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
• University of Minnesota
• 6 May 2008

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The Standard Model

• One of the great scientific achievements of all
  time
• Allows us to understand nature at the most
  fundamental level
• Allows us to look back into the beginning of time
  itself

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Standard Model Incomplete New Physics

• Theory
• EW symmetry breaking ? Higgs?
• Quantum theory of gravity ? strings?
• Hierarchy problem ? SUSY?
• Cosmology
• Non-zero baryon number of the universe ? baryon
  nonconservation, new sources of CP violation
• Dark matter ? new particles, SUSY?
• Dark energy
• Experiment
• Neutrino mass
• Other hints, g-2, NuTeV, etc.

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Quark Flavor not Conserved
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What about Lepton Flavor?
Neutrino Oscillations ? neutrino flavor not
conserved
Lepton flavor is conserved at the charged W
vertex because the neutrinos in the theory are
assumed massless
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Charged Current Performs Alchemy
Al ? Mg 24 mg
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What about the Neutral Current?
Flavor changing neutral currents do not occur!
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Why Search for Charged Lepton Flavor Violation?

• In Standard Model not there
• Neutrino oscillation 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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Where to Search for Lepton Flavor Violation

• Many different techniques have been used
• Most sensitive searches come from
• muon decays
• kaon decays
• Muon decays more promising for future searches
  because muons (pions) are more copiously produced
• All proposed high-sensitivity searches are
  focused on muon decays

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History of Muon Flavor Violation Searches
Mu2e intends to improve sensitivity by 10,000!
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m?eg vs m-N?e-N
Model independent CLFV Lagrangian
L gt 3,000 TeV!
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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Two Methods are Complementary
Observation of CLFV in both m-N?e-N and m?eg
could elucidate SUSY parameters
MSSM/msugra/seesaw
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Example Sensitivities
Supersymmetry
Compositeness
Predictions at 10-15
Second Higgs doublet
Heavy Neutrinos
Heavy Z, Anomalous Z coupling
Leptoquarks
After W. Marciano
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Specific Model Examples
SU(5) GUT SUSY (k ltlt 1)

• Examples with k ltlt 1 (no m?eg signal)
• Leptoquarks
• Z-prime
• Compositeness
• Heavy neutrino

Randall-Sundrum (k 1)
Littlest Higgs (k 1)
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Isidoris Summary Talk from HQL-2006
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For Many Processes t?mg Much More Sensitive
However, SuperB Factories will be needed to make
them competitive with muon LFV searches
Calibbi, Faccia, Masiero, Vempati SUSY-GUT model
with seesaw
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What about m?eee-?

• Currently best limit 1.0x10-12 (SINDRUM I)
• In general much less sensitive to new physics
• B(m?eee) lt B(mN ? eN) or B(m ? eg)

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m?eg

• Signature
• time coincident back-to-back electron and photon
• Ee Eg 52.8 MeV

• Backgrounds
• m?enmne random g
• m?enmneg

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MEG Experiment at PSI

• Goal BR(m?eg) 10-13 100X MEGA limit
• Running shortly!
• 1x108 m/s, t2.2x107 s/yr, W/4p0.09
• 100 duty factor
• Hope to push sensitivity down an order of
  magnitude further

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m-N?e-N

• Signature
• muon stopped in atom
• rapidly (10-16s) cascades down to the 1S state
• coherently interacts with nucleus leaving it in
  ground state
• single isolated electron
• Ee mm ENR - Eb 104.97 MeV (Al)

• Backgrounds
• muon decay in orbit
• m-NA,Z?e-nmneNA,Z Ee lt mc2-ENR-Eb
• radiative muon capture
• m-NA,Z?nmgNA,Z-1, g?ee- mZ-1 gt mZ
• radiative pion capture
• p-NA,Z?gNA,Z-1 , g?ee-

Eelt½mmc2
E(E-E0)5
Measure ratio of conversion rate to capture rate
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SINDRUM II Result

• Best limits on
• m?ee-e 1.2x10-11 (SINDRUM I)
• m-N?e-N 4.3x10-12 (SINDRUM II)
• Rate limited by need to veto prompt backgrounds!
• Note not background limited

High energy tail of coherent Decay-in-orbit (DIO)
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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, 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
Currently 50 scientists 11 institutions
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Signal

• Need good energy resolution ? 0.200 MeV
• Need particle ID
• Need bunched muon beam 50x109 sm/s
• Need turn off detector for tmN
• Need lt 10-9 interbunch contamination

Single, monoenergetic electron with E 105 MeV,
coming from the target, produced 1 ms (tmAl
864ns) after the m is stopped in the foils
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Producing 1018 Bunched Muons
Recycler
Main Injector

• Energy
• Fermilab produces beams with energies
• Booster 8 GeV, 1/15 s or 0.067 s
• Main Injector 150 GeV 20/15 s or 1.33 s
• Tevatron 900 GeV
• 8 GeV booster energy is the optimal
• Any higher energy produces too many anti-protons
• Structure
• Beam must be bunched with spacing on the order of
  the muon lifetime 1ms

Debuncher
Accumulator
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Boomerang Scheme
new detector hall and beamline
Minimal modifications only a new switch magnet
needed
6 batches x 4x1012 /1.33 s x 2x107 s/yr
3.6x1020 protons/yr
Cycle time determined by Main Injector magnet
ramp rate
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Rebunching in Accumulator/Debuncher
Momentum stack 6 Booster batches directly in
Accumulator (i.e. reverse direction)
Capture in 4 kV h1 RF System. Transfer to
Debuncher
Phase Rotate with 40 kV h1 RF in Debuncher
Recapture with 200 kV h4 RF system
st40 ns
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Resonant Extraction

• 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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Several Possible Locations for Experimental Hall

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

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Mu2e Apparatus
for every incident proton 0.0025 m-s are stopped
in the 17 0.2 mm Al target foils
MECO spectrometer 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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Production Solenoid

• R 75 cm
• 23kW beam
• 0.8 mm x 160 mm water-cooled 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

2.0 T
5.0 T
Cu and W Heat shield
Target
Coils
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Solenoidal Magnetic Fields

• 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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Choice of StoppingTarget Material

• Rate ? ZFp2, where Fp is the form factor
  describing the nuclear charge
• Advantage in larger rate with higher Z targets
  offset by shorter lifetime
• Binding energy also increases with Z for Au it
  is 10.08 MeV, Ec95.56
• Need mZ-1 gt mZ to place max. energy of radiative
  capture muons below signal electrons

S, V, D dependence unique to mN?eN

• 17 Al disks, separated by 50 mm
• each 200 mm thick
• 83 mm to 65 mm radius
• in graded magnetic field

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What we get at the Stopping Target

• 0.0043 m incident on target per proton on
  production target
• 0.58 stops in target
• 50x109 m stops per spill second
• 85,000 m stops per microbunch

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Mu2e Detector

• No detector element in region of tranported 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
• Straw tubes 2,800, 5 mm diam., 2.6-3.0 m long,
  25mm thick
• Cathode strips 17,000
• 0.2 MeV energy resolution
• Resolution dominated by multiple scattering
• 50 geometrical acceptance 9030
• Most DIO electrons miss straw vanes

End View
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Electromagnetic Calorimeter

• Needed for
• trigger 5 energy resolution
• particle ID
• confirm the electron position and energy
  measurements of the straws
• 2000 30x30x120mm3 PbWO4 crystals
• APD readout

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What we Get
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Three Types of Backgrounds
1. Stopped Muon Induced Backgrounds

• Muon decay in orbit
• m- ? e-nn
• Ee lt mmc2 ENR EB
• N ? (E0 - Ee)5
• Fraction within 3 MeV of endpoint ? 5x10-15
• Defeated by good energy resolution
• Radiative muon capture
• m-Al ? gnMg
• g endpoint 102.5 MeV
• 10-13 produce e- above 100 MeV
• Defeated by good energy resolution

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Backgrounds (continued)
2. Beam Related Backgrounds

• Muon decay in flight
• m- ? e-nn
• Since Ee lt mmc2/2, pm gt 77 GeV/c
• Radiative p- capture
• p-N ?Ng, gZ ? ee-
• Beam electrons
• Pion decay in flight
• p- ? e-ne
• Suppressed by minimizing beam between bunches
• Need ? 10-9 extinction
• Get 10-3 for free

3. Time Dependent Background

• Cosmic rays
• suppressed by active and passive shielding

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Background Fractions
Blue text interbunch related
Roughly half of background is beam related, and
half 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 History and Status
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Pushing Beyond 10-17 Project X
10X increase in 8 GeV protons/s 1.8x1013 p/s
?2.3x1014 p/s
120 GeV fast extraction spill 1.5 x 1014
protons/1.4 sec 2 MW
8 GeV extraction 1 second x 2.25 x 1014
protons/1.4 sec 200 kW
Recycler 3 linac pulses/fill
Main Injector 1.4 sec cycle
8 GeV H- Linac 9mA x 1 msec x 5 Hz
Stripping Foil
Single turn transfer _at_ 8 GeV
0.6-8 GeV ILC Style Linac
0.6 GeV Front End Linac
3 to MI 4 to 8 GeV program
7 Linac pulses per Main Injector cycle
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Two Project X Workshops held at Fermilab
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Project X Layout
Project X would include a rich program of
neutrino, muon, and kaon physics
Mu2e
8 GeV Linac
Rare Ks
g-2
n factory
m test area
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Exploiting the Project X Rates

• Getting an order of magnitude improvement is
  easy
• Mu2e designed to run at rates 3X possible using
  the Fermilab booster/accumulator/debuncher.
• 4 year run at 3X intensity would provide 2x10-18
  single event sensitivity if backgrounds can be
  kept at bay.
• If signal seen at lower rates in booster era,
  Mu2e can
• Explore target dependence
• Improve precision on Rme
• Going beyond 2x10-18 will be challenging
• A new muon production target and production
  solenoid are needed to handle increase heating
  and radiation loads
• Individual straw rates gt500 kHz
• Backgrounds may prove to be insurmountable ?
  senstivity scales as square root rather than
  linearly with NmC
• New ideas are needed!

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PRISM (J-PARC)

• Two ways to increase the rates of a muon capture
  experiment
• Increase distance from production target to
  stopping target to improve m/p ratio
• Narrow the muon momentum spread to increase the
  stopping rate
• 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
• PRISM FFAG under construction!
• 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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Slide from Oddones P5 Presentation
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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
• All possible avenues need be explored!
• Mu2e quest for charged lepton flavor violation
  greatly expand our window into New Physics

FNAL ? LHC 7X LEP ? ILC 2.5X
Mass reach
Mass reach 10X
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It will be a Campaign not an Experiment!
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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
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Stacking the Booster Beam

• Inject a booster batch every 67 ms into the
  accumulator
• Accelerate to the core orbit where it is is
  merged and debunched
• Momentum stack with 3-6 booster batches
• Transfer to debuncher and bunch into a single
  40ns wide bunch

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Project X Physics
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Relative Rates in Microbunch
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Radiative Muon Capture Gamma Spectrum
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mN?eN / m?eg
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(No Transcript)
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Pion Arrival Time at Stopping Target
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Radiative Muon Capture
m-NA,Z?gnmNA,Z-1
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Measuring the Resolution Function

• Need to know Gaussian shape and non-Gaussian
  tails
• Measure the shape of the DIO endpoint spectrum
• Look at monoenergetic electrons from p ? e n
• only 70 MeV ? need to reverse magnet polarity and
  reduce field (not easy to do!)
• note positive beam more likely quieter than
  negative beam!
• BR 1 x 10-4
• t 26 ns ? not many left
• Look at spectrum as a function of beam intensity
• Beat part of the tracking detector against the
  other part
• Insert electrons with a 100 MeV LINAC

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Beam Electrons
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Mu2e reach
Calibbi, Faccia, Masiero, Vempati SUSY-GUT model
with seesaw
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(No Transcript)
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The 26 Fundamental Constants of Nature

• masses of the 6 quarks up, down, charm strange,
  top bottom
• 4 parameters describing the CKM matrix
• masses of the 6 leptons electron, muon, tau, and
  their associated neutrinos
• 4 parameters describing the MNS matrix
• the mass of the Higgs boson
• the expectation value of the Higgs field
• the U(1) coupling constant
• the SU(2) coupling constant
• the strong coupling constant
• the cosmological constant