Why are muon conversion experiments interesting?

California State University Los AngelesDepartment of Physics and Astronomy Colloquium Detecting Muon Number Violation with Sensitivity of 10-17:Resetting the Standardin Searches for Extremely Rare Processes W. Molzon - University of California, IrvineOctober 16, 2003 Why are muon conversion experiments interesting? Why do we think muon conversion might be discovered? What are the experimental difficulties? What experiments are proposed search for this process? When can we expect results?

Why is Muon to Electron Conversion Interesting? In 1947, examples of a cosmic ray tracks were discovered in which a charged particle decayed, with the daughter particle decaying to an electron. It was shown that the first particle was the pion, proposed by Yukawa to be responsible for strong binding forces between nucleons, and that the intermediate particle had all the properties of the electron except that its mass was 200 times that of the electron. Why there exists a massive copy of the electron was a mystery, prompting the question attributed to Rabi, “Who ordered that?” In some sense the question has not been answered. The absence of radiative decays of the muon indicated that something prevented these decays. The discovery of neutrinos and the demonstration that different neutrinos are associated with electrons and muons prompted the idea that there is an additive quantum number associated with each type, or flavor, of lepton: M conserving - seen m-  e-me M violating - not seenm- N e-Nm-  e-g W. Molzon California State University Los Angeles Physics Department Colloquium

Why Might Muon Number Not Be Conserved? In our experience, conserved quantities are associated with symmetries,and interactions are described by local gauge theories. • A continuous symmetry implies the existence of a conserved quantity: translation symmetry  momentum conservation. • Interactions that are described by local gauge theories have a conserved charge and a massless gauge boson that mediates the interaction. For electromagnetism, electric charge is conserved and the photon is the force carrier. • Attempts to identify muon number as a charge associated with a local gauge symmetry don’t work. • No force coupled to muon number can be identified; lepton universality is very well tested. • No massless particle that might be associated with such a force is known. • Perhaps the massless force carrier has developed a very heavy mass by some symmetry breaking mechanism as do the W and Z bosons; what is this mass scale? Unless there is a reason for a quantity to be conserved, it usually is not. Perhaps muon number is not conserved and me conversion will be found. This discovery would have important implications for understanding fundamental particles. W. Molzon California State University Los Angeles Physics Department Colloquium

Muon Number in the Standard Model • Particle physicists have now identified three families of both leptons and quarks and have a model that describes their known interactions. • The matter particles are spin ½ particles that are weak isospin doublets. Three families of fermions are known; only three light families are allowed. • Interactions are described by gauge theories, and gauge bosons that mediate the forces have been discovered and their properties measured. • The fermion eigenstates that have definite mass and lifetime are linear combinations of the states that have definite flavor. • The description of the matter sector is incomplete: • It does not explain why there is more than one type of lepton and quark. • It doesn’t explain why the families of quarks and leptons have different masses or even why they have mass. • The model says nothing about the structure of the unitary matrix that quantifies the mixing between the mass and flavor eigenstates. Understanding the relationships among the families of quarks and leptons is among the most important issues in particle physics. W. Molzon California State University Los Angeles Physics Department Colloquium

 q2/3 q2/3 q-1/3 q q W W n q-1/3 q2/3 Z li lj W li lj li nj us uu W ee   e Flavor Mixing in the Standard Model Flavor ChangingCharged Current Flavor ChangingNeutral Current Effective Flavor ChangingNeutral Current liU†M Ulj liU†M Dj liU†M Dj iD†MUlj This diagram is not zerosince U†D  I. U†D is thephysical observable. The sum over neutrino flavors of these diagrams is not zero only if neutrinos have different masses andU†D  I This diagram is zero sinceM is proportional to I (lepton universality)and U†U = I. K- 0e- e mixes first and second generation quarks  e oscillation mixes first and second generation leptons Discovery of neutrino oscillations necessitated changing the Standard Model to incorporate neutrino masses and mixing. W. Molzon California State University Los Angeles Physics Department Colloquium

q2/3 q2/3 q-1/3 W W n m e nm m W lmd u d led How are Neutrino Flavor-Oscillations and -N  e-NRelated? • Neutrino flavor-oscillations have now been found. This requires non-degenerate neutrino masses and non-diagonal mixing matrix. • Neutrino flavor-oscillations have limited implications for flavor mixing (lepton flavor violation or LFV) in the charged sector. • Charged LFV processes will occur through intermediate states with n mixing. However,the small upper limits on n mass2 differences imply that theexpected rate is well below what is experimentally accessible. • Because the Standard Model contribution to muon conversion processes is so small, their discovery will be an unambiguous signal of physics beyond the Standard Model. • In many scenarios for physics beyond the SM charged LFV processes might be detectable. Effective mass reach of new experiments is enormous, well beyond reach of direct searches. 10-31 Ratio of coupling strength to weak coupling strength W. Molzon California State University Los Angeles Physics Department Colloquium

Heavy Neutrinos Heavy Z’, Anomalous Z coupling Leptoquarks Sensitivity to Different Muon Conversion Mechanisms Supersymmetry Compositeness Predictions at 10-15 Second Higgs After W. Marciano W. Molzon California State University Los Angeles Physics Department Colloquium

10 -11 10 -13 Re 10 -15 10 -17 10 -19 MECO single event sensitivity 10 -21 100 200 300 100 200 300 Supersymmetry Predictions for m e Conversion • Supersymmetry postulatesexistence of a fermion for every boson and a boson for every fermion • Observable levels of LFV are expected in some supersymmetric grand unified models • Level of LFV related to quark mixing W. Molzon California State University Los Angeles Physics Department Colloquium

Now required(SNO, Kamland) 10-3 MEGA limit 10-4 MSW large angle 10-11 MSW small angle 10-5 MSW large angle 10-6 10-12 MSW large anglesmall mass B(e) PSI-MEG goal 10-7 m2 [eV2] MSW small angle Possible m2 and sin22 from early SuperK 10-13 10-8 Just so 10-9 10-14 just so 10-10 MECO goal 10-15 10-11 1012 1013 1014 10-3 10-2 10-1 1 MR2 [GeV] sin2 2 Rates for LFV Processes Linked to n Oscillations Solar neutrino oscillation – ne disappearance 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 California State University Los Angeles Physics Department Colloquium

Current Limits on Lepton Flavor Violating Processes Branching Fraction Limit Lower limit on MX 150 TeV/c231 TeV/c237 TeV/c2 Mass limit assumes electroweak coupling strength K decays changeboth quark flavorand lepton flavor. 86 TeV/c2 m decays changeonly lepton flavor. 21 TeV/c2 365 TeV/c2 W. Molzon California State University Los Angeles Physics Department Colloquium

History of Lepton Flavor Violation Searches 1 - N  e-N +  e+ +  e+ e+ e- 10-2 10-4 10-6 10-8 MEGA 10-10 E871 10-12 K0 +e-K+ + +e- SINDRUM2 PSI-MEG Goal  10-14 10-16 MECO Goal  1940 1950 1960 1970 1980 1990 2000 2010 W. Molzon California State University Los Angeles Physics Department Colloquium

PSI-MEG m+e+g Experiment Search for m+e+gwith sensitivity of 1 event forB(meg) = 10-14 W. Molzon California State University Los Angeles Physics Department Colloquium

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 California State University Los Angeles Physics Department Colloquium

The PSI-MEG Apparatus W. Molzon California State University Los Angeles Physics Department Colloquium

Eg / Emax 0.90 0.95 1.00 0.90 0.95 1.00 Ee / Emax Calculation of m＋ → e＋g Accidental Backgrounds Signal for B(m＋→e＋g) = 10-13 • Backgrounds calculated with Gaussian resolution functions with conservatively chosen widths • About 0.5 background events expected for a sensitivity of 1 event for a branching fraction of 10-14 W. Molzon California State University Los Angeles Physics Department Colloquium

Muon to Electron COnversion (MECO) Experiment Boston University J. Miller, B. L. Roberts, O. Rind Brookhaven National Laboratory K. Brown, M. Brennan, G. GreeneL. 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 • 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 California State University Los Angeles Physics Department Colloquium

What is Coherent Muon-to-Electron Conversion (-Ne-N)? • Muons are stopped in matter, losing energy by ionization, and form a muonic atom. • They cascade down to the 1S state in less than 10-16 s. • They coherently interact with a nucleus and convert to an electron without emitting neutrinos. The coherence increases the rate by a factor of Z with respect to processes like inverse  decay.The conversion electrons have energy nearly equal to the muon mass;momentum is conserved by nuclear recoil with nearly negligible recoil energy. • 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 is at least 2 X 10-17, • with one event providing compelling evidence of a discovery. W. Molzon California State University Los Angeles Physics Department Colloquium

How Big a Number is 1017? • The number of stars within 500 million light years of the earth • The ratio of the surface area of North America to that of a postage stamp • The number of cents in 500 years of the federal budget (about 80 years with inflation) W. Molzon California State University Los Angeles Physics Department Colloquium

The First -N  e-NExperiment – 1955 • 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 California State University Los Angeles Physics Department Colloquium

What Drives the Design of the Experiment? Considerations of potential sources of fake signals specify much of the design of the beam and experimental apparatus. Look at data from the SINDRUM2 experiment, which currently has thebest limit on this process: Cosmic raybackground Prompt background Expected signal Experimental signature is105 MeV e- originating ina thin stopping target Muon decay in orbit W. Molzon California State University Los Angeles Physics Department Colloquium

The Basis of the Experimental Technique • Produce a very intense beam of low energy muons (1000x other beams) • 1011 muons per second • 107 seconds • 10% detection efficiency • Transport the muons to a thin target and stop them • Reduce flux of unwanted particles • Minimize thickness of stopping target • Detect conversion electrons with high efficiency • Must operate in very high rate environment (1011 m decays or captures per second) • Must distinguish conversion electrons from other electrons • Understand and eliminate other potential sources of electrons so that a single event is strong evidence for a signal.  1 detected event if Re = 2  10-17 The conversion electron has fixed energy: Ee = mc2 – Ebinding – Erecoil = 105.6 – 0.25 – 0.25 MeV W. Molzon California State University Los Angeles Physics Department Colloquium

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 Plus many others – cosmic rays, anti-proton interactions … W. Molzon California State University Los Angeles Physics Department Colloquium

Features of the MECO Experiment • 1000 fold increase in muon intensity • 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 • suppress high momentum negatives and all positives and neutrals • Pulsed beam to eliminate prompt backgrounds • 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 • Improved acceptance • Improved rate handling • Improved background rejection • Very high resolution spectrometer W. Molzon California State University Los Angeles Physics Department Colloquium

MECO at Brookhaven National Laboratory W. Molzon California State University Los Angeles Physics Department Colloquium

Pulsed Proton Beam from AGS for MECO • Accelerate 41013 protons each second to 8 GeV – 50 kW beam power. • Revolution time in the AGS is 2.7 ms – protons are accelerated in 2 RF buckets separated by 1.35 ms. • 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. Quiet detection time Promptbackgrounds Proton pulse W. Molzon California State University Los Angeles Physics Department Colloquium

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 California State University Los Angeles Physics Department Colloquium

MIT Plasma Science and Fusion Center Conceptual Design of MECO Magnet System 5 T 2.5 T • Very detailed CDR completed (300+ pages) • Complete 3D drawing package prepared • TS and SOW for commercial procurement developed • Industrial studies contracts let 2 T 1 T 1 T • 150 MJ stored energy • 5T maximum field • Uses surplus SSC cable • Can be built in industry W. Molzon California State University Los Angeles Physics Department Colloquium

Muons Production and Capture in Graded Magnetic Field • Pions produced in target in axially graded magnetic field: • 50 kW beam incident on W target • Charged particles are trapped in 5 – 2.5 T, axial magnetic field • Axially graded field reflects pions and muons moving away from the experiment • Superconducting magnet is protected by Cu and W heat and radiation shield 150 W load on cold mass15 W/g on superconductor20 Mrad integrated dose Superconducting coil 2.5T 5T Azimuthal position Productiontarget Heat Shield Axial position W. Molzon California State University Los Angeles Physics Department Colloquium

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 California State University Los Angeles Physics Department Colloquium

Sign and Momentum Selection in the Curved Transport Solenoid Transport in a section of a torus results in particle charge and momentum selection: positive particles and low momentum particles absorbed in collimators. Detection Time W. Molzon California State University Los Angeles Physics Department Colloquium

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 California State University Los Angeles Physics Department Colloquium

Magnetic Spectrometer for Conversion Electron Momentum Measurement • 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 California State University Los Angeles Physics Department Colloquium

Spectrometer Performance Calculations • Performance calculated using Monte Carlo simulation of all physical effects • Resolution dominated by multiple scattering in tracker • Resolution function of spectrometer convolved with theoretical calculation of muon decay in orbit to get expected background. • Prototypes demonstrate required resolution 10 1.0  0.1 FWHM~900 keV 0.01 103 104 105 106 W. Molzon California State University Los Angeles Physics Department Colloquium

Expected Sensitivity of the MECO Experiment MECO expects ~ 5 signal events for 107 s (2800 hours) running if Rme = 10-16 W. Molzon California State University Los Angeles Physics Department Colloquium

Expected Background in MECO Experiment MECO expects ~0.45 background events for 107 s with ~ 5 signal events for Rme = 10-16 W. Molzon California State University Los Angeles Physics Department Colloquium

Current Status of MECO Approval, Review and Funding Scientific approval status: • Approved by Brookhaven National Laboratory • Approved by the National Science Foundation through the level of the Director • Approved by the National Science Board for a Major Research Equipment Grant • Endorsed by the HEPAP Subpanel charged with studying the long term scientific goals of the particle physics community Technical and management review status: • Positively reviewed by multiple NSF and Laboratory appointed panels • Some pieces (primarily magnet system) positively reviewed by external expert committees appointed by MECO leadership Funding status: • MECO is currently operating on three R&D grants for a total of $2.6M • A fourth R&D grant is pending, awaiting report from an NSF panel review. • We expect significant funding ($5M) in FY04, currently in the House appropriations bill. We expect a formal project start in FY05. We work actively with the NSF and with Congress to help make that happen. W. Molzon California State University Los Angeles Physics Department Colloquium

MECO PSI MEG Summary and Prospects • Despite 50+ years of experiments, no evidence for muon and electron number violation has been found in charged lepton interactions. • Discovery of neutrino oscillations adds motivation for improved experiments. • Theoretical motivation has increased recently, particularly in grand unified supersymmetric scenarios. • New ideas for much improved muon beams and experiments promise very large improvement in sensitivity. The large expected increase in experimental sensitivity and the expectations from many models provide optimism for making a significant discovery in the next few years. W. Molzon California State University Los Angeles Physics Department Colloquium

Frederick Reines Hall Home of the Department of Physics & Astronomy W. Molzon California State University Los Angeles Physics Department Colloquium

Located 5 miles from the Pacific Ocean W. Molzon California State University Los Angeles Physics Department Colloquium

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Why are muon conversion experiments interesting?