# Presentation: 506258

## Mu2e Experiment*

- Eric Prebys
- For the Mu2e Collaboration

- *A Letter of Intent to Search for Charged Lepton Flavor Violation in Nuclear Muon Capture

## Goals of Experiment

- Initial Phase (MECO design, No Project X):
- Exploit NOvA accelerator modifications and post-Run II availability of Accumulator and Debuncher rings to mount a ->e conversion experiment patterned after MECO
- 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
- Exploit Project X and improved muon transport to achieve dramatically increased sensitivity
- If no signal: set limit Re<1x10-18
- If signal: measure target dependence, etc

## Mu2e Collaboration

- Currently:   50 Scientists   11 Institutions

- *Co-contact persons

**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*

## 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-

- The discovery of neutrino oscillations naturally raises the question:
- What is the rate of charged lepton flavor violation in nature?
- 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

## Broadly Sensitive to New High Energy Dynamics

- Courtesy: A. de Gouvea

- At Re~10-16 (first phase, this LOI), the sensitivity is already very compelling, well above the reach of colliders
- At 10-18 (potentially, with upgraded apparatus and higher muon flux), energy scales probed would be difficult to access by other means

- Sindrum II

- MEGA

## Specific Model Examples

- 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)

## 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

## 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

## Mu2e History

- 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

## Mu2e at Fermilab

- If the current suite of proton source upgrades is effective, there should be at least enough excess 8 GeV protons during the NOvA era 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.*

## Available Protons: NOvA Timeline

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

## 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 septum and Lambertson 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

## Mu2e and Project X

- We have described the initial phase of mu2e, which is based on the proposed data sample of 4x1020 protons.
- 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 flux.
- Slow extraction directly from Recycler ruled out by Project X Working Group
-  Will need to load beam from Recycler to Accumulator as we are planning to do for Phase I
- A preliminary scheme to exploit this additional flux will be included in our proposal.

## Mu2e and Muon Collider/Neutrino Factory

- There are a number of synergies between this project and muon cooling efforts
- The Debuncher beam could be extracted in a single turn to produce the short, intense bunch needed by muon production experiments
- Muon cooling studies have increased the understanding of solenoidal transport.
- It is possible that a “helical cooling channel”, of the sort envisioned for muon cooling, could generate a significantly higher muon yield for this experiment.
- We will investigate these in more detail for the proposal.

- A combination of increased flux from Project X and a more efficient muon transport line could potentially result in a sensitivity as low as 10-18

## 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
- Improve momentum resolution for the ~100 MeV electrons to reject high energy tails from ordinary DIO electrons.
- Operate with higher background levels.
- 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

## Required Resources for Proposal

- ~$100K of FESS time
- for a preliminary cost estimate of the experimental facility and beam line civil construction
- ~1/2 FTE beam line design expert
-  to produce a preliminary design of the primary proton line, including extinction channel
- ~1/2 FTE of ES&H radiation safety expert
-  to help us produce a plan to deal with the increased flux in the pBar enclosure (VERY important!)
- ~1/2 FTE of a TD magnet expert
- to evaluate the MECO magnet design, and advise of possible improvements.
- A dedicated postdoc and guest scientist position
- Focus on Monte Carlo work.
- Could also be supplemented with PPD resources

## 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

## Backup Slides

## 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 Scheme

**Accumulator **
- Momentum stack 1 to 6 booster batches
- Adiabatically bunch at h=1- **4kV**
- 500ns gap for kicker
- Some beam/halo cleaning in Acc and transfer
- Adiabatic easier at T=5.5
- Transfer to Debuncher
**Debuncher**
- h=1 90-degree phase rotation at fixed voltage
**40kV** - 0.007s
- Capture and store at h=4
- ~**200** to 250 kV ~0.02s;   = 42ns
- Beam halo cleaning also  ....

## Attractive Features of Debuncher

- Large Acceptance
- Low chromaticity
- Long, dispersion-free segments
- Lots of open straight sections after cooling hardware removed
- Problem:
- Getting protons there

- Production Region

- Axially graded 5 T solenoid captures 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 long transit time trajectories

**Collimators**

**Curvature Drift**

- 2.5 T

- 2.1 T

## Detector Region

- 1 T

- 1 T

- 2 T

- Axially-graded field near stopping target to increase acceptance and       reduce cosmic ray background
- 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**

## Expected Sensitivity of the MECO Experiment

- We expect ~ 5 signal events for 10**7** s (2800 hours) running if Re = 10**-16**

| Contributions to the Signal Rate | Factor |
| --- | --- |
| Running time (s)  | 107 |
| Proton flux (Hz) (50% duty factor, 740 kHz micropulse) | 4 1013 |
|  entering transport solenoid / incident proton  | 0.0043 |
|  stopping probability | 0.58 |
|  capture probability | 0.60 |
| Fraction of  capture in detection time window |  0.49 |
| Electron trigger efficiency | 0.90 |
| Fitting and selection criteria efficiency | 0.19 |
| Detected events for Re = 10-16 | 5.0 |

## Expected Background in MECO Experiment

- We expect ~ 0.45 background events for 107 s running with sensitivity of  ~ 5 signal events for Re = 10-16

| 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  |