# Presentation: 103220

## A Muon to Electron Experiment at Fermilab

- Eric Prebys
- 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
- A Brief History of MECO
- Mu2e at Fermilab
- Now
- The future

## 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

## Search for Charged Lepton Flavor Violation (CLFV)

- Standard Model rate via Dirac neutrino mixing is too small to be observed (~10-52)
- Very common feature of Beyond Standard Model physics at much larger rates
- Similar to ewith important advantages:
- No combinatorial background
- Sensitive to other types of BSM physics
- Relative rate depends on details of physics

- 

- 105 MeV e-

- There has always been an interest in the search for charge lepton flavor violation (CLFV)
- 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

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

## e Conversion vs. e 

- This type of diagram gives rise to small CLFV through virtual neutrino mixing
- Also contributes to. e if photon real
- Relative rate easy to calculate

- Corresponds to exchange of a new, massive flavor-changing neutral current particle
- Does not produce e  signal

- In general, two types of diagrams contribute to +Ne+N:

- Dipole (“penguin”) transition:

- Four-fermi contact interaction:

## e Conversion Broadly Sensitive

- e Conversion Broadly Sensitive

- Courtesy: A. de Gouvea

- At Re~10-16 (first phase, this LOI), the sensitivity is already very compelling
- ~four orders of magnitude improvement over existing limit

- Sindrum II

- MEGA

- MEG proposal

## Some Example Sensitivies*

- *W. Molzon, Fermilab Wine and Cheese, 9/06

## 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

- 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

- ~100 ns

- ~1.5 s

- Prompt backgrounds

**live window**

## Detector Layout

- 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

## Beam Related Rates

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

## 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

## 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  |

## Brief History of MECOMu2e

- Brief History of MECOMu2e

- 1997
- MECO proposed for the AGS at Brookhaven
- Approved, along with KOPIO, as part of RSVP program
- 1998-2005
- Design refined
- Frequent favorable reviews
- 2005
- June: final reviews, very positive
- Physics goals: HEPAP RSVP Subpanel
- Cost and schedule: “Wojcicki Panel”
- July: RSVP cancelled for financial reasons
- MECO and KOPIO “charged” for entire cost of continued AGS operation.
- 2006
- January: First informal meeting at BNL
- September: First meeting at Fermilab
- 2007
- June: Mu2e expression of interest submitted to Fermilab Directorate
- August: First Mu2e collaboration meeting
- October: Letter of Intent submitted to Directorate

## Doing e Conversion at Fermilab

- Doing e Conversion at Fermilab

- Following the end of the collider program (~2010), the immediate focus on the lab will be the neutrino program, particularly the NOvA off-axis neutrino experiment.
- The proton intensity to the 120 GeV neutrino program is ultimately limited by the capacity of the Main Injector.
- If the current suite of proton source upgrades is effective, there should be at least enough excess 8 GeV protons to do an experiment with similar sensitivity to MECO in a reasonable amount of time
- The resonant operation of the 8 GeV Booster makes it impossible to directly generate the desired time structure.
- There is a scheme to generate this time structure using the antiproton Accumulator and Debuncher rings, which will become available after the termination of the collider program.
- This scheme requires only modest modifications beyond those planned for NOvA, *with which **it is fully compatible.*

## 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.

## 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

## 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)

## 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

## 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

## Proposed Location

- Requires new building.
- Minimal wetland issues.
- Can tie into facilities at MiniBooNE target hall.

## 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

## Post-Collider: The High Intensity Frontier

- There has long been an interest in an intense 8 GeV proton source to take the place of the aging Linac/Booster
- Increase power to long baseline neutrino program
- Provide excess capacity for diverse experimental program
- Many have favored an 8 GeV linac which could take advantage of superconducting RF technology developed for the ILC, *however*
- The number of protons needed for 2MW Main Injector operation corresponds to ~3 times the integrated single pulse intensity of the ILC
- This has significant implications for the design.
- Precludes “plug compatible”  design for high energy linac and ILC
- Also, if ion stripping is in the Main Injector, then the excess 8 GeV protons are in the form of “useless” 1 ms pulses of H- ions.

## “Project X” Scheme

- 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.

## 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**

## Future Upgrades to mu2e (Phase II)

- Our Phase I proposal is based on a data sample of 4x1020 protons, obtained with modest modifications to the existing complex
- 90% C.L. limit of  Re<6x10-17 (improvement over existing limit of more than 4 orders of magnitude).
- The Project X linac would provide roughly a *factor of ten* increase in excess 8 GeV flux.
- In addition to the increased flux, alternate solenoidal tranport channels have been proposed, based on the “helical cooling channel” which is being developed for the neutrino factory/muon collider
- Potentially a factor of five incrase in muon production efficiency
- Together, these could potentially produce enough stopped muons for a sensitivity on the order of Re~10-18
*However, there are many challenges*

## 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.

## 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

## Status and Plans

- The Mu2e Collaboration submitted a Of Intent in October, 2007, which was presented to the Fermilab PAC in November
- The PAC encouraged the collaboration to work toward a Phase I proposal in the Fall of 2008.
- We are working toward that goal with our primary efforts focused on:
- Resurrecting the MECO simulation software to adapt and upgrade it for our situation
- Continuing on the Accumulator/Debuncher beam delivery scheme and beam line design
- We are actively involved in the Project X planning process, with an eye toward designing and upgraded experiment, and possibly designing parts of the experiment fo the higher rates from the beginning.

## Not a good time for concrete planning!

*(from Dep. Director Y-K Kim)*

## Conclusions

- The mu2e experiment is an opportunity for Fermilab to make an important measurement
- In the initial phase (without project X) we would either
- Reduce the limit for Re by more than four orders of magnitude (Re<6x10-17 @ 90% C.L.)
- 				OR
- Discover unambiguous proof of Beyond Standard Model physics
- This experiment benefits greatly from both the voluminous work done for the MECO proposal and by fortuitous configuration and availability of Fermilab accelerator components.
- With a combination of Project X and/or improved muon transport, we could either
- Extend the limit by up to two orders of magnitude
- 					OR
- Study the details of new physics