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Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation

Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation. W. Molzon University of California, Irvine April 8, 2002. 20 th ICFA Advanced Beam Dynamics Workshop High Intensity High Brightness Hadron Beams Fermilab. Outline.

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Physics with Very Intense Muon Beams: Search for Lepton Flavor Violation

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  1. Physics with Very Intense Muon Beams:Search for Lepton Flavor Violation W. MolzonUniversity of California, Irvine April 8, 2002 20th ICFA Advanced Beam Dynamics WorkshopHigh Intensity High Brightness Hadron Beams Fermilab

  2. Outline • Motivation for improved lepton flavor violation searches • Current experimental status • Prospects for improved searches • PSI-MEG e experiment • MECO -N e-N experiment • Comments on possible improvements beyond currently approved experiments W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  3. e  W What Will Observation of -N  e-N or   e gTeach Us? Discovery of -N  e-Nor a similar charged lepton flavor violating (LFV) process will be unambiguous evidence for physics beyond the Standard Model. • For non-degenerate neutrino masses, n oscillations can occur. Discovery of neutrino oscillations required changing the Standard Model to include massive . • Charged LFV processes occur through intermediate states with n mixing. Small n mass differences and mixing angles  expected rate is well below what is experimentally accessible. • Charged LFV processes occurin nearly all scenarios for physics beyond the SM, in many scenarios at a level that MECO or PSIMEG will detect. • Effective mass reach of sensitivesearches is enormous, well beyondthat accessible with direct searches. One example of new physics, with leptoquarks lmd led  W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  4. Sensitivity to Different Muon Conversion Mechanisms Supersymmetry Compositeness Predictions at 10-15 Second Higgs doublet Heavy Neutrinos Heavy Z’, Anomalous Z coupling Leptoquarks After W. Marciano W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  5. 10 -11 10 -10 10 -13 10 -12 10 -15 10 -14 10 -17 10 -16 10 -19 10 -18 10 -20 10 -21 Supersymmetry Predictions for LFV Processes • From Hall and Barbieri Large t quark Yukawa couplingsimply observable levels of LFV insupersymmetric grand unified models • Extent of lepton flavor violation in grand unified supersymmetry related to quark mixing • Original ideas extended by Hisano, et al. Current MEGA bound Current SINDRUM2 bound B(  e g) Re PSI-MEG single event sensitivity MECO single event sensitivity 100 200 300 100 200 300 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  6. Rates for LFV Processes Linked to n Oscillations MEGA Bound MSW large angle PSI-MEG goal MSW small angle Possible solutions to solar n oscillations MECO goal Just so From the model of J. Hisano and D. Nomura, Phys. Rev. D59 (1999): SU(5) grand unified model with heavy, right-handed neutrinos W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  7. What Does Muon g-2 Say About -N  e-N and   e g ? • Same SUSY loop diagrams contribute to both. -N  e-Nalso requires LFV in SUSY sector: • g-2 sets scale for chirality-changing loop contributions:are effective L- and R-handed couplings for loop contributions. • Reported 2.6s discrepancy with theory raised possibility that supersymmetric contributions might be large. •   e g (-N  e-N)also have loop contributions, with couplings defined similarly. • Going beyond this requires an assumption about the LFV mechanism. • -N  e-N may also have contributions from chirality-conserving diagrams that do not contribute to g-2 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  8. Possible Constraints on m0 and m1/2 From g-2 With 2.6s discrepancy in g-2: • m > 0 is preferred • With one choice for LFV texture, very large -N  e-N rates would be expected • Taking the difference as a measurement, limits on allowed m0 , m1/2 space can be derived • With correction to error in calculation of hadronic correction, discrepancy is ~1.6s and allowed region is very large • Uncertainty in higher order corrections may not allow confrontation between experiment and Standard Model (Wise and Ramsey-Musolf) 10-14 10-17 2s limit Carvalho, Ellis, Gomez, Lola (since superseded) W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  9. Current Limits on Muon Number Violating Processes Mass limit G=0 G=1 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  10. History of Lepton Flavor Violation Searches 1 - N  e-N +  e+ +  e+ e+ e- 10-2 10-4 10-6 Branching Fraction Upper Limit 10-8 MEGA 10-10 SINDRUM2 10-12 K0 +e-K+ + +e- PSI-MEG goal 10-14 MECO goal 10-16 1940 1950 1960 1970 1980 1990 2000 2010 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  11. -N  e-N vs. me g as Probes of LFV • -N  e-N is more sensitive for essentially all processes not mediated by photon • -N  e-N is more sensitive than is e g to chirality conserving processes • e g is more sensitive for processes mediated by photons • B(e )  300  Rmefor these processes • The motivation is sufficiently strong that both experiments should be done • Relative rates for e g and -N  e-N would give information on underlying mechanism • A significant rate for e g with polarized muons could give additional information on mechanism • Experimental considerations are different • e g is rate-limited due to accidental physics backgrounds physics (correctly measured e and g) at about 10-14level • -N  e-N is rate-limited only in the sense that high detector rates will contaminate events and cause measurement errors W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  12. Coherent Conversion of Muon to Electrons (-Ne-N)? • Muons stop in matter and form a muonic atom. • They cascade down to the 1S state in less than 10-16 s. • They coherently interact with a nucleus (leaving the nucleus in its ground state) and convert to an electron, without emitting neutrinos  Ee = Mm - ENR- EB. • More often, they are captured on the nucleus: or decay in the Coulomb bound orbit: ( = 2.2 s in vacuum, ~0.9 s in Al) • Rate is normalized to the kinematically similar weak capture process: MECO goal is to detect -Ne-N if Re isat least 2 X 10-17, with one event providing compelling evidence of a discovery. Re  (-Ne-N) / (-NN(Z-1)) W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  13. The First -N  e-NExperiment – Steinberger and Wolf After the discovery of the muon, it was realized it could decay into an electron and a photon or convert to an electron in the field of a nucleus. Without any flavor conservation, the expected branching fraction for +e+ is about 10-5. Steinberger and Wolf looked for -N  e-Nfor the first time, publishing a null result in 1955, with a limit Re < 2  10-4 Absorbs e- from - decay 9” Conversion e- reach this counter W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  14. MECO Collaboration Boston University J. Miller, B. L. Roberts, O. Rind Brookhaven National Laboratory K. Brown, M. Brennan, 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, B. W. Mayes, L. S. Pinsky, J. Wilson University of Massachusetts, Amherst K. Kumar Institute for Nuclear Research, Moscow V. M. Lobashev, V. Matushka, A. N. Toropin 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 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  15. What Drives the Design of the MECO Experiment? Considerations of potential sources of fake backgrounds specify much of the design of the beam and experimental apparatus. Cosmic raybackground Prompt background Expected signal SINDRUM2 currently has thebest limit on this process: Muon decay Experimental signature is105 MeV e-originating in a thin stopping target. W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  16. Potential Sources of Background • Muon Decay in Orbit – • Emax = Econversion when neutrinos have zero energy • dN/dEe (Emax – Ee)5 • Sets the scale for energy resolution required: ~200 keV • Radiative Muon Capture: - N   N(Z-1)  • For Al, Egmax = 102.5 MeV/c2, P(Eg > 100.5 MeV/c2) = 4  10-9 • P(g e+e-, Ee> 100.5 MeV/c2) = 2.5  10-5 • Restricts choice of stopping targets:Mz-1 > Mz • Radiative Pion Capture: • Branching fraction ~ 1.2% for Eg > 105 MeV/c2 • P(g e+e-, 103.5 < Ee< 100.5 MeV/c2) = 3.5  10-5 • Limits allowed pion contamination in beam during detection time Muon decay in vacuum: Ee < mc2/2 Muon decay in bound orbit: Ee < mc2 - ENR - EB W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  17. Other Potential Sources of Backgrounds • Muon decay in flight + e- scattering in stopping target • Beam e- scattering in stopping target • Limits allowed electron flux in beam • Antiproton induced e- • Annihilation in stopping target or beamline • Requires thin absorber to stop antiprotons in transport line • Cosmic ray induced e- – seen in earlier experiments • Primarily muon decay and interactions • Scales with running time, not beam luminosity • Requires the addition of active and passive shielding W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  18. Features of the MECO Experiment • 1000 fold increase in muon intensity using an idea from MELC at MMF • High Z target for improved pion production • Graded solenoidal field to maximize pion capture • Produce 10-2m-/p at 8 GeV (SINDRUM2 10-8, MELC 10-4,Muon Collider 0.3) • Muon transport in curved solenoid suppressing high momentum negatives and all positives and neutrals (new for MECO) • Pulsed beam to eliminate prompt backgrounds following PSI method (A. Badertscher, et al. 1981) • Beam pulse duration << tm • Pulse separation  tm • Large duty cycle (50%) • Extinction between pulses < 10-9 • Improved Detector Resolution and Rate Capability • Detector in graded solenoid field for improved acceptance, rate handling, background rejection following MELC concept • Spectrometer with nearly axial components and very high resolution(new for MECO) W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  19. Promptbackgrounds Pulsed Proton Beam from AGS for MECO Proton pulse • Machine will operate at 8 GeV with 41013 protons per second – 50 kW beam power. • Cycle time of 1.0 s with 50% duty factor • Revolution time = 2.7 ms with 6 RF buckets in which protons can be trapped and accelerated • Fill 2 RF buckets for 1.35 ms pulse spacing • 2 1013 protons / RF bucket - twice current bunch intensity • Resonant extraction of bunched beam • To eliminate prompt backgrounds, we require< 10-9 protons between bunches for each proton in bunch. We call this the beam extinction. Detection time W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  20. Producing Pulsed beam with 10-9 Extinction Extinction Measurements: • Initial test at 24 GeV with one RF bucket yielded <10-6 extinction between buckets and 10-3 in unfilled buckets • A second test at 7.4 GeV with a single filled bucket found <10-7 extinction Multiple means of improving extinction: • AGS internal cleanup with 40 kHz AC dipole and fast kicker magnets (field shown inverted). • External cleanup and extinction monitoring with RF modulatedmagnet and septum magnets W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  21. The MECO Apparatus Straw Tracker Muon Stopping Target Muon Beam Stop Superconducting Transport Solenoid (2.5 T – 2.1 T) Crystal Calorimeter Superconducting Detector Solenoid (2.0 T – 1.0 T) Superconducting Production Solenoid (5.0 T – 2.5 T) Muon Production Target Collimators Proton Beam Heat & Radiation Shield W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  22. MIT Plasma Science and Fusion Center Conceptual Design of MECO Magnet System 5 T 2.5 T 2 T 1 T 1 T W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  23. Muons Production and Capture in Graded Magnetic Field Pions produced in a target located in an axially graded magnetic field: • 50 kW beam incident on W target • Charged particles are trapped in 5 – 2.5 T, axial magnetic field • Pions and muons moving away from the experiment are reflected • Superconducting magnet is protected byCu and W heat and radiation shield 150 W load on cold mass15 W/g in superconductor20 Mrad integrated dose Superconducting coil mW/gm in coil 2.5T 5T Azimuthal position Productiontarget Heat Shield Axial position W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  24. Production Target for Large Muon Yield Production target region designed for high yield of low energy muons: • High Z target material • No extraneous material in bore to absorb p/m • Cylinder with diameter 0.8 mm, length 160 mm • ~5 kW of deposited energy Two target configurations being considered • Radiation cooled tungsten • Requires high emissivity coating • Segmented into 40 disks • Maximum temperature is ~2100 K • Well below melting point, low evaporation • Thermally induced stresses near yield stress • Requires a uniform proton beam profile on the target • Water cooling in narrow channels in target • Required flow rate is reasonable • Temperature rise is below 100 K • Detailed design and prototype underway Temperature distribution in target segment W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  25. Muon Beam Transport with Curved Solenoid Goals: • Transport low energy m-to the detector solenoid • Minimize transport of positive particles and high energy particles • Minimize transport of neutral particles • Absorb anti-protons in a thin window • Minimize long transittime trajectories Curved sections eliminate line of site transport of photons and neutrons. Toroidal sections causes particles to drift out of plane;used to sign and momentum select beam. dB/dS < 0 to avoid reflections 2.5T 2.4T 2.4T 2.1T 2.1T 2.0T W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  26. Sign and Momentum Selection in the Curved Transport Solenoid Transport in a torus results in charge and momentum selection: positive particles and low momentum particles absorbed in collimators. Detection Time Relative particle flux Relative particle rate in mbunch W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  27. Muon Beam Studies • Muon flux estimated with Monte Carlo calculation including models of p- production and simulation of decays, interactions and magnetic transport. • Estimates scaled to measured pion production on similar targets at similar energy • Expected yield is about 0.0025 -stops perproton Stopping Flux Total flux at stopping target Relative yield Data Model 0 50 100 0 0.5 1.0 Muon Momentum [MeV/c] Pion Kinetic Energy [GeV] W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  28. Stopping Target and Experiment in Detector Solenoid • Graded field in front section to increase acceptance and reduce cosmic ray background • Uniform field in spectrometer region to minimize corrections in momentum analysis • Tracking detector downstreamof target to reduce rates 1T Electron Calorimeter 1T Tracking Detector 2T Stopping Target: 17 layers of 0.2 mm Al W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  29. Magnetic Spectrometer to Measure Electron Momentum • Measures electron momentum with precision of about 0.3% (RMS) – essential to eliminate muon decay in orbit background Electron starts upstream, reflects in field gradient • Must operate in vacuum and in high rate environment – 500 kHz rates in individual detector elements. • Implemented in straw tube detectors – • 2800 nearly axial detectors, 2.6 m long, 5 mm diameter,0.025 mm wall thickness – minimum material to reduce scattering • position resolution of 0.2 mm in transverse direction, 1.5 mm in axial direction W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  30. Spectrometer Performance Calculations Performance calculated using Monte Carlo simulation of all physical effects Resolution dominated by multiple scattering in tracker and energy loss in target Resolution function of spectrometer convolved with theoretical calculation of muon decay in orbit to get expected background. FWHM ~850 keV  W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  31. Tracking Performance Potentially Compromised by High Rates There are about 1011 muon decays and muon captures per second • electrons from - decay mostly trapped at small radius in the detectorsolenoid and don’t reach detectors • ~1011 neutrons and 1011 photons from nuclear de-excitation following muon capture can interact in detectors • ~1010 protons per second produced from muon capture • potentially serious background from low energy electrons contaminated with noise signals producing fake signal events real e- from - decay fake e- trajectory W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  32. Scintillating Crystal Absorption Calorimeter • Provides prompt signal proportional to electron energy for use in online event selection • Provides position measurement to confirm electron trajectory • Provides energy measurement to ~ 5% to confirm electron momentum measurement • Consists of ~2000 3 cm x 3 cm x 12 cm (PbWO4 or BGO) crystals W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  33. Expected Sensitivity of the MECO Experiment W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  34. Expected Background in MECO Experiment Background calculated for 107 s running time at intensity yielding 1 signal event for Rme = 2  10-17. Sources of background will be determined directly from data. W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  35. Current Status of MECO Approval, Review, Funding, Schedule Scientific approval status: • 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 review status: • Positively reviewed by many NSF and Laboratory appointed panels • Some pieces (primarily magnet system) positively reviewed by external expert committees appointed by MECO leadership Funding status: • Currently operating on 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 MRE funding Construction schedule • Construction schedule driven by superconducting solenoids – estimate from the MIT Conceptual Design Study is 41 months from signing of contract for engineering design and construction until magnets are installed and tested More information at http://meco.ps.uci.edu W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  36. PSI-MEG m+e+g Experiment Search for m+e+gwith sensitivity of 1 event forB(meg) = 10-14 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  37. Kinematics qeg= 180° g e m Ee = 52.8 MeV Eg = 52.8 MeV • Main source of background: • Accidental　coincidences of e＋ from Michel decay (m＋→e＋νeνμ) + random g from radiative decay or other sources • Eedistribution peaks near 50 MeV ( x =Ee / Emax) • Egdistribution in interval dy near y=1 given by dNg dy2 (y = Eg / Emax) •  background/signal  Ee(Eg)2  t  ()2  Rate Crux of experiment is improving resolution in all measured quantities The MEGA experiment (which currently has the best limit) was background limited due to unanticipated tails in resolution functions of these quantities. Principal Features of m＋ → e＋g Experiment • Stop m＋ in thin target • Measure energies of e＋ (Ee)andg (Eg) • Measure angle betweene＋ and g() • Measure time betweene＋ and g(t) W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  38. Overview of the PSI-MEG Experiment Getting desired sensitivity is a tradeoff between increased rate (and background) and increased acceptance (typically giving lower resolution)PSI-MEG has optimized these tradeoffs with following parameters: • Rm = 1 x 108 Hz • T = 2.2 x 107sec • Solid angle W/4p = 0.09 • Acceptances eg = 0.7, ee = 0.95 esel = 0.8 Reducing accidental background to less than 1 event requires resolution functions that are Gaussian with widths given below: One event for B(m＋→e＋g) ~ 0.94 x 10-14 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  39. Eg / Emax 0.90 0.95 1.00 0.90 0.95 1.00 Ee / Emax Calculation of m＋ → e＋g Accidental Backgrounds • Backgrounds calculatedwith Gaussian resolutionfunctions with conservatively chosen widths. • ~0.5 background events expected Signal for B(m＋→e＋g) = 10-13 W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  40. Producing the Required m＋ Beam m+from p+ decay at rest Fluxes of p and m at pE5 Paul Scherrer Institute590 MeV proton cyclotron: Operating current ~1.8 mA (Max > 2.0 mA) – 1.2 MW DC muon beam rate above 108 m/sat pE5 beam line W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  41. Muon Beam Transport System W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  42. The PSI-MEG Apparatus W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  43. 53 MeV Photon Detection Detector requirements: • Energy resolution ~0.6 – 0.9% (sRMS) • Timing resolution ~ 65 psec　(sRMS) • Position resolutions ~ 1.0 mm (sRMS) • Ability to operate in high rate environment Detector design • Active volume of LXe: 600 liter • Scintillation light collected by ~800 PMTs immersed in LXe • Effective photo-cathode coverage: ~35% • Detector outside magnet  Liquid xenon scintillation detector W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  44. Photon Detector Based on Liquid Xenon Scintillator • High light yield (75% of NaI(Tl)) • Fast signals to avoid accidental pileup • Spatially uniform response - no need for segmentation W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  45. Small Prototype of Liquid Xenon Calorimeter • 32 x PMTs • Active Xe volume 116 x 116 x 174 mm3 (2.3 liter) • Energy, position and timing resolutionfor g up to 2 MeV W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  46. Small Prototype Test Results Tests done: • At low rate • With small detector • At low energy Simple extrapolation to 53 MeV implies: senergy ~ 0.8% (0.6-0.9%) sposition ~ a few mm st ~ 50 psec W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  47. Large Prototype Photon Detector Tests Purpose • Measure resolutions with high energy g • Check of cryogenics and other detector components • Measure absorption length Construction • 228 PMTs, 69 liter LXe 1 m Results: • Cryogenic performance is good • First test with 40 MeV g beam completed • Analysis in progress W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  48. Positron Detection and Energy Measurement COBRA spectrometer Thin superconducting magnet with gradient magnetic field Drift chamber for positron tracking Scintillation counters for timing measurement W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  49. Principle of the COBRA Spectrometer Bending radius independent of emission angles Uniform field Gradient field Low energy positrons quickly swept out Uniform field Gradient field e+ from m+e+g W. Molzon, UC Irvine Physics with Very Intense Muon Beams

  50. PSI-MUEG Superconducting Spectrometer Magnet • Bc = 1.26T, Bz=1.25m=0.49T, operating current = 359A • Five coils with three different diameters to realize gradient field • Compensation coils to suppress the residual field around the LXe detector • High-strength aluminum-stabilized superconductor  very thin coil (3 g/cm2) • Design completed, conductor produced W. Molzon, UC Irvine Physics with Very Intense Muon Beams

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