1. The Mu2e Experiment at Fermilab Jim Miller Boston University for the Mu2e Collaboration

2. What is the measurement? ► Detect charged lepton flavor non-conservation in the coherent, neutrinoless conversion of a muon to an electron in the field of a nucleus: ► Measure the ratio of conversion relative to ordinary muon capture on the nucleus: (where X=(A, Z-1); or (A’,Z’)+ protons, neutrons, gammas) (where X=(A, Z-1); or (A’,Z’)+ protons, neutrons, gammas) ► Current limits: R<4.3x10 -12 (Ti), R<7x10 -13 (Au) (SINDRUM II at PSI) ► Goals of Mu2e:  Stage I: R<6x10 -17 (Al, 90% c.l.). An improvement over existing limit by four orders of magnitude!  Stage II: (Project X) R<10 -18 (Al). Most sensitive foreseeable CLFV process.

3. Beyond the Standard Model ► Charged Lepton Flavor Violation (CLFV) is a nearly universal feature of extensions to the Standard Model, 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 and in some cases more powerful than 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: unmistakable signal of new physics

4. Example Sensitivities* Compositeness Second Higgs doublet Heavy Z’, Anomalous Z coupling Predictions at 10 -15 Supersymmetry Heavy Neutrinos Leptoquarks *After W. Marciano

5. The Method ► Muon beam line: Low energy pions decay to low energy muons ► Muon stops in an appropriate target, quickly forming a muonic atom with muon in 1s state ► Bohr radius (1s) much less than 1s orbit of innermost atomic electrons ► Bohr energy (1s) which is about 475 keV for Al ► 3 things can occur 1process of interest 2muon can decay ► Endpoint of bound muon same as conversion energy: 3muon can capture on nucleus ► For muonic Aluminum, muon lifetime is 0.88  s; partial decay lifetime=2.2  s, partial capture lifetime=1.5  s

6. Some potential backgrounds 1 Electrons from muon decay bound in atomic orbit: max energy is same as conversion electron energy  Probability falls rapidly near endpoint,  This background can be separated from conversion electrons with good electron energy resolution: Require <1 MeV FWHM for Mu2e, R<10 -16  Vast majority of decay electrons are < 53 MeV, well below conversion electron energy- big advantage over  This is an example of a ‘Delayed’ background 2 Radiative pion capture, followed by photon conversion, E  ~ 140 MeV, E  ~ 140 MeV  This is an example of a ‘Prompt’ background  Possibility of ~105 MeV conversion electrons strong suppression of pions is required 3 Flux of low energy protons, neutrons, gammas from ordinary muon capture on stopping target nuclei- can lead to tracking errors. 4 Beam electrons ~100 MeV  Suppress with collimators in muon beam line  Most traverse beam line quickly during muon injection 5 Cosmic rays- suppress with shielding and 4  veto

7. Past Experimental Method ► Beamline collects muons from pion decay; beam line length large enough so that most pions decay ► Pass beam through material to degrade energy: range out pions, muons continue on to stop in target ► Use a scintillator to detect charged particles entering stopping target. ► Detect electrons delayed relative to incident particle- includes DIO and conversion electrons ► Reject prompt electrons- could be prompt from beam  or e ► ‘One-at-a-time’ method limits rates to ~10 7 /s: It would take ~ 100 years to get the statistics necessary to approach Mu2e goal of R<10 -16

8. 8 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) After background suppression, there are no counts in the region of interest.

9. The New (Mu2e) Approach ► Based on MELC/MECO approach ► Go to a temporally narrow pulsed muon beam. Delay measurement period after pulse until almost all pions have decayed and other beam particles have dissipated (after about 700 ns). ► Establish high level of between-burst beam suppression (extinction) to avoid prompt background from  or e. ► Select a stopping target having a muon lifetime which is matched to this delay time (Aluminum is a good choice: lifetime = 0.88  s). ► Beam pulse spacing of ~ 1.7  s is a good match for Al: collect data from.7-1.7  s after muon injection pulse ► Ideally, there would be a continuous stream of muon pulses, ~100% d.f.

10. High Flux Muon Beam ► Following the idea from the MELC expt., use a negative gradient solenoid, 5 T to 2.5 T, around the production target to mirror upstream- going low energy pions and muons back downstream into the beam line ► Use an ‘S’ shaped solenoid to transport the beam to the aluminum stopping target  Avoids line-of-sight between detectors and production target  Curved (toroidal) solenoid sections move beam vertically depending on charge and momentum: select low energy negative particles, attenuate everything else. ► Place the aluminum stopping target in a negative field gradient, mirroring potential conversion electrons travelling upstream back toward the target, increasing acceptance. ► Rates: Booster Era: 23 kW average current, 4x10 10 stopped muons/s, (Project X: 10 times more beam)

11. Mu2e Muon Beam Line and Detector

23. Stopping Target and Detectors ► Solenoid, 1m radius, B=2 T-> 1T from 0 to 4 m, B=1 T from 4 to 10 m ► Negative field gradient at target creates mirror increasing detector acceptance. ► Stopping target: thin to reduce energy loss and loss of energy resolution ► Copious low energy charged particles (e.g. electrons from in-orbit muon decays) spiral down the hollow axis of the tracker, missing it entirely. ► Calorimeter after the tracker: provides fast trigger, confirms energy and position information on tracks.

24. 24 Delivering Protons: “Boomerang” Scheme ► Deliver beam to Accumulator/Debuncher enclosure with minimal beam line modifications and minimal civil construction. ► Use Booster batches which would not otherwise be used for NoVA Recycler (Main Injector Tunnel) MI-8 -> Recycler done for NOvA New switch magnet extraction to P150 (no need for kicker)

25. Proton flux, per second muon stopping rate, per second 1.8x10 13 5x10 10 Running time2x10 7 s Total protons per year3.6x10 20   stops/incident proton 0.0025   capture probability 0.60 Time window fraction0.49 Electron trigger efficiency0.90 Reconstruction and selection efficiency 0.19 Detected events for R  e = 10 -16 4.5 Event Rates

26. Time Scale ► Stage I, Booster-era, 20 kW proton beam  R<6x10 -17 : readiness determined primarily by the four to five years to construct the beam line after funding is available  Commissioning + data collection, ~3-4 years ► Stage II, Project X, 200 kW proton beam, R<10 -18  Depends on Project X schedule  Extensive upgrade studies needed: ► Primary target upgrade to handle increased heat load ► Production solenoid upgrade to handle increased heat and radiation loads on superconducting magnet. ► Improved extinction ► Improved detector to handle higher rates with impprved energy resolution

27. Tasks ► Tracker: two candidates, need R&D (simulations, prototypes) to choose ► Calorimeter: Lead tungstate is baseline- limited initial prototype work- need to evaluate new crystal materials, new photo-sensitive devices ► Cosmic ray veto system- candidate system has been proposed- needs to be developed- a challenge to get to 99.9% efficiency, 4  coverage ► Simulations:  Have GEANT3, working; full GEANT4 simulation being developed, > manpower  Beam line optimization, background studies ► Calibration systems for all detectors ► Extinction monitor- ideas exist- needs to be developed and built ► Muon stopping rate monitor- Measure xray rate from muonic aluminum ► Solenois magnets (big project): joint effort of physicists and engineers. Initial design work done, needs further development for full design. ► Develop readout electronics for calorimeter, tracker, cosmic veto,… ► Identify a viable upgrade path to get to R<10 -18 with Project X. ► Develop the proton source, with needed extinction ► …radiation shielding, building siting,… ► If you might be interested in working on some of these tasks, let’s talk!

28. End

29.  e Conversion vs.  e  29 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 

30. 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 - K 0    + e - K +    +  + e - SINDRUM II Initial MEG Goal  10 -4 10 -16

31. 31 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)

32. 32 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)