A Muon to Electron Experiment at Fermilab

1
A Muon to Electron Experiment at Fermilab

• Eric PrebysFermilab
• For the Mu2e Collaboration

2
Mu2e Collaboration
R.M. Carey, K.R. Lynch, J.P. Miller, B.L.
Roberts Boston University 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
Co-contact persons
3
Acknowledgement

• This effort has benefited greatly (and
  plagiarized shamelessly) from over a decade of
  voluminous work done by the MECO collaboration,
  not all of whom have chosen to join the current
  collaboration.

4
Outline

• Theoretical Motivation
• Experimental Technique
• Making Mu2e work at Fermilab
• Sensitivities
• Future Upgrades
• Conclusion

5
General

• The study or rare particle decays allows us to
  probe mass scales far beyond those amenable to
  direct searches.
• Among these decays, rare muon decays offer the
  possibility of experimentally clean and
  unambiguous evidence of physics beyond the
  current Standard Model.
• Such searches are a natural part of the
  Intensity Frontier, which is being proposed for
  Fermilab after the end of the current collider
  program.
• In the case of muon conversion, we can take
  advantage of a great deal of work that has
  already been done in the planning of the Muon to
  Electron Conversion Experiment (MECO), which was
  proposed at Brookhaven.

6
m-gte CLFV in the SM
First Order FCNC
Higher order dipole penguin

• Forbidden in Standard Model

• Observation of neutrino mixing shows this can
  occur at a very small rate
• Photon can be real (m-gteg) or virtual (mN-gteN)
• Standard model B.R. O(10-50)

7
Beyond the Standard Model

• Because extensions to the Standard Model couple
  the lepton and quark sectors, lepton number
  violation is virtually inevitable.
• Charged Lepton Flavor Violation (CLFV) is a
  nearly universal feature of such models, and the
  fact that it has not yet been observed already
  places strong constraints on these models.
• CLFV is a powerful probe of multi-TeV scale
  dynamics complementary to direct collider
  searches
• Among various possible CLFV modes, rare muon
  processes offer the best combination of new
  physics reach and experimental sensitivity

8
Generic Beyond Standard Model Physics
Flavor Changing Neutral Current
Dipole (penguin)

• Can involve a real photon
• Or a virtual photon

• Mediated by massive neutral Boson, e.g.
• Leptoquark
• Z
• Composite
• Approximated by four fermi interaction

9
Muon-to-Electron Conversion mN ? eN

• When captured by a nucleus, a muon will have an
  enhanced probability of exchanging a virtual
  particle with the nucleus.
• This reaction recoils against the entire nucleus,
  producing the striking signature of a
  mono-energetic electron carrying most of the muon
  rest energy

• Similar to m?eg??with important advantages
• No combinatorial background
• Because the virtual particle can be a photon or
  heavy neutral boson, this reaction is sensitive
  to a broader range of BSM physics
• Relative rate of m?eg and mN?eN is the most
  important clue regarding the details of the
  physics

10
m?e Conversion vs. m?eg

• We can parameterize the relative strength of the
  dipole and four fermi interactions.
• This is useful for comparing relative rates for
  mN?eN and m?eg

Courtesy A. de Gouvea
MEG proposal
Sindrum II
MEGA
11
History of Lepton Flavor Violation Searches
1
K0?? ?e- K?? ? ?e-
10-2
?- N ? e-N ? ? e? ? ? e e e-
10-4
10-6
10-8
10-10
SINDRUM II
10-12
Initial MEG Goal ?
10-14
Initial mu2e Goal ?
10-16
10-16
1940 1950 1960 1970
1980 1990 2000 2010
12
Example Sensitivities
Supersymmetry
Compositeness
Predictions at 10-15
Second Higgs doublet
Heavy Neutrinos
Heavy Z, Anomalous Z coupling
Leptoquarks
After W. Marciano
13
Sensitivity (contd)
SU(5) GUT Supersymmetry ? ltlt 1
Littlest Higgs ? ? 1
Randall-Sundrum ? ? 1
R(mTi?eTi)
R(mTi?eTi)

• Examples with kgtgt1 (no m?eg signal)
• Leptoquarks
• Z-prime
• Compositeness

10-10
MEG
10-10
10-12
10-12
10-14
10-14
10-16
10-16
mu2e
10-9
10-11
10-11
10-15
10-13
10-13
Br(m?eg)
Br(m?eg)
14
Decay in Orbit (DIO) Backgrounds Biggest Issue
Ordinary
Coherent

• Nucleus coherently balances momentum
• Rate approaches conversion (endpoint) energy as
  (Es-E)5
• Drives resolution requirement.

• Very high rate
• Peak energy 52 MeV
• Must design detector to be very insensitive to
  these.

15
Previous muon decay/conversion limits (90 C.L.)
m-gte Conversion Sindrum II
LFV m Decay
High energy tail of coherent Decay-in-orbit (DIO)

• Rate limited by need to veto prompt backgrounds!

16
Mu2e (MECO) Philosophy

• Eliminate prompt beam backgrounds by using a
  primary beam with short proton pulses with
  separation on the order of a muon life time
• Design a transport channel to optimize the
  transport of right-sign, low momentum muons from
  the production target to the muon capture target.
• Design a detector to strongly suppress electrons
  from ordinary muon decays

17
Signal
Single, monoenergetic electron with E105 MeV,
coming from the target, produced 1 ms (tmAl
880ns) after the m bunch hits the target foils

• Need good energy resolution ? 0.200 MeV
• Need particle ID
• Need a bunched beam with less than 10-9
  contamination between bunches

18
Choosing the Capture Target

• Dipole rates are enhanced for high-Z, but
• Lifetime is shorter for high-Z
• Decreases useful live window
• Also, need to avoid background from radiative
  muon capture

?Want M(Z)-M(Z-1) lt signal energy
?Aluminum is nominal choice for Mu2e
19
mu2e Muon Beam and Detector
for every incident proton 0.0025 m-s are stopped
in the 17 0.2 mm Al target foils
MECO spectrometer design
20

Production Region

• Axially graded 5 T solenoid captures low energy
  backward and reflected pions and muons,
  transporting them toward the stopping target
• Cu and W heat and radiation shield protects
  superconducting coils from effects of 50kW
  primary proton beam

Superconducting coils
2.5 T
5 T
Proton Beam
Heat Radiation Shield
Production Target
21
Transport Solenoid

• Curved solenoid eliminates
  line-of-sight transport of photons and neutrons
• Curvature drift and collimators sign and momentum
  select beam
• dB/ds lt 0 in the straight sections to avoid
  trapping which would result in long transit times

Collimators and pBar Window
2.1 T
2.5 T
22
Detector Region

• Axially-graded field near stopping target to
  sharpen acceptance cutoff.
• Uniform field in spectrometer region to simplify
  momentum analysis
• Electron detectors downstream of target to reduce
  rates from g and neutrons

Straw Tracking Detector
Stopping Target Foils
2 T
1 T
1 T
Electron Calorimeter
23
Magnetic Field Gradient
Production Solenoid
Transport Solenoid
Detector Solenoid
24
Transported Particles
Vital that e- momentum lt signal momentum
E3-15 MeV
25
Tracking Detector/Calorimeter

• 3000 2.6 m straws
• s(r,f) 0.2 mm
• 17000 Cathode strips
• s(z) 1.5 mm
• 1200 PBOW4 cyrstals in electron calorimeter
• sE/E 3.5
• Resolution .19 MeV/c

26
Sensitivity

• Rme 10-16 gives 5 events for 4x1020 protons on
  target
• 0.4 events background, half from out of time
  beam, assuming 10-9 extinction
• Half from tail of coherent decay in orbit
• Half from prompt

Coherent Decay-in-orbit, falls as (Ee-E)5
27
A long time coming
1992 MELC proposed at Moscow Meson Factory
1997 MECO proposed for the AGS at Brookhaven as part of RSVP (at this time, experiment incompatible with Fermilab)
1998-2005 intensive work on MECO technical design magnet system costed at 58M, detector at 27M
July 2005 RSVP cancelled for financial reasons
2006 MECO subgroup Fermilab physicists work out means to mount experiment at Fermilab
June 2007 mu2e EOI submitted to Fermilab
October 2007 LOI submitted to Fermilab
Fall 2008 mu2e submits proposal to Fermilab
2010 technical design approval start of construction
2014 first beam
28
Enter Fermilab

• Fermilab
• Built 1970
• 200 GeV proton beams
• Eventually 400 GeV
• Upgraded in 1985
• 900GeV x 900 GeV p-pBar collisions
• Most energetic in the world ever since
• Upgraded in 1997
• Main Injector-gt more intensity
• 980 GeV x 980 GeV p-pBar collisions
• Intense neutrino program
• Will become second most energetic accelerator (by
  a factor of seven) when LHC comes on line 2009
• What next???

29
The Fermilab Accelerator Complex
MiniBooNE/BNB
NUMI
30
Preac(cellerator) and Linac
New linac (HEL)- Accelerate H- ions from 116
MeV to 400 MeV
Preac - Static Cockroft-Walton generator
accelerates H- ions from 0 to 750 KeV.
Old linac(LEL)- accelerate H- ions from 750 keV
to 116 MeV
31
Booster

• Accelerates the 400 MeV beam from the Linac to 8
  GeV
• Operates in a 15 Hz offset resonant circuit
• Sets fundamental clock of accelerator complex
• From the Booster, 8 GeV beam can be directed to
• The Main Injector
• The Booster Neutrino Beam (MiniBooNE)
• A dump.
• More or less original equipment

32
Main Injector/Recycler

• The Main Injector can accept 8 GeV protons OR
  antiprotons from
• Booster
• The anti-proton accumulator
• The Recycler (which shares the same tunnel and
  stores antiprotons)
• It can accelerate protons to 120 GeV (in a
  minimum of 1.4 s) and deliver them to
• The antiproton production target.
• The fixed target area.
• The NUMI beamline.
• It can accelerate protons OR antiprotons to 150
  GeV and inject them into the Tevatron.

33
Present Operation of Debuncher/Accumulator

• Protons are accelerated to 120 GeV in Main
  Injector and extracted to pBar target
• pBars are collected and phase rotated in the
  Debuncher
• Transferred to the Accumulator, where they are
  cooled and stacked
• Not used for NOvA

34
Available Protons NOvA Timeline
MI uses 12 of 20 available Booster Batches per
1.33 second cycle
Preloading for NOvA
Recycler
Available for 8 GeV program
Recycler ? MI transfer
15 Hz Booster cycles
MI NuMI cycle (20/15 s)

• Roughly 6(4x1012 batch)/(1.33 s)(2x107
  s/year)3.6x1020 protons/year available

35
Delivering Protons Boomerang Scheme
MI-8 -gt Recycler done for NOvA
Recycler(Main Injector Tunnel)
New switch magnet extraction to P150 (no need for
kicker)

• Deliver beam to Accumulator/Debuncher enclosure
  with minimal beam line modifications and no civil
  construction.

36
Momentum Stacking

• Inject a newly accelerated Booster batch every 67
  mS onto the low momentum orbit of the Accumulator
• The freshly injected batch is accelerated towards
  the core orbit where it is merged and debunched
  into the core orbit
• Momentum stack 3-6 Booster batches

Tlt133ms
T134ms
37
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
38
Resonant Extraction

• Exploit 29/3 resonance
• Extraction hardware similar to Main Injector
• Septum 80 kV/1cm x 3m
• LambertsonC magnet .8T x 3m

39
Proposed Location

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

40
What we Get
Proton flux 1.8x1013 p/s
Running time 2x107 s
Total protons 3.6x1020 p/yr
m- stops/incident proton 0.0025
m- capture probability 0.60
Time window fraction 0.49
Electron trigger efficiency 0.90
Reconstruction and selection efficiency 0.19
Detected events for Rme 10-16 4.5
41
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

42
Backgrounds (continued)
2. Beam Related Backgrounds

• Suppressed by minimizing beam between bunches
• Need ? 10-9 extinction
• Get 10-3 for free
• 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

3. Asynchronous Backgrounds

• Cosmic rays
• suppressed by active and passive shielding

43
The Bottom Line
Blue text beam related.
Roughly half of background is beam related, and
half interbunch contamination related Total
background per 4x1020 protons, 2x107 s 0.43
events Signal for Rme 10-16 5 eventsSingle
even sensitivity 2x10-1790 C.L. upper limit
if no signal 6x10-17
44
Possible Future Project X

• Three 5 Hz pulses every 1.4 s Main Injector cycle
  2.3MW at 120 GeV
• This leaves four pulses (200 kW) available for 8
  GeV physics
• These will be automatically stripped and stored
  in the Recycler, and can also be rebunched there.

45
Experimental Challenges for Increased Flux

• Achieve sufficient extinction of proton beam.
• Current extinction goal directly driven by total
  protons
• Upgrade target and capture solenoid to handle
  higher proton rate
• Target heating
• Quenching or radiation damage to production
  solenoid
• Improve momentum resolution for the 100 MeV
  electrons to reject high energy tails from
  ordinary DIO electrons.
• Limited by multiple scattering in target and
  detector planes
• Requirements at or beyond current state of the
  art.
• Operate with higher background levels.
• High rate detector
• Manage high trigger rates
• All of these efforts will benefit immensely from
  the knowledge and experience gained during the
  initial phase of the experiment.
• If we see a signal a lower flux, can use
  increased flux to study in detail
• Precise measurement of Rme
• Target dependence
• Comparison with m?eg rate

46
However, the future has some uncertainty!
47
Conclusions

• We have proposed a realistic experiment to
  measure
• Single event sensitivity of Rme2x10-17
• 90 C.L. limit of Rmelt6x10-17
• This represents an improvement of more than four
  orders of magnitude compared to the existing
  limit, or over a factor of ten in effective mass
  reach. For comparison
• TeV -gt LHC factor of 7
• LEP 200 -gt ILC factor of 2.5
• Potential future upgrades could increase this
  sensitivity by one or two orders of magnitude
• ANY signal would be unambiguous proof of physics
  beyond the Standard Model
• The absence of a signal would be a very important
  constraint on proposed new models.

48
Backup Slides
49
Project X Linac
Single 3 MW JPARC Klystron
PULSED RIA Front End Linac 325 MHz 0-110 MeV
0.5 MW Initial 8 GeV Linac
Modulator
11 Klystrons (2 types) 449 Cavities 51
Cryomodules
Multi-Cavity Fanout at 10 - 50 kW/cavity Phase
and Amplitude Control w/ Ferrite Tuners
H-
RFQ
MEBT
RTSR
SSR
DSR
DSR
ßlt1 ILC LINAC
10 MW ILC Multi-Beam Klystrons
Modulator
1300 MHz 0.1-1.2 GeV
48 Cavites / Klystron
2 Klystrons 96 Elliptical Cavities 12 Cryomodules
ß.81
ß.81
ß.81
ß.81
ß.81
ß.81
8 Cavites / Cryomodule
ILC LINAC
8 Klystrons 288 Cavities in 36 Cryomodules
1300 MHz ß1
Modulator
Modulator
Modulator
Modulator
10 MW ILC Klystrons
36 Cavites / Klystron
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Modulator
Modulator
Modulator
Modulator
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50
Helical Cooling Channel
A helical cooling channel (similar to a Siberian
Snake) provides transverse cooling of muon beam
This, together with an ionizing degrader could
allow the forward muons to be used, for a much
higher efficiency.
51
The Big Picture Goals of Experiment

• Initial Phase
• Exploit post-collider accelerator modifications
  at Fermilab to mount a m-gte conversion experiment
  patterned after proposed MECO experiment at BNL
• 4x1020 protons in 2 years
• Measure
• Single event sensitivity of Rme2x10-17
• 90 C.L. limit of Rmelt6x10-17
• ANY signal Beyond Standard Model physics
• Ultimate goal
• Take advantage of intense proton source being
  developed for Fermilab (Project X) as well as
  muon collider RD
• If no signal set limit Rmelt1x10-18
• If signal measure target dependence, etc

52
Beam Related Rates

• Cut 700 ns after pulse to eliminate most serious
  prompt backgrounds.

53
Proton Timeline Now and Post-Collider
Present Operation
wasted loading time
15 Hz Booster cycles

• In order to increase protons to the NOvA neutrino
  experiment after the collider program ends,
  protons will be stacked in the Recycler while
  the Main Injector is ramping, thereby eliminating
  loading time.

54
Momentum Stacking

• Inject a newly accelerated Booster batch every 67
  mS onto the low momentum orbit of the Accumulator
• The freshly injected batch is accelerated towards
  the core orbit where it is merged and debunched
  into the core orbit
• Momentum stack 3-6 Booster batches

Tlt133ms
T134ms
55
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
56
Resonant Extraction

• Exploit 29/3 resonance
• Extraction hardware similar to Main Injector
• Septum 80 kV/1cm x 3m
• LambertsonC magnet .8T x 3m

57
Beam Extinction

• Need 10-9
• Get at least 10-3 from beam bunching
• Remainder from AC Dipole in beam line
• Working with Osaka (FNALUS-Japan funds) to
  develop AC dipole design, as well as explore
  measurement options

58
Expected Background (from MECO TDR)

• For 4x1020 protons on target

Source Events Comments
m decay in orbit 0.25 S/N 20 for Rme 10-16
Tracking errors lt 0.006
Radiative m decay lt 0.005
Beam e- lt 0.04
m decay in flight lt 0.03 Without scattering in stopping target
m decay in flight 0.04 With scattering in stopping target
p decay in flight lt 0.001
Radiative p capture 0.07 From out of time protons
Radiative p capture 0.001 From late arriving pions
Anti-proton induced 0.007 Mostly from p-
Cosmic ray induced 0.004 Assuming 10-4 CR veto inefficiency
Total Background 0.45 Assuming 10-9 inter-bunch extinction
Signal Events 5 For Rme 10-16
59
Cost and Time Scale

• A detailed cost estimate of the MECO experiment
  had been done just before it was cancelled
• Solenoids and cryogenics 58M
• Remainder of experimental apparatus 27M
• Additional Fermilab costs have not been worked
  out in detail, but are expected to be on the
  order of 10M.
• Hope to begin Accelerator work along with NOvA
  upgrades
• 2010 (or 2011 if Run II extended)
• Based on the original MECO proposal, we believe
  the experiment could be operational within five
  years from the start of significant funding
• Driven by magnet construction.
• 2014
• With the proposed beam delivery system, the
  experiment could collect the nominal 4x1020
  protons on target in about one to two years, with
  no impact on NOvA
• NOvA rate limited by Main Injector

Costs in 2005 dollars, including contingency
60
Lepton Number Conservation

• The concept of Lepton Number Conservation dates
  back to the earliest experiments and models for
  the Weak Interaction, originally involving only
  electrons and electron neutrinos. Example
• After the discovery of the muon, it was
  discovered that Lepton number was separately
  conserved for each lepton generation
• These conservation laws were an important
  constraint in formulating what is now the
  Standard Model

61
The Standard Model

• In the Standard Model, both quarks and leptons
  are arranged in generations.
• In weak eigenspace, the weak interaction causes
  transition within generations
• Because the mass eigenstates are superpositions
  of the weak eigenstates, transitions between
  physical generations can occur, iff
• The mixing element is nonzero
• The masses are nonzero (otherwise unitarity will
  force the amplitude to sum to zero)
• Thus, to first order (where neutrinos are equally
  massless), generational transtions are
• Allowed for quarks
• Forbidden for leptons

62
Producing 1018 m-
Note 8 GeV booster energy is the optimal energy
for mu2e muon beam
mu2e
6 batches x 4x1012 /1.33 s x 2x107 s/yr
3.6x1020 protons/yr
63
Beam Extinction

• Need 10-9
• Get at least 10-3 from beam bunching
• Remainder from AC Dipole in beam line
• Working with Osaka (FNALUS-Japan funds) to
  develop AC dipole design, as well as explore
  measurement options