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A Muon to Electron Experiment at Fermilab

A Muon to Electron Experiment at Fermilab. Eric Prebys* For the Mu2e Collaboration. *U of A ’84 (Eng. Phys.). 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

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A Muon to Electron Experiment at Fermilab

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  1. A Muon to Electron Experiment at Fermilab Eric Prebys* For the Mu2e Collaboration *U of A ’84 (Eng. Phys.)

  2. 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 Currently: 50 Scientists 11 Institutions *Co-contact persons

  3. 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.

  4. Outline • Theoretical Motivation • Experimental Technique • Making Mu2e work at Fermilab • Sensitivities • Future Upgrades • Conclusion

  5. 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.

  6. 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”

  7. 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

  8. m->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 (m->eg) or virtual (mN->eN) • Standard model B.R. ~O(10-50) First Order FCNC: Higher order dipole “penguin”: Virtual n mixing

  9. 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

  10. ? ? ? Generic Beyond Standard Model Physics • Mediated by massive neutral Boson, e.g. • Leptoquark • Z’ • Composite • Approximated by “four fermi interaction” • Can involve a real photon • Or a virtual photon Flavor Changing Neutral Current Dipole (penguin) ? ? ?

  11. Similar to meg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 meg and mNeNis the most important clue regarding the details of the physics Muon-to-Electron Conversion: m+N e+N • 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 m 105 MeV e-

  12. ? ? ? me Conversion vs. meg • We can parameterize the relative strength of the dipole and four fermi interactions. • This is useful for comparing relative rates for mNeN and meg Courtesy: A. de Gouvea MEG proposal Sindrum II MEGA

  13. History of Lepton Flavor Violation Searches 1 K0+e- K+++e- 10-2 - N  e-N +  e+ + e+ e+ e- 10-4 10-6 10-8 10-10 SINDRUM II 10-12 Initial MEG Goal 10-14 Initial mu2e Goal  10-16 10-16 19401950 1960 1970 1980 1990 2000 2010

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

  15. Examples with k>>1 (no meg signal): Leptoquarks Z-prime Compositeness Heavy neutrino Sensitivity (cont’d) SU(5) GUT Supersymmetry:  << 1 Littlest Higgs:   1 Randall-Sundrum:   1 R(mTieTi) R(mTieTi) 10-10 MEG 10-10 10-12 10-12 10-14 10-14 10-16 10-16 mu2e 10-9 10-11 10-11 10-15 10-13 10-13 Br(meg) Br(meg)

  16. 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: N

  17. Previous muon decay/conversion limits (90% C.L.) m->e Conversion: Sindrum II • Rate limited by need to veto prompt backgrounds! LFV m Decay: High energy tail of coherent Decay-in-orbit (DIO)

  18. 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 ms Prompt backgrounds live window

  19. Signal Single, monoenergetic electron with E=105 MeV, coming from the target, produced ~1 ms (tmAl ~ 880ns) after the “m” 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

  20. Nucleus Re(Z) / Re(Al) Bound lifetime Atomic Bind. Energy(1s) Conversion Electron Energy Prob decay >700 ns Al(13,27) 1.0 .88 s 0.47 MeV 104.97 MeV 0.45 Ti(22,~48) 1.7 .328 s 1.36 MeV 104.18 MeV 0.16 Au(79,~197) ~0.8-1.5 .0726 s 10.08 MeV 95.56 MeV negligible Choosing the Capture Target • 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 Aluminum is nominal choice for Mu2e

  21. mu2e Muon Beam and Detector for every incident proton 0.0025 m-’s are stopped in the 17 0.2 mm Al target foils MECO spectrometer design

  22. 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 2.5 T 5 T Proton Beam Heat & Radiation Shield Production Target

  23. 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 2.1 T 2.5 T

  24. Detector Region • 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 g and neutrons Straw Tracking Detector Stopping Target Foils 2 T 1 T 1 T Electron Calorimeter

  25. Magnetic Field Gradient Production Solenoid Transport Solenoid Detector Solenoid

  26. Transported Particles Vital that e- momentum < signal momentum E~3-15 MeV

  27. Tracking Detector/Calorimeter Coherent Decay-in-orbit, falls as (Ee-E)5

  28. A long time coming

  29. 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???

  30. The Fermilab Accelerator Complex MiniBooNE/BNB NUMI

  31. microBooNE, August 20th, 2007 - Prebys Preac(cellerator) and Linac “New linac” (HEL)- Accelerate H- ions from 116 MeV to 400 MeV “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

  32. 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

  33. 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.

  34. 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

  35. Producing ~1018m- Note: 8 GeV booster energy is the optimal energy for mu2e muon beam mu2e 6 batches x 4x1012 /1.33 s x 2x107 s/yr = 3.6x1020 protons/yr

  36. Requires new building. Minimal wetland issues. Can tie into facilities at existing experimental hall. Proposed Location

  37. What we Get

  38. Three Types of Backgrounds 1. Stopped Muon Induced Backgrounds • Muon decay in orbit: m-→ e-nn • Ee < mmc2 – ENR – EB • N  (E0 - Ee)5 • Fraction within 3 MeV of endpoint  5x10-15 • Defeated by good energy resolution • Radiative muon capture: m-Al→ gnMg • g endpoint 102.5 MeV • 10-13 produce e- above 100 MeV

  39. 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: • m-→ e-nn • Since Ee < mmc2/2, pm > 77 GeV/c • Radiative p- capture: • p-N→N*g, gZ → e+e- • Beam electrons • Pion decay in flight: • p-→ e-ne 3. Asynchronous Backgrounds • Cosmic rays • suppressed by active and passive shielding

  40. The Bottom Line Blue text: beam related. Roughly half of background is beam related, and half interbunch contamination related Total background per 3.4x1020 protons, 2x107 s: 0.43 events Signal for Rme = 10-16: 5 eventsSingle even sensitivity: 2x10-1790% C.L. upper limit if no signal: 6x10-17

  41. 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.

  42. 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 Rme • Target dependence • Comparison with meg rate

  43. However, the future has some uncertainty! (from Dep. Director Y-K Kim)

  44. Conclusions • We have proposed a realistic experiment to measure • Single event sensitivity of Rme=2x10-17 • 90% C.L. limit of Rme<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.

  45. Backup Slides

  46. Project X Linac Single 3 MW JPARC Klystron “PULSED RIA” Front End Linac 325 MHz 0-110 MeV 0.5 MW Initial 8 GeV Linac Modulator 11 Klystrons (2 types) 449 Cavities 51 Cryomodules Multi-Cavity Fanout at 10 - 50 kW/cavity Phase and Amplitude Control w/ Ferrite Tuners H- RFQ MEBT RTSR SSR DSR DSR Modulator β<1 ILC LINAC 10 MW ILC Multi-Beam Klystrons Modulator Elliptical Option 1300 MHz0.1-1.2 GeV β=.47 β=.47 β=.61 β=.61 β=.61 β=.61 48 Cavites / Klystron 2 Klystrons 96 Elliptical Cavities 12 Cryomodules or… 325 MHz Spoke Resonators β=.81 β=.81 β=.81 β=.81 β=.81 β=.81 8 Cavites / Cryomodule ILC LINAC 8 Klystrons 288 Cavities in 36 Cryomodules 1300 MHz β=1 Modulator Modulator Modulator Modulator 10 MW ILC Klystrons 36 Cavites / Klystron β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 Modulator Modulator Modulator Modulator β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1 β=1

  47. 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.

  48. The Big Picture: Goals of Experiment • Initial Phase: • Exploit post-collider accelerator modifications at Fermilab to mount a m->e conversion experiment patterned after proposed MECO experiment at BNL • 4x1020 protons in ~2 years • Measure • Single event sensitivity of Rme=2x10-17 • 90% C.L. limit of Rme<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 Rme<1x10-18 • If signal: measure target dependence, etc

  49. Beam Related Rates • Cut ~700 ns after pulse to eliminate most serious prompt backgrounds.

  50. 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. Present Operation: “wasted” loading time 15 Hz Booster cycles

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