The Mu2e Experiment at Fermilab Jim Miller Boston University for the Mu2e Collaboration June 6, 2008

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 50 Scientists 11 Institutes Recent LOI for Stage I measurement favorably reviewed by the Fermilab PAC

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<4.3x (Ti), R<7x (Au) (SINDRUM II at PSI) ► Goals of Mu2e:  Stage I: R<6x (Al, 90% c.l.). An improvement over existing limit by four orders of magnitude!  Stage II: (Project X) R< (Al).

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  offers ultimate in LFV sensitivity because of experimental advantages over other reactions ► Detection of muon to electron conversion is an unmistakable signal of new physics

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 ~470 keV for Al ► 3 muon interactions can occur 1Muon to electron conversion  Conversion electron is monoenergetic 2muon can decay ► Endpoint of bound muon same as conversion energy: 3muon can capture on nucleus ► For muonic Aluminum, muon lifetime is 0.86  s; partial decay lifetime=2.2  s, partial capture lifetime=1.5  s

Some potential backgrounds 1 Electrons from decay of muon 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<6x  Vast majority of decay electrons are < 53 MeV, well below conversion electron energy- big experimental advantage over  This is an example of a ‘Delayed’ background 2 Radiative pion capture, followed by photon conversion, E  up to 140 MeV, E  up to 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 ~105 MeV 5 Cosmic rays- suppress with shielding and 4  veto

SINDRUM II Experimental Method ► Low energy beamline collects muons from pion decay; beam line length is 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- Conversion electrons + background from muon interactions ► Reject prompt electrons- could be from beam  radiative capture or beam electron ► ‘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 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.

The New Approach ► Based on the proposed MELC and MECO approach ► Go to a temporally narrow pulsed primary proton beam. Delay beginning of measurement period after proton injection 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 ~10 -9 ) to avoid  or e production leading to false conversion electrons during the measurement period. ► Select a stopping target having a muon lifetime which is matched to this delay time (Aluminum is a good choice: lifetime = 0.86  s). ► Beam pulse spacing of ~1.7  s is a good match for Al: collect data from  s after muon injection pulse ► Ideally, there would be a continuous stream of muon pulses, ~100% duty factor

6/3/ Production Solenoid Transport Solenoid Detector Solenoid Proton Target Shielding Muon Beam Collimators Tracker Calorimeter PionsElectrons Muons Muon Stopping Target Mu2e Muon Beamline Simulations with G4beamline (Code developed by Muons Inc)

6/3/ Proton Target and Superconducting Production Solenoid Proton Target Shielding (Copper) Pions Muons Target Shielding (Tungsten) Protons enter here B=5T B=2.5T

6/3/ Transport Solenoid ++ -- Collimator 3 (Copper) Collimator 3 (Copper) (Showing muons only)

6/3/ Detector Solenoid Tracker Calorimeter 105 MeV/c Electrons Muons Muon Stopping Target B=2 T B=1T

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 ► Tracker measures momentum of electrons to <1MeV FWHM: 2.6 m long, 0.5 cm ► 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.

15 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)

Proton flux, per second muon stopping rate, per second 1.8x x10 10 Running time2x10 7 s Total protons per year3.6x10 20   stops/incident proton   capture probability 0.60 Time window fraction0.49 Electron trigger efficiency0.90 Reconstruction and selection efficiency 0.19 Detected events for R  e = Event Rates (Stage I, from Booster)

Time Scale ► Stage I, Booster-era, ~20-25 kW proton beam  MECO proposal is baseline design, vetted in several reviews  Readiness determined primarily by the four to five years to construct the solenoidal beam line after funding is available  Commissioning + data collection, ~3-4 years  LOI in Fall 2007, strong physics stamp of approval from PAC  Full proposal being prepared now for Fall 2008 PAC  Plan for a reasonable upgrade path to Stage II ► Stage II, Project X, 200 kW proton beam, R<  Depends on Project X schedule and lessons learned in Stage I  Extensive upgrade studies will be 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 improved energy resolution

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(p beam)- ideas exist- needs to be developed and built ► Muon stopping rate monitor- Measure xray rate from muonic aluminum ► Solenoid 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< 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! Contact persons: Jim Miller: Bob Bernstein:

End

Conclusions ► Muon to electron conversion is a powerful probe of new physics, complementary to LHC ► A baseline design exists on paper. It needs to be updated and holes filled in. Need prototypes; much development work remains ► P5 has given a strong endorsement, and FNAL is gearing up to do this experiment.

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

High Flux Muon Beam ► Following the idea from the MELC and MECO proposals, 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. ► Negative gradient causes p(longtudinal) to increase as particle moves downstream ► Use an ‘S’ shaped solenoid to transport the beam to the aluminum stopping target  Avoids line-of-sight between detectors and production target: eliminate neutral bkg.  Curved (toroidal) solenoid sections move beam vertically depending on charge and momentum: select low energy negative particles, attenuate everything else. ► Rates- Booster Era: 8 GeV protons, 23 kW average current, 4x10 10 stopped muons/s, (Project X: 10 times more beam)

Mu2e Muon Beam Line and Detector

Toroidal section: vertical separation of  + and  -

Production Solenoid: pions, muons, electrons

Mu2e Solenoids

 e Conversion vs.  e  34 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 Initial mu2e Goal   - N  e - N  +  e +   +  e + e + e - K 0    + e - K +    +  + e - SINDRUM II Initial MEG Goal 

36 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)

37 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 R(  Ti  eTi) Br(  e  ) R(  Ti  eTi)