Searching for Muon to Electron Conversion Below the 10 -16 Level

1. Searching for Muon to Electron Conversion Below the 10-16 Level Michael Hebert UC Irvine PANIC02 Osaka, Sept. 29 – Oct. 4, 2002

2. Institute for Nuclear Research, Moscow V. M. Lobashev, V. Matushka, New York University R. M. Djilkibaev, A. Mincer, P. Nemethy, J. Sculli, A.N. Toropin Osaka University M. Aoki, Y. Kuno, A. Sato University of Pennsylvania W. Wales Syracuse University R. Holmes, P. Souder College of William and Mary M. Eckhause, J. Kane, R. Welsh Boston University J. Miller, B. L. Roberts, O. Rind Brookhaven National Laboratory K. Brown, M. Brennan, G. Greene, L. Jia, W. Marciano, W. Morse, Y. Semertzidis, P. Yamin University of California, Irvine M. Hebert, T. J. Liu, W. Molzon, J. Popp, V. Tumakov University of Houston E. V. Hungerford, K. A. Lan, L. S. Pinsky, J. Wilson University of Massachusetts, Amherst K. Kumar MECO Collaboration Eleven Institutions worldwide at present. Substantial growth expected following formal project start M. Hebert, UC Irvine PANIC02

3.  Charged Lepton Flavor Violation • Lepton Flavor is NOT conserved in the neutral sector as shown by recent neutrino mixing results • Lepton Flavor Violation (LFV) in the charged sector should occur via n mixing, but far below the experimentally accessible range, meaning that any observed signal necessarily requires new physics • Fortunately a wide variety of proposed extensions to the Standard Model predict observableLFV processes in charged lepton sector M. Hebert, UC Irvine PANIC02

4.  One Example • SU(5) SUSY- GUT predicts a signal only a few orders of magnitude below the current experimental limit • SO(10) prediction is enhanced by Hisano, et. al. ‘97 MECO single event sensitivity M. Hebert, UC Irvine PANIC02

5. History of Charged LFV Searches 1 Goal: A four order of magnitude leap in sensitivity to the 2  10 –17 level for a single event Effective mass reach is enormous, e.g. for leptoquark exchange -N  e- N +  e+  +  e+ e+ e- K0 + e- K+ ++e- 10-4 10-8 Sensitivity to Lepton Flavor Violation 10-12 MECO Goal Kuno and Okada ‘01 10-16 1940 1950 1960 1970 1980 1990 2000 2010 Year M. Hebert, UC Irvine PANIC02

6. Muon to Electron Conversion • Low energy muons stopped in Ti or Al foils, forming muonic atoms • Three possible fates for the muon • Nuclear capture • Three body decay in orbit • Coherent LFV decay • Signal is a single mono-energetic electron • Single particle signal avoids “accidental” backgrounds at high rate • Rate is normalized to the kinematically similar weak capture process: M. Hebert, UC Irvine PANIC02

7. MECO Features • 1000–fold increase in muon beam intensity (from MELC at MMF) • High Z target for improved pion production • Axially-graded 5 T solenoidal field to maximize pion capture • Muon transport in a curved solenoid suppressing neutrals, positives, and high momentum negatives (new for MECO) • Pulsed beam to eliminate prompt backgrounds (A. Baertscher et al.) • Beam pulse duration << muon lifetime • Pulse separation ~ muon lifetime • Extinction between pulses < 10-9 • High rate capability and improved acceptance electron detectors • Axially-graded 2 T solenoidal field for improved acceptance (from MELC) • Spectrometer with axial components and good resolution (new for MECO) M. Hebert, UC Irvine PANIC02

8. Potential Backgrounds • Muon Decay in Orbit • The dominant background • Steeply falling spectrum near endpoint, e.g. • Sets required energy resolution: Nbkgd = 0.25 for Rme= 2  10-17 DEe= 900 keV (FWHM) • Radiative Muon Capture – also eliminated by energy resolution • Radiative Pion Capture – requires beam extinction < 10-9 • Muon decay in flight + e- scattering – negligible with pulsed beam • Beam e- scattering in stopping target – eliminated by pulsed beam • Antiproton induced e- – requires thin stopping window • Cosmic ray induced e- – requires active and passive shielding M. Hebert, UC Irvine PANIC02

9. The MECO Apparatus Superconducting Detector Solenoid contains conversion electron detectors Superconducting Production Solenoid captures muons Superconducting Transport Solenoid selects low momentum m- M. Hebert, UC Irvine PANIC02

10. 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 Production Region Superconducting coils 2.5 T Proton Beam Production Target Heat & Radiation Shield 5 T M. Hebert, UC Irvine PANIC02

11. 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 Transport Solenoid 2.1 T Collimators 2.5 T Curvature Drift M. Hebert, UC Irvine PANIC02

12. Detector Region • 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 g and neutrons Electron Calorimeter Straw Tracking Detector Stopping Target Foils 1 T 1 T 2 T M. Hebert, UC Irvine PANIC02

13. Electron Spectrometer Performance Side View 900 keV resolution dominated by • Energy loss in muon stopping target (640 keV FWHM) • Tracker intrinsic resolution (350 keV FWHM) Axial View Background with Detector Response Conversion electron produced in the stopping target, detected in the Tracker, and triggered in the Calorimeter 900 keV FWMH Full GEANT Simulation Signal M. Hebert, UC Irvine PANIC02

14. MECO Sensitivity & Background Expected Sensitivity Expected Background M. Hebert, UC Irvine PANIC02

15. Current Status Scientific approval • Approved by BNL and by the NSF through level of the Director • Approved (with KOPIO) by the NSB as an MREFC Project (RSVP) • Endorsed by the recent HEPAP Subpanel on long-range planning Technical and management reviews • Positively reviewed by many NSF and Laboratory appointed panels • Magnet system design positively reviewed by external expert committees appointed by MECO leadership Funding • Currently operating on $2.1M R&D funds from the NSF • Project start awaits Congressional action; RSVP (MECO + KOPIO) is not in the FY03 budget – efforts in Congress to improve NSF MREFC funding Construction schedule • Construction schedule driven by superconducting solenoids – estimate from the Conceptual Design Study is 41 months from signing of contract until magnets are installed and operational M. Hebert, UC Irvine PANIC02

16. Outlook • The physics potential for MECO is extremely robust. Numerous extensions of the Standard Model predict an observable me signal if we are able to achieve the predicted four order of magnitude increase in sensitivity • We expect to make this leap forward using several advances in the muon conversion state of the art • 1000–fold increase in rate of the m- stops • Muon beam line that minimizes contamination while maximizing yield • Improved detector acceptance, high rate capability, and good resolution • We expect to move into the detailed design and construction phase very soon, meaning now is the perfect time for people to get involved! More information visit http://meco.ps.uci.edu M. Hebert, UC Irvine PANIC02