, P. Kammel University of
Illinois, Urbana-Champaign V. Lobashev Institute
for Nuclear Research, Moscow, Russia D.M.
Kawall, K.S. Kumar University of Massachusetts,
Amherst R.J. Abrams, M.A.C. Cummings, R.P.
Johnson, S.A. Kahn,S.A. Korenev, T.J. Roberts,
R.C. Sah Muons, Inc. R.S. Holmes, P.A.
Souder Syracuse University M.A. Bychkov, E.C.
Dukes, E. Frlez, R.J. Hirosky, A.J. Norman, K.D.
Paschke, D. Pocanic University of Virginia J.
Kane College of William Mary
Currently 50 scientists 12 institutions
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Bunched Beam Technique Needed
Signal single, monoenergetic electron with E
105 MeV, coming from the target, produced 1 ms
(tmAl 864ns) after the m is stopped in the foils

• Need bunched muon beam 50x109 m/s
• Need turn off detector for tmN (800 ns) while
  bad stuff (pions, electrons) is around
• Need lt 10-9 interbunch contamination

bunched beam arrives with ms, ps, and electrons
a huge amount of stuff comes off the target from
scatters, captures etc.
about 50 of the muons stop in target
need to be sure it isnt a scattered electron or
p capture electron
look for a delayed 105 MeV electron
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Producing 1018 Bunched Muons
Recycler
Main Injector

• Energy
• Fermilab produces beams with energy 8 GeV
  (Booster), 150 GeV (Main Injector), 900 GeV
  (Tevatron)
• 8 GeV booster energy is optimal any higher
  energy produces too many anti-protons
• Structure
• Beam must be bunched with spacing on the order of
  the muon lifetime 1ms ?Booster, Antiproton
  Accumulator, Debuncher Ring 1.7 ms
• Three 8 GeV storage rings available Recycler
  Ring, Antiproton Accumulator, Debuncher Ring

Debuncher
Accumulator
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Producing 1018 Bunched Muons
new detector hall and beamline

• Minimal accelerator modifications needed
• new switch magnet needed to transfer beam from
  Recycler to Antiproton Accumulator
• upgrade for 15 Hz Booster rate

6 batches x 4x1012 /1.33 s x 2x107 s/yr
3.6x1020 protons/yr
Cycle time determined by Main Injector magnet
ramp rate no NOvA neutrinos lost!
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Stacking and Bunching the Proton Beam
Energy

• Inject a booster batch every 67 ms into the
  accumulator
• Accelerate to the core orbit where it is merged
  and debunched
• Momentum stack with 3 booster batches
• Transfer to debuncher and bunch into a single
  40ns wide bunch

T67ms
T0ms
T135ms
1st batch is injected onto the injection orbit
1st batch is accelerated to the core orbit
2nd Batch is injected
2nd Batch is accelerated
3rd Batch is injected
3nd Batch is accelerated
Dave Neuffer simulation
Capture in 4 kV h1 RF System. Transfer to
Debuncher
Momentum stacked bunches
Phase Rotate with 40 kV h1 RF in Debuncher
Recapture with 200 kV h4 RF system
st40 ns
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Conventional Slow Extraction Scheme

• Exploit 29/3 resonance
• Extraction hardware similar to Main Injector
• Septum 80 kV/1cm x 3m
• LambertsonC magnet .8T x 3m
• Produces a single bunch every 1.7 ms

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Removing Interbunch Protons Extinction

• Interbunch protons cause backgrounds
• 1. Muon decay in flight
• m- ? e-nn
• Since Ee lt mmc2/2, pm gt 77 MeV/c
• 2. Radiative p- capture
• p-N ?Ng, gZ ? ee-
• 3. Beam electrons
• 4. Pion decay in flight
• p- ? e-ne
• Suppressed by minimizing beam between bunches
• Need ? 10-9 extinction
• Get 10-3 for free
• Special kickers needed for rest
• US-JAPAN collaboration on RD

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Possible Experimental Hall Location

• Requires new building.
• Minimal wetland issues.
• Can tie into facilities at existing experimental
  hall.

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Mu2e Apparatus

• Salient Features
• Graded solenoidal field to maximize pion capture
  (MELC)
• Muon transport in curved solenoid to eliminate
  neutral and positive particles
• Pulsed proton beam to eliminate prompt backgrounds

for every incident proton 0.0025 m-s are stopped
in the 17 0.2 mm Al target foils
MECO Apparatus Design
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Solenoid Magnets

• Advanced engineering design done by MIT PSFC
• Solenoidal fields monotonically decrease to avoid
  magnetic traps

• 5T max field
• 4 cryostats PS, TS1, TS2, DS
• 27 m total length
• SSC NbTi wire
• Stored energy 150 MJ
• Commercially fabricated

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Capture Solenoid

• R 75 cm
• 23kW beam
• 0.8 mm x 160 mm gold target
• 2.5T 5.0T graded magnetic field
• Forward moving pions and muons with q gt 30 and
  pz lt 180 MeV/c reflected back in graded field

• Graded solenoidal field to maximize pion capture
• 2.5x10-3 m-/p
• SINDRUMII 10-8
• MELC 10-4
• Muon collider 0.3

2.0 T
5.0 T
Cu and W Heat shield
Target
Coils
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Water-Cooled Target Design

• Average beam power 23 kW
• Max. beam power 26 kW
• 1.8x1013 p/s (90 duty factor)
• Pt orAu cylinder
• L 16.0 cm, R 3.0 mm
• Water cooled
• Ti coolant cylinder 0.5 mm wall thickness
• 3.8 liter/min flow rate
• Max. temps
• tgt 124 C
• water 71 C

Steady State Temperature vs Axial Position
400
390
Target Center
380
370
360
350
Temperature (K)
340
Target/Water Interface
330
320
310
Water Channel Center
300
290
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
z (m)
397 K
Titanium Tube Inner Surface
Water Inlet
Target Core
293 K
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Transport Solenoidal Magnet

• Curved solenoid
• separates charges by charge sign
• reduces line-of-sight transport of neutrals
• Collimators eliminate wrong-sign particles and
  particles with too large momentum

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What we get at the Stopping Target

• 1/230 incident protons produce a muon at the
  stopping target
• 58 of muons stop in target
• 50x109 m stops per spill second
• 85,000 m stops per microbunch

• 17 Al disks
• each 200 mm thick
• 83 mm to 65 mm radius
• in graded magnetic field

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Note the Vast Difference in Muon Range

• The large spread in muon energy dictates a vacuum
  for the Detector Solenoid
• tracker must operate in vacuum
• pion absorber cannot be used

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Choice of StoppingTarget Material

• Large Z
• rate ? ZFp2 (Fp is the form factor)
• can reveal nature of interaction
• Small Z
• longer lifetime
• higher endpoint energy
• Note Need mZ-1 gt mZ to place max. energy of
  radiative capture muons below signal electrons

S, V, D dependence unique to mN?eN
Initial Choice
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Mu2e Detector

• Salient Features
• No detector element in region of transported beam
• Small acceptance for DIO electrons
• Minimal amount of material ? detector elements in
  vacuum

Electromagnetic calorimeter
Beam dump
Stopping target
Straw tracker
1T
2T
Proton absorber
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Magnetic Spectrometer

• Must operate in rates up to 200 kHz in individual
  detector elements
• Must operate in vacuum lt 10-3 Torr
• Must have low acceptance for DIO electrons
• Straw tubes 2,800, 5 mm diam., 2.6 m long, 25mm
  thick
• Cathode strips 17,000
• 50 geometrical acceptance 9030
• 0.2 MeV intrinsic energy resolution
• Resolution dominated by multiple scattering

20X DIO rate
End View
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Electromagnetic Calorimeter

• Needed for
• trigger 5 energy resolution ? 1,000 triggers/s
• particle ID
• confirm the electron position and energy
  measurements of the straws
• 2000 30x30x120mm3 PbWO4 crystals
• Dual APD readout

PWO-II (PANDA)
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Cosmic Ray Shield Design

• Passive
• 1 m thick concrete shielding blocks
• 0.50 m thick iron return yoke
• yet-to-be-determined overburden
• Active
• scintillator strips w embedded fibers
• three layers with 99 coverage
• 10-4 inefficiency

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What we Get
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Background Fractions
Blue text beam related.
Roughly half of background is interbunch
contamination related Total background per
3.4x1020 protons, 2x107 s 0.43 events Signal for
Rme 10-16 5 events
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Mu2e Status
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Pushing Beyond 10-17 Project X
10X increase in 8 GeV protons/s 1.8x1013 p/s
?1.6x1014 p/s
3 to MI 4 to 8 GeV program
7 Linac pulses per Main Injector cycle
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Project X Layout
To handle increased rates the Accumulator/Debunche
r would probably have to be replaced by a new 8
GeV ring
Milorad Popovic and Chuck Ankenbrandt
Mu2e
8 GeV Linac
Rare Ks
g-2
n factory
m test area
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Exploiting the Project X Rates

• Mu2e designed to run at rates 3X possible with
  Fermilab booster/accumulator/debuncher
• Going beyond 3X will be challenging
• A new muon production target and production
  solenoid are needed
• Individual straw rates gt500 kHz
• New ideas are needed to defeat backgrounds
• more monoenergetic muons
• ? thinner, fewer targets
• ? less target scatter, better energy resolution
• ? pion absorber for less pion stops
• COMET, PRISM
• helical cooling channel
• (Muons Inc)

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PRISM (J-PARC)

• Does two things for you
• Eliminates p capture and decay backgrounds
• Narrow the muon momentum spread to increase the
  stopping fraction and target scattering
• Phase Rotated Intense Slow Muon source proposed
  for J-PARC
• Fixed-Field Alternating Gradient synchrotron
  (FFAG) as phase rotator
• Intensity 1011 1012 m/s
• Kinetic energy 20 MeV (68 MeV/c)
• Momentum spread 30 ?3
• Beam rep rate 100-1000 Hz
• Osaka FFAG almost complete.
• Detailed simulations lacking
• No experiment approved

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Staged Approach
Kuno, Project X Workshop, Nov 2007
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Helical Cooling

• Muons, Inc. (Mu2e collaborators)

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Summary

• We live in an exciting time in particle physics ?
  we know something great, New Physics, is just
  around the corner
• Several ways to look for New Physics
• Direct Searches High-Sensitivity Searches
• The present and next generation of LFV
  experiments will make enormous sensitivity
  improvements ?even non-observation of LFV will be
  a great discovery
• New ideas are need to exploit the full beam power
  from proposed high-intensity accelerators

FNAL ? LHC 7X LEP ? ILC 2.5X
Mass reach
Mass reach 10X
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Expected Backgrounds
per 3.4x1020 protons, 2x107 s
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Selection Criteria
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Mu2e MECO Comparison