# Presentation: 649180

## A Muon to Electron Experiment at Fermilab

- Eric PrebysFermilab
- For the Mu2e Collaboration

## Mu2e Collaboration

- Currently:   50 Scientists   11 Institutions

- *Co-contact persons

**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*

## 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.

## Outline

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

## 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.

## ->e CLFV in the SM

- Forbidden in Standard Model

- Observation of neutrino mixing shows this can occur at a *very small *rate
- Photon can be real (->e) or virtual (N->eN)
- Standard model B.R. ~O(10-50)

- First Order FCNC:

- Higher order dipole “penguin”:

## 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

## Generic Beyond Standard Model Physics

- Flavor Changing Neutral Current

- Mediated by massive neutral Boson, e.g.
- Leptoquark
- Z’
- Composite
- Approximated by “four fermi interaction”

- Dipole (penguin)

- Can involve a real photon
- Or a virtual photon

## Muon-to-Electron Conversion: +N e+N

- Muon-to-Electron Conversion: +N e+N

- Similar to e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 eand NeNis the most important clue regarding the details of the physics

- 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

## e Conversion vs. e 

- Courtesy: A. de Gouvea

- Sindrum II

- MEGA

- MEG proposal

- We can parameterize the relative strength of the dipole and four fermi interactions.
- This is useful for comparing relative rates for NeN and e

## History of Lepton Flavor Violation Searches

- 1

- 10-2

- 10-16

- 10-6

- 10-8

- 10-10

- 10-14

- 10-12

- 1940         1950        1960         1970         1980        1990        2000       2010

- Initial mu2e Goal ****

-     - N  e-N
-     +  e+
-     +  e+ e+ e-

-    K0 +e-
-    K+ + +e-

- SINDRUM II

- Initial MEG Goal ****

- 10-4

- 10-16

## Example Sensitivities*

- Compositeness

- Second Higgs doublet

- Heavy Z’,        Anomalous Z    coupling

- Predictions at 10-15

- Supersymmetry

- Heavy Neutrinos

- Leptoquarks

- *After W. Marciano

## Sensitivity (cont’d)

- Examples with >>1 (no e signal):
- Leptoquarks
- Z-prime
- Compositeness
- Heavy neutrino

- SU(5) GUT Supersymmetry:  << 1

- Littlest Higgs:   1

- Br(e)

- Randall-Sundrum:   1

- MEG

- mu2e

- 10-12

- 10-14

- 10-16

- 10-11

- 10-13

- 10-15

- R(TieTi)

- 10-13

- 10-11

- 10-9

- Br(e)

- 10-16

- 10-10

- 10-14

- 10-12

- 10-10

- R(TieTi)

## Decay in Orbit (DIO) Backgrounds: Biggest Issue

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

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

- Ordinary:

- Coherent:

## Previous muon decay/conversion limits (90% C.L.)

- Rate limited by need to veto prompt backgrounds!

- >e Conversion: Sindrum II

- LFV Decay:

- High energy tail of coherent Decay-in-orbit (DIO)

## 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

## Signal

- Single, monoenergetic electron with E=105 MeV, coming from the target, produced ~1 s (Al ~ 880ns) after the “” 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

## Choosing the Capture Target

- Aluminum is nominal choice for Mu2e

- 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) < signal energy

## mu2e Muon Beam and Detector

- MECO spectrometer design

- for every incident proton 0.0025 ’s are stopped in the 17 0.2 mm Al target foils

- 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**

**Production Target**

**Heat & Radiation Shield**

**Proton Beam**

- 5 T

- 2.5 T

## Transport Solenoid

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

**Collimators and pBar Window**

- 2.5 T

- 2.1 T

## Detector Region

- 1 T

- 1 T

- 2 T

- 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  and neutrons

**Stopping Target Foils**

**Straw Tracking Detector**

**Electron Calorimeter**

## Magnetic Field Gradient

- Production Solenoid

- Transport Solenoid

- Detector Solenoid

## Transported Particles

- E~3-15 MeV

- Vital that e- momentum < signal momentum

## Tracking Detector/Calorimeter

- 3000 2.6 m straws
- (r,) ~ 0.2 mm
- 17000 Cathode strips
- z) ~ 1.5 mm
- 1200 PBOW4 cyrstals in electron calorimeter
- E/E ~ 3.5%
- Resolution: .19 MeV/c

## Sensitivity

- Re = 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

## 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 |

## 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-> 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???

## The Fermilab Accelerator Complex

- MiniBooNE/BNB

- NUMI

## Preac(cellerator) and Linac

**“****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

**“****New linac” (HEL)-** Accelerate H- ions from 116 MeV to 400 MeV

## 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

## 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.

## 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

## Available Protons: NOvA Timeline

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

- MI uses 12 of 20 available Booster Batches per 1.33 second cycle

- Preloading for NOvA

- Available for 8 GeV program

**Recycler**

- Recycler  MI transfer

- 15 Hz Booster cycles

- MI NuMI cycle (20/15 s)

## Delivering Protons: “Boomerang” Scheme

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

- Recycler(Main Injector Tunnel)

- MI-8 -> Recycler done for NOvA

- New switch magnet extraction to P150 (no need for kicker)

## 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

- T<133ms

- T=134ms

## Rebunching in Accumulator/Debuncher

- Momentum stack 6 Booster batches directly in Accumulator (i.e. reverse direction)

- Capture in 4 kV h=1 RF System.
- Transfer to Debuncher

- Phase Rotate with 40 kV h=1 RF in Debuncher

- Recapture with 200 kV h=4 RF system

- t~40 ns

## Resonant Extraction

- Exploit 29/3 resonance
- Extraction hardware similar to Main Injector
- Septum: 80 kV/1cm x 3m
- Lambertson+C magnet ~.8T x 3m

## Proposed Location

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

## What we Get

| Proton flux | 1.8x1013 p/s |
| --- | --- |
| Running time | 2x107 s |
| Total protons | 3.6x1020 p/yr |
|  stops/incident proton | 0.0025 |
|  capture probability | 0.60 |
| Time window fraction | 0.49 |
| Electron trigger efficiency | 0.90 |
| Reconstruction and selection efficiency | 0.19 |
| Detected events for Re = 10-16 | 4.5 |

## Three Types of Backgrounds

- Muon decay in orbit:
-  → e
- Ee < mc2 – ENR – EB
- N  (E0  - Ee)5
- Fraction within 3 MeV of endpoint  5x10-15
- Defeated by good energy resolution
- Radiative muon capture:
- Al → Mg
-   endpoint 102.5 MeV
- 10-13 produce e- above 100 MeV

- 1. Stopped Muon Induced Backgrounds

## 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:
-  → e
- Since Ee < mc2/2, p > 77 GeV/c
- Radiative  capture:
- 	N →N*, Z → ee
- Beam electrons
- Pion decay in flight:
- 	 → ee

- 3. Asynchronous Backgrounds

- Cosmic rays
- suppressed by active and passive shielding

## The Bottom Line

- Roughly half of background is beam related, and half interbunch contamination related
- Total background per 4x1020 protons, 2x107 s:	0.43 events
- Signal for Re = 10-16:	5 eventsSingle even sensitivity:  	2x10-1790% C.L. upper limit if no signal:	6x10-17

- Blue text: beam related.

## 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.

## 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 Re
- Target dependence
- Comparison with e rate

## However, the future has some uncertainty!

## Conclusions

- We have proposed a realistic experiment to measure
- Single event sensitivity of Re=2x10-17
- 90% C.L. limit of Re<6x10-17
- ANY signal = Beyond Standard Model physics
- 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 -> LHC = factor of 7
- LEP 200 -> 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.

## Backup Slides

## Project X Linac

- β=1

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- Modulator

- 36 Cavites / Klystron

**ILC LINAC**

*8 Klystrons*
*288 Cavities in 36 Cryomodules *

- 1300 MHz  β=1

**β****<1**** ILC LINAC**

*2 Klystrons*
*96 Elliptical Cavities*
*12 Cryomodules** *

- _1300 MHz_     0.1-1.2 GeV

- β=1

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- β=1

- Modulator

- β=1

- β=1

- β=1

- β=1

- Modulator

- 10 MW
- ILC
- Multi-Beam
- Klystrons

- 48 Cavites / Klystron

- β=.81

- Modulator

- β=.81

- β=.81

- β=.81

- β=.81

- β=.81

- 8 Cavites / Cryomodule

**0.5 MW Initial 8 GeV Linac**

- 11  Klystrons  (2 types)
- 449 Cavities
- 51  Cryomodules

**“****PULSED RIA”**
- Front End Linac
- _325 MHz_
- 0-110 MeV

- H-

- RFQ

- MEBT

- RTSR

- SSR

- DSR

- Single
- 3 MW
- JPARC
- Klystron

- Multi-Cavity Fanout at 10 - 50 kW/cavity
- Phase and Amplitude Control w/ Ferrite Tuners

- DSR

- Modulator

**10 MW**
**ILC**
**Klystrons**

## 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.

## The Big Picture: Goals of Experiment

- Initial Phase:
- Exploit post-collider accelerator modifications at Fermilab to mount a ->e conversion experiment patterned after proposed MECO experiment at BNL
- 4x1020 protons in ~2 years
- Measure
- Single event sensitivity of Re=2x10-17
- 90% C.L. limit of Re<6x10-17
- 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 R&D
- If no signal: set limit Re<1x10-18
- If signal: measure target dependence, etc

## Beam Related Rates

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

## Proton Timeline: Now and Post-Collider

- 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.

- 15 Hz Booster cycles

- Present Operation:

- “wasted” loading time

## 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

- T<133ms

- T=134ms

## Rebunching in Accumulator/Debuncher

- Momentum stack 6 Booster batches directly in Accumulator (i.e. reverse direction)

- Capture in 4 kV h=1 RF System.
- Transfer to Debuncher

- Phase Rotate with 40 kV h=1 RF in Debuncher

- Recapture with 200 kV h=4 RF system

- t~40 ns

## Resonant Extraction

- Exploit 29/3 resonance
- Extraction hardware similar to Main Injector
- Septum: 80 kV/1cm x 3m
- Lambertson+C magnet ~.8T x 3m

## Beam Extinction

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

## Expected Background (from MECO TDR)

- For 4x1020 protons on target:

| Source | Events | Comments |
| --- | --- | --- |
|  decay in orbit | 0.25 | S/N = 20 for Re = 10-16  |
| Tracking errors | < 0.006 |  |
| Radiative  decay  | < 0.005 |  |
| Beam e- | < 0.04 |  |
|  decay in flight | < 0.03 | Without scattering in stopping target |
|  decay in flight |  0.04 | With scattering in stopping target |
| decay in flight | < 0.001 |  |
| Radiative capture | 0.07 | From out of time protons |
| Radiative capture | 0.001 | From late arriving pions |
| Anti-proton induced | 0.007 | Mostly from  |
| 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 Re = 10-16  |

## 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

## 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”

## 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

## Producing ~1018 

- Producing ~1018 

- 6 batches x 4x1012 /1.33 s x 2x107 s/yr
- = 3.6x1020 protons/yr

- mu2e

- Note: 8 GeV booster energy is the optimal energy for mu2e muon beam

## Beam Extinction

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