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Mu2e: A New Search for Charged Lepton Flavor Violation at Fermilab Jim Miller Boston University for the Mu2e Collaboration 26 June 2009 J. Miller, BU PAVI 09 June, 2009 1 Outline Brief introduction and theoretical motivation Description of main backgrounds, experimental technique and proposed apparatus Description of Fermilab Accelerator Potential Future Upgrades Conclusions J. Miller, BU PAVI 09

June, 2009 2 What is ee Conversion? A muon converts to an electron in the presence of a nucleus, with no neutrinos. The nucleus is required to conserve energy and momentum! A( Z , N ) e A( Z , N ) Mu2e goal: R e (Aluminum) 6 10 17 (90% c.l.) Charged Lepton Flavor Violation (CLFV) Related Processes: e e, e, , e e+e-e , e+e-e , J. Miller, BU PAVI 09 June, 2009 3 Muon to Electron Conversion Current limits: Au e Au

R e <7x10 13 (SINDRUM II) Au capture Also: Ti e Ti R e <4.3x10 12 (SINDRUM II) Ti capture Ti e Ti R e <4.6x10 12 (TRIUMF) Ti capture Al e Al New Mu2e proposal: Re 6x10 17 (90% c.l.) Al capture x10000 improvement over current limit J. Miller, BU PAVI 09 June, 2009 4

Experimental Signal A( Z , N ) e A( Z , N ) A Single Monoenergetic Electron Ee=mc2BERecoil For Aluminum, Ee = 105 MeV nucleus electron energy depends somewhat on Z eJ. Miller, BU PAVI 5 09 June, 2009 Endorsed in US Roadmap A muon-electron conversion program at FNAL: Strongly endorsed by P5 The

experiment could go forward in the next decade with a modest evolution of the Fermilab accelerator complex. Such an experiment could be the first step in a world-leading muon-decay program eventually driven by a nextgeneration high-intensity proton source. The panel recommends pursuing the muon-to-electron conversion experiment... under all budget scenarios considered by the panel Mu2e is a central part of the future US program Mu2e received Stage I approval from the FNAL PAC and Directorate in November, 2008. J. Miller, BU PAVI 09 June, 2009 6 Collaboration Boston University: R.M. Carey, K.R. Lynch, J.P. Miller*, B.L. Roberts BNL: W. Marciano,Y. Semertzidis, P. Yamin UC Berkeley: Yu.G. Kolomensky FNAL: C.M. Ankenbrandt , R.H. Bernstein*, D. Bogert, S.J. Brice, D.R. Broemmelsiek, R. Coleman, D.F.

DeJongh, S. Geer, D. Glenzinski ,R. Kutschke, M. Lamm, , P.J. Limon, M.A. Martens, S. Nagaitsev, D.V. Neuffer, M. Popovic, E.J. Prebys, V. Rusu , P. Shanahan, M. Syphers, R.E. Ray, R. Tschirhart, H.B. White, K. Yonehara, C.Y. Yoshikawa Idaho State University: D. Dale, K.J. Keeter, E. Tatar UC Irvine: W. Molzon University of Illinois/Champaign-Urbana: P.T. Debevec, G. Gollin,D.W. Hertzog, P. Kammel INFN/Universit Di Pisa: F. Cervelli, R. Carosi, M. Incagli,T. Lomtadze, L. Ristori, F. Scuri, C. Vannini INR Moscow: V. Lobashev U Mass Amherst: D. Kawall, K. Kumar Muons, Inc: R.J. Abrams, M.A.C. Cummings, R.P. Johnson, S.A. Kahn,S.A. Korenev, T.J. Roberts, R.C. Sah

City University of New York: J.L. Popp Northwestern University: A. DeGouvea 70 Collaborators Rice University: M. Corcoran Syracuse University: R.S. Holmes, P.A. Souder 16 Institutions University of Virginia: M.A. Bychkov, E.C. Dukes, E. Frlez, R.J. Hirosky, A.J. Norman, K.D. Paschke, D. Pocanic J. Miller, BU PAVI 09 June, 2009 7 History of CLFV Searches J. Miller, BU PAVI 09 June, 2009 8

Lepton Flavor Violation Searches Current and Planned Expts Neutrino Oscillations! decays at Babar, Belle. Future decays: Super B factories MEG at PSI: ee L. Calibbi, A. Faccia, A. Masiero, S. Vempati hep-ph/0605139 e conversion: Mu2e at FNAL COMET at JPARC J. Miller, BU BR(e in Au) <7x10-13 BR(l+l-l) <(few)x10-8 BR() <4.2x10-8 PAVI 09 June, 2009 9 Neutrino Oscillations and e s have mass! Individual lepton numbers are not conserved

This implies lepton flavor violation also occurs in charged leptons. In the SM (extended to handle neutrino oscillations): SM: Branching ratio(e)<10-52 o This is way below any experimental sensitivity Other CLFV reactions are similarly suppressed Any observation of CLFV is a definite sign of new physics Many models predict CLFV at levels just beyond current limits J. Miller, BU PAVI 09 June, 2009 10 New Physics Contributions to e Conversione Conversion From W. Marciano. also see Flavour physics of leptons and dipole moments, arXiv:0801.1826 J. Miller, BU PAVI 09 June, 2009 11 Power of Signal in Muon-Electron Conversion

Neutrino-Matrix Like (PMNS) Minimal Flavor Violation(CKM) BR(e) vs M1/2 for tan=10 BR(e)x1012 Current e limit Proposed e limit M1/2(GeV) L. Calibbi, A. Faccia, A. Masiero, S. Vempati hep-ph/0605139 neutrino mass via the see--saw mechanism,analysis in SO(10) framework J. Miller, BU PAVI 09 June, 2009 12 Similar Plots for and e BR() x107 vs M1/2 for tan=10 L. Calibbi, A. Faccia, A. Masiero, S. Vempati hep-ph/0605139: neutrino mass via the see-saw mechanism,analysis in SO(10) framework

BR(e) x1011vs M1/2 for tan=10 J. Miller, BU PAVI 09 June, 2009 13 ee Conversion versus ee MEG Little Higgs Model w/T parity M. Blanke, A. J. Buras, B. Duling, A. A. Poschenrieder and C. Tarantino, JHEP 0705, 013 013 (2007). MEG Mu2e BR(iei) BR(e) Constrained Minimal SUSY SO(10) models

MEGA C. C. Albright Albright and and M. M. Chen, Chen, arXiv:0802.4228, arXiv:0802.4228, PRD PRD D77:113010, D77:113010, 2008. 2008. Mu2e J. Miller, BU PAVI 09 June, 2009 14 SUSY: Minimal SU(5) >0 <0

J. Hisano, T. Moroi, K. Tobe and M. Yamaguchi, Phys. Lett. B 391, 341 (1997). [Erratum-ibid. B397, 357 (1997).] J. Miller, BU PAVI 09 June, 2009 15 Sensitivity of Mu2e Single-event sensitivity= 2.5 x 10-17 Ree < 6 x 10-17 90% CL For Re Conversione = 10-15 ~40 events / 0.4 bkg (LHC SUSY?) For Re Conversione = 10-16 ~4 events / 0.4 bkg J. Miller, BU PAVI 09 June, 2009 16 Outline Brief introduction and theoretical motivation Description of main backgrounds,

experimental technique and proposed apparatus Description of Fermilab Accelerator Potential Future Upgrades Conclusions J. Miller, BU PAVI 09 June, 2009 17 The Measurement Method in a Nutshell Stop negative muons in an aluminum target The stopped muons form muonic atoms hydrogenic 1S level in aluminum nucleus Bohr radius ~20 fm, Binding E~500 keV nucleus n2 mZ 2 r , E 2 mZ

n Nuclear radius ~ 4 fm muon and nuclear wavefunctions overlap Muon lifetime in 1S orbit of aluminum ~864 ns compared to 2.2 sec in vacuum 40% decay, A( N , Z ) A( N , Z ) e e 60% nuclear capture, A ( N , Z ) X ( N 1, Z 1)

( p n) (capture is ~ sum of reactions over protons in nucleus) Look for a monoenenergetic electron from the neutrinoless conversion of a muon to an electron, leaving the nucleus in the ground state: 27 27 Measuredquantity: ratio 13 Althe 13 AlRe:e Electron energy~105 MeV 27 1327 Al 13 Al e R e 27 , where X=A'(N,Z)+neutrons, protons,... 13 Al X + (capture) J. Miller, BU

PAVI 09 June, 2009 18 Backgrounds from Stopped Muons: Muon Decay in Atomic Orbit (DIO) S [ A( N , Z )]1bound A( N , Z ) e e Conversion electrons of interest: lbound leeV Electrons from decay of bound muons (DIO) -- kinematic endpoint equals conversion electron energy: prob ( E E )5 endpt Decay in Orbit electrons Simulation (Assume Re~10-16) e FWHMMeV, MeV on high side tail

J. Miller, BU PAVI 09 June, 2009 19 Backgrounds from Stopped Muons(Contd) Ordinary muon capture on the nucleus In neutron capture, secondary particles can be produced A( N , Z ) A '( N ', Z ') an bp c, ~2, ~0.1, ~2 n, p are low energy, are mostly low energy, well below conversion electron energy: create high rate background in detectors, potential track recognition errors Neutral background (n, ) is reduced by displacing detectors downstream from the stopping target Protons are reduced by placing thin absorbers in their path Radiative Muon Capture, energy above 55 MeV, prob ~ few x 10-5; endpoint for aluminum 102.4 MeV, 2.5 MeV below conversion electron energy. Event rate comparable to DIO at 102.5 MeV, rises much slower than DIO at lower energies. J. Miller, BU PAVI 09

June, 2009 20 Radiative Pion Capture Background , like , stop in stopping target and form atoms Radiative Pion Capture A( N , Z ) A '( N ', Z ') X 27 1327 Al 12 Mg , E ~137 MeV Reaction of with nucleus is fast: occurs part way through atomic cascade BR 2% for photon > 55 MeV, peak prob ~110 MeV, endpoint~137 MeV + material (e.g. target) eeemay have energy > 100 MeV J. Miller, BU PAVI 09 June, 2009 21 Previous Data, NeN

From SINDRUM Experiment High energy tail of Decay-in-orbit (DIO) electrons. Simulated conversion peak Ti e Ti 12 Re 4 . 3 10 Ti capture DC beam J. Miller, BU Rate limited by need to veto prompt backgrounds! pulsed beam

PAVI 09 June, 2009 22 Dealing with radiative pion capture background Mu2e plans a pulsed proton beam Well-matched to 864 ns muonic Al lifetime Simulation Time distribution of pions arriving at target after proton strikes the production target Wait ~700 ns to start measurement, pion stopping rate is reduced by ~10 11 ~0.0007 events background, compared to ~4 events signal at R e=10-16 Extinction (=between-pulse proton rate) < 10 -9 gives ~0.07 counts Recognized and studied by time dependence, presence of e+ J. Miller, BU PAVI 09 June, 2009 23 Mu2e Muon Beamline and Detector Requirements Pulsed beam- 109 extinction High flux beam to stopping target

At FNAL, high proton flux ~20x1012 Hz, 8 GeV Mu2e: use solenoidal muon collection and transfer scheme muons ~5 x 1010 Hz , 1018 total needed Muon properties low momentum and/or narrow momentum spread stop max # muons in thin target avoid ~105 MeV e from in-flight decay (keep p<75 MeV/c) Background particles from beam line must be minimized especially ~105 MeV e and high momentum a major factor driving design of muon beamline Detector should have high resolution and acceptance for 105 MeV electrons J. Miller, BU PAVI 09 June, 2009 24 Mu2e Muon Beamline- follows MECO design Muons are collected, transported, and detected in superconducting solenoidal magnets Muon Stopping Target Tracker

Proton Beam Muon Beam Target Proton Target Shielding Calorimeter Collimators B=1 T B=1 T Pions Muons B=2.5 T Transport B=5 T Production Detector Electrons Solenoid Solenoid Delivers 0.0025 stopped muons per 8 GeV proton Solenoid

J. Miller, BU B=2 T PAVI 09 June, 2009 25 J. Miller, BU PAVI 09 June, 2009 26 Production Solenoid 8GeV Incident Proton Flux 3107 p/pulse (34ns width) p Primary production off gold target

ield F d i o n Sole t n e i d a Gr 2.5T e decays to e e is captured into the transport solenoid and proceeds to the stopping targets

5T Magnetic Mirror Effect Transport Solenoid Inner bore radius=25 cm Length=13.11 m Toroid bend radius=2.9 m B=2.5T Beam particles Pitch pl p cos Curved sections eliminate line of sight transport of n, . 1 q s 1 D p (

cos ). Radial gradients (dB /dR) in toroidal s 2 0.3 B R cos sections cause particles to drift vertically; off-center collimator signs and momentum selects beam B=2.4T Goals: Transport low energy dB/dS < 0 in straight sections to avoid slow transiting particles to the detector solenoid B=2.4T Minimize transport of positive particles and high energy B=2.1T particles Minimize transport of neutral particles Absorb anti-protons in a thin window Minimize particles with long transit time trajectories

Collimation designed to greatly suppress transport of e greater than 100 MeV J. Miller, BU PAVI 09 Length decreases flux, by decay, of pions arriving at stopping target in measurement period B=2.1 T June, 2009 B=2.0 T To stopping target 28 Separation of from pitch pl p

cos 1 q s 1 D p ( cos ). 2 0.3 B R cos 29 J. Miller, BU PAVI 09 June, 2009 29 The Detector Graded Field fo Magnet r ic Mirro

r Effect 1T 1T Beam in 2T The detector is specifically design to look for the helical trajectories of 105 MeV electrons Each component is optimized to resolve signal from the Decay in Orbit Backgrounds Detector Solenoid Solenoid, 1m radius, B=2 T1T 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 loss of energy resolution due to energy straggling Tracker Calorimeter Stopping Target Muon Beam Line B=1T

B=1T Muons Field gradient B=2T J. Miller, BU PAVI 09 Uniform field Large flux of electrons from low energy portion of muons decaying in target (DIO) spiral harmlessly through the centers of the detectors June, 2009 31 Straw Tracker (In Vacuum) Barrel Octagonal+Vanes geometry is optimized Van for reconstruction of 105MeV

e helical trajectories Extremely low mass Acceptance for DIO tracks Electron < 10-13 track Trajectories Pt > 90MeV Low Energy DIODIO Trajectories Tail Target Foils > 57MeV R=57MeV Calorimeter Function: provide initial trigger to system (E>80 MeV gives trigger rate ~1 kHz), and redundant position, timing, and energy information 1800 PbWO4 crystals, 3 x 3 x 12 cm3 arranged in four vanes. Density 8.3g/cm3, Rad. Length 0.89 cm, R(Moliere)=2.3 cm, decay time 25 ns

Each crystal is equipped with two large area Avalanche PhotoDiodes: gives larger light yield and allows identification of events with charged particles traversing photodiode Both the front end electronics (amplifier/shapers) and the crystals themselves are cooled to -240 C to improve PbWO4 light yield and reduce APD dark current. Single crystal performance has been demonstrated by MECO with cosmic rays: 38 p.e./MeV, electronic noise 0.7 MeV. Estimated performance with electrons, ~5-6 MeV at 100 MeV, position<1.5 cm 33 J. Miller, BU PAVI 09 June, 2009 33 Cosmic Ray Veto and Shielding Active shielding goal: inefficiency Simulation study has shown that 10- 4 inefficiency in scintillator veto 0.016 background events / 2x107s. Three overlapping layers of scintillator consisting of 10 cm x 1 cm x 4.7 m strips. Veto= signals in 2

or more layers. Cost-efficient MINOS approach: extruded (not cast) scintillator,1.4 mm wavelength-shifting fiber. Use multi-anode PMT readout of WLS fiber Passive shielding: heavy concrete plus 0.5 m magnet return steel. Steel also shields CRV scintillator from neutrons coming from the stopping target. J. Miller, BU PAVI 09 June, 2009 34 Extinction Scheme Need to achieve (out-of-time flux)/(in-time flux)<10-9 Sweep protons out of beam between proton pulses: Under development Continuous Extinction monitoring techniques under study Telescope to measure secondary proton production as in MECO Joint effort with COMET to develop in-beam proton veto

counter. J. Miller, BU PAVI 09 June, 2009 35 Background Summary (DIO) * Due to out-of-time protons, depends on extinction, 10 -9 assumed (From MECO simulations, in process of being repeated by Mu2e) Total run time, 2x107 seconds Total protons, 4x1020 Total stopped muons, 1x1018 Total conversion electrons (if Re=10-16)=4 counts J. Miller, BU PAVI 09 June, 2009 36 Outline Brief introduction and theoretical motivation

Description of main backgrounds, experimental technique and proposed apparatus Description of Fermilab Accelerator Potential Future Upgrades Conclusions J. Miller, BU PAVI 09 June, 2009 37 FNAL Beam Delivery FNAL has unique, major strength: Multiple Rings no interference with NOvA neutrino oscillation experiment reuse existing rings with only minor modifications J. Miller, BU PAVI 09 June, 2009 38

Mu2E & NOA/NuMI How do we deliver O(1018) bunched Mu2e s? Detector Hall To NuMI Use NuMI cycles in the Main injector to slow spill to Mu2e. No Impact on NOvA Results in: 6 batches x 4x1012 /1.33 s x 2x107 s/yr = 3.6 x1020protons/yr Boomerang Scheme Booster Batches transported partway through Recycler and injected directly into Accumulator Stack batches in Accumulator

Transfer to Debuncher Rebunch into Single Bunch: Mu2e and Recycler 38 nsec RMS, 200 MeV Period~ 1700 ns Slow resonant extraction: transverse, yields bunch train: desired pulsed beam J. Miller, BU PAVI 09 June, 2009 40 Beam Sharing

time to ramp allows us to fit eight extra Booster batches for Mu2e (can use 6) J. Miller, BU PAVI 09 June, 2009 41 Outline Brief introduction and theoretical motivation Description of main backgrounds, experimental technique and proposed apparatus Description of Fermilab Accelerator Potential Future Upgrades Conclusions J. Miller, BU PAVI 09 June, 2009 42

Experimental Advantage of e and Upgrade Path Production of lots of muons is relatively easy Conversion electron energy, 105 MeV, is far above the bulk of low energy decay electron background. Considerable improvement in the ultimate sensitivity is quite possible. With additional improvements in detectors, beam line, fluxes, it may be possible to get Re <10-18 or better. Contrast with e Method: look for back-to-back 53 MeV electron and photon e and energies are right at the maximum flux of electron energies from ordinary muon decay. There can be a significant rate of accidental coincidence between Michel electrons and photons from other events, or from radiative muon decay. These backgrounds are believed to limit future improvements in achievable limits on the branching ratio. J. Miller, BU PAVI 09 June, 2009 43

What is Project X? Project X is a concept for an intense 8 GeV proton source that provides beam for the Fermilab Main Injector and an 8 GeV physics program. The source consists of an 8 GeV superconducting linac that injects into the Fermilab Recycler J. Miller, BU PAVI 09 June, 2009 44 Mu2e and Project X First establish a signal or set a strong limit -what do we do next? Project X gives us a chance to upgrade the experiment by up to x100 available 8 GeV Power for intensity frontier 20 kW (current)

200 kW (Project X) 2000 kW (Project X Upgrades) J. Miller, BU PAVI 09 June, 2009 45 Upgrade Plans... Signal? Yes Yes No Change Z of Target to determine source of new physics 1. Both Prompt and DIO backgrounds must drop -18

to measure Reeee ~ 10-18 2. Detector, Muon Transport, Cosmic Ray Veto, Calorimeter J. Miller, BU PAVI 09 June, 2009 46 Schedule Received Stage-1 Approval and DOEs CD-0 anticipated shortly Technically Driven Schedule (wholly magnet driven) results in 2016 start of data taking Opportunities for Significant R&D, Test Beam, and Auxiliary Measurement work for students and university groups Mu2e Experiment Technically Driven Schedule 2009 2010 2011

2012 2013 2014 2015 2016 2017 2018 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Mag. CDR Final Magnet Design PSI Test Beam Magnet Construction, Installation, Commissioning First Physics Run 2E-17 SES Conclusions

Strong physics case for muon to electron conversion: Lepton flavor conservation properties are at the core of understanding why we have three generations A positive signal indicates new physics e can be large in most extensions to the SM e likely has the greatest potential experimental sensitivity for CLFV Energy reach can be well beyond forseeable accelerators Addresses P5 goal of Terascale (and often well beyond) sensitivity to new physics complementary to LHC, and has strong P5 endorsement Mu2e experiment is based on MECO design Innovative design to obtain the muon beam Many successful MECO reviews Physics Experimental design and Costing Exceptional fit at FNAL Desired beam can be had with modest modifications to existing facilities Operation with minimal impact on NoVA program Possibility of x100 improvement with Project X and detector upgrades J. Miller, BU PAVI 09 June, 2009

48 END J. Miller, BU PAVI 09 June, 2009 49 Vertical Drift Motion in a Toroid Toroidal Field: Axial field Bs=constant x 1/r. This gives a large dBs/dr Particle spiral drifts vertically (perpendicular to the plane of the toroid bend): D= vertical drift distance R=major toroid radius=2.9 m, pl=longitudinal momentum s/R = total toroid bend angle=900 D[m]=distance, B[T], p[GeV/c] pt=transverse momentum Define pitch pl B p

Toroid B field line 1 q s 1 D p ( ). 2 0.3 B R J. Miller, BU PAVI 09 R June, 2009 pl 50 Cosmic Ray Background Well-studied and controlled by previous muon conversion experiments Reactions which could produce a fake conversion electron:

Scattered electrons from cosmic ray muons in target or detectors Muons decaying in-flight in the Detector Solenoid Muon scattering from the target, then mistaken for an electron Muons interacting in shielding, producing hadrons or photons which may not register in a veto counter. MECO design 0.5 m steel, 2 m concrete shielding Active 4 scintillator veto, 2 out of three layers report, 10-4 inefficiency From simulations with ~70x cosmic flux, expect 0.021 events in 2x107 seconds running at FNAL J. Miller, BU PAVI 09 June, 2009 51 Sensitivity of Mu2e For Re Conversione = 10-15 ~40 events / 0.4 bkg (LHC SUSY?) For Re Conversione = 10-16

~4 events / 0.4 bkg Ree < 6 x 10-17 90% CL Source Number/3.6 x 1020 POT Decay-In-Orbit 0.225 Radiative capture 0.063 Muon Decay-In-Flight 0.063 Scattered e- 0.036 Decay-In-Flight < 0.004 J. Miller, BU PAVI 09

June, 2009 52 Electrons from Production Target Electrons present largest flux of particles during the proton injection Most traverse the beam line quickly compared to muons, pions, etc. Suppression depends on extinction + suppression in the beam line. Beamline and collimators are designed to strongly suppress electrons > 100 MeV from arriving at stopping target Simulation: With 107 electrons starting at the production target, none made it to the stopping target. Electrons >100 MeV entering the Detector Solenoid from the Transport Solenoid will have the wrong (too large) pitch when arriving at the detector compared to a conversion electron, due to gradient field in the stopping target region, except for target scatter. max< 60 degrees for electrons from stopping target max< 45 degrees for electrons from entrance of Detector Solenoid Estimate: 0.04 background events

J. Miller, BU PAVI 09 June, 2009 53 Power of Signal in Muon-Electron Conversion L. Calibbi, A. Faccia, A. Masiero, S. Vempati hep-ph/0605139 neutrino mass via the see--saw mechanism,analysis in SO(10) framework Neutrino-Matrix Like (PMNS) Minimal Flavor Violation(CKM) tan =10 Current measurement can distinguish between PMNS and MFV J. Miller, BU Mu2e 0 800

PAVI 09 1600 June, 2009 54 Detector Solenoid:Stopping Target and Detectors Tracker measures energy of electrons to <1MeV FWHM, high-side tail keV Calorimeter after the tracker: provides fast trigger, confirms energy and trajectory 2.4-2.9 m long, 0.5 cm diameter straws Specsz~1.5 mm, r~~200 m Target pt=55MeV/c Uniform B=1 T B=2 T 55 J. Miller, BU

PAVI 09 Graded B June, 2009 55 Detector Solenoid:Stopping Target and Detectors Tracker measures energy of electrons to <1MeV FWHM, high-side tail keV Calorimeter after the tracker: provides fast trigger, confirms energy and trajectory 2.4-2.9 m long, 0.5 cm diameter straws Specsz~1.5 mm, r~~200 m Uniform B=1 T B=2 T 56 J. Miller, BU PAVI 09 Graded B June, 2009

56 Tracking Projection of helical track Conversion electron has high momentum (pT) and has R large enough to pass outside octagon and be tracked DIO (pT < 55 MeV/c) does not! J. Miller, BU PAVI 09 June, 2009 57 Lepton Flavor Violation BABAR Pub07057 < 9 x 10-8-8 Belle and BABAR J. Miller, BU

PAVI 09 June, 2009 58 Energy Calibration Mu2e resolution: 300 keV on high-side tail, FWHM ~ 1 MeV Response function needs to be well established Proposed energy integration region: 103.6-105.1 MeV Proposed absolute energy calibration: ~0.1-0.2 MeV Calibration approaches: e , E( e ) ~ 70 MeV

e Stop in the stopping target Lower solenoid field to improve geometric overlap with detector Gives energy response function and energy calibration but at lower energy Reverse field to transport positive particles, or rotate collimator Use DIO spectrum to monitor calibrations Calibrate with a 100 MeV electrons from a dedicated accelerator J. Miller, BU PAVI 09 June, 2009 59 Antiproton-induced background Cross section for production by 8 GeV protons is small Only very low energy antiprotons are transported. Can move very slowly through beam line. When material is encountered, forms atom, annihilates producing energetic pions, gammas, etc.

Potentially dangerous background source. Eliminated by stopping all antiprotons in a very thin window in the middle of the Transport Solenoid Simulation: Secondary particles from antiproton annihilation tracked. Estimate: 0.006 counts Background is fairly continuous in time, unlike muons and pions J. Miller, BU PAVI 09 June, 2009 60 Magnetic Spectrometer: Rates vs. Time Rates start at 6 MHz/wire but 180 kHz/wire in live time window Each muon capture produces 2, 2n, 0.1p J. Miller, BU PAVI 61 09 June, 2009

Muon decay and nuclear reaction modes Ordinary decay modes of a free muon eeE(e-)max=52.6 MeV B.R. = .9986 ee B.R. = 1.4x10-2 ee+ee B.R. = 3.4x10-5 Muon reaction and decay modes when muon is bound in atomic orbit Muon decay in orbit(DIO), e-e+ E(e-)max=105.6 MeV Atomic BE- Nuclear recoil E Muon capture on the nucleus, - + ( + Highly allowed when muon wavefunction has significant overlap with nuclear wavefunction: significant even for low Z nuclei Radiative muon capture (RMC) on the nucleus, - +A (followed by ee,gives E(e)max=105.6 MeV Atomic BE 2(Nuclear recoil E) (mA-mA)c2

J. Miller, BU PAVI 09 June, 2009 62 Extinction Monitor Requirement: 10-9 extinction of proton beam between pulses MECO Concept: Monitor 1-2 GeV proton production rate 1-2 cm diameter collimators 5 kG-m field Momentum-select 1-2 GeV protons Measure energy, TOF Good shielding to suppress background Run occasionally during microbunch to normalize the calorimeter Expect ~1 proton every few minutes for 10 -9 extinction J. Miller, BU PAVI 09 Alternative being developed by COMET experiment with support of US-Japan agreement:

pressurized gas Cerenkov detector placed in front of production target. June, 2009 63 Muon to electron conversion Measure rate of the lepton flavor violating (LFV) reaction of neutrinoless muon to electron conversion in the field of a nucleus, relative to the ordinary muon capture rate on a nucleus, several thousand times better than previous measurements. Goal: Re< 6 x10-17, where Re is the measured ratio in a muonic atom, Re={Rate(-+A(N,Z) e-+A(N,Z)} / {Rate(-+A(N,Z) +A(N+1,Z-1)} Lm =+1, Le=0 -> Lm=0, Le=+1 In SM, suppressed far below experimental accessibility. Experimentally accessible rates are commonly predicted in new physics models excellent place to search for new physics. Current limit from SINDRUM II: Re<6.1x10-13 (unpublished) J. Miller, BU PAVI 09 June, 2009 64

Motion in a Solenoid with a Gradient Field In a magnetic field, low momentum charged particles follow helical paths along the field lines. The magnetic moment of the particle associated with the helical motion is approximately constant. For a relativistic particle, pt2/B=constant, pt B pt pt 0 B B0 , pl p 2 pt20 B B0 pl is continuously increasing in the direction of decreasing field Particle pitch increases when spiraling to lower field: pt decreases and pl increases. Particle pitch decreases when spiraling to higher field: pt increases and |pl| decreases. Particles are pushed in the direction of lower field 1 Bz B0 | Gz | z Br | Gz | r 2 Note that net qp t xB r points downstream regardless of q (if q flips sign, pt reverses direction) J. Miller, BU Br

PAVI 09 pt (Bz points out of page. Field decreases moving out of page, Gz <0. ) June, 2009 65 Magnetic Mirror If a particle spirals in the direction of higher field, pt increases and |pl| decreases: pt pt 0 B B0 , pl p 2 pt20 B B0 p Solenoid axis (~+B or ~-B) If the field becomes large enough, pt p, pl 0 and the particle is reflected, spiraling back toward lower field For a particle born in the middle of the PS, where B~3.5 T, the maximum pitch which can be reflected in the maximum 5T field upstream is (define anglebetween momentum and solenoid

axis) sin p / p B B 3.5 5 0.84 min t0 0 max 1230 Increases downstream flux of muons If a particle is born near the target where B~3.5 T, then the maximum (corresponding to minimum pitch) at the downstream end of the PS, where B=2.5 T, will be about 600. J. Miller, BU PAVI 09 June, 2009 66 Long Transit Time Background Particles with low longitudinal velocity will take a long time to traverse the beam line, arriving at the stopping target during the measurement period Antiprotons and radiative pion capture: Antiprotons are stopped by a thin window in middle of transport

Adjust detector start time until most long-transit time pions decay Example of a potential problem Pion decays into a muon early in the transport solenoid Muon can have small pitch and progress very slowly downstream Muon can decay after a long time into an electron Decay electron can be >100 MeV if p>75 MeV/c Electron could scatter in collimators a couple times, arriving at the target during the measurement period, where it could scatter into the detector acceptance To suppress this All straight sections of solenoids have <0 gradient, |dBs/ds|>0.02 T/m Greatly reduces number of particles (e.g) with small pitch Gradient criterion not necessary in curved solenoid sections, low pitch particles are swept away vertically by dBs/dr field gradient. J. Miller, BU PAVI 09 June, 2009 67

Who ordered that? I.I. Rabi, 1936 After the was discovered, it was logical to think the was just an excited electron. Early on, expected a large BR(ee) Then it was discovered that lepton flavor conservation prevented it.. To this day, the muon appears to be in every way an electron except for its mass and lepton flavor. Understanding lepton flavor conservation or violation is at the heart of understanding why we have generations of leptons Neutrino oscillations lepton flavor non-conservation Lepton flavor violation not yet seen in charged lepton interactions J. Miller, BU PAVI 09 June, 2009 68 Vertical Drift Motion in a Toroid

Toroidal Field: Bs=constant x 1/R. This gives a gradient dBs/dR. Particle spiral drifts vertically (perpendicular to the plane of the toroid bend): D= vertical drift distance R = major toroid radius, p s/R = toroid arc angle=900 pitch l cos( ) Bs D[m]= distance, B[T], p[GeV/c] p q s 1 1 D p( cos ). 0.3 B R 2 cos R pl (Note: particles with small pitch have large D, and particles with opposite sign drift in opposite directions cm Production Target

J. Miller, BU ) B field line in toroid Central Collimator PAVI 09 Stopping Target June, 2009 69 cm Physics Case Mu2e is compelling with or without the discovery of new physics at the LHC A range of models predict a signal just beyond current limits Tight constraints on models in the case of observations at the LHC (e.g. PMNS or MFV) Complementary information on any new LHC physics Probes up to 104 TeV for new physics if no new physics at LHC (e.g. SM Higgs and nothing else)

Offers a unique window to study CLFV Mu2e Will Either: Reduce the limit for Ree by ~ four orders of magnitude (Ree <6x10-17 @ 90% C.L.) Discover unambiguous proof of Beyond Standard Model physics at a scale directly relevant for the LHC With future more powerful muon sources, 10-18 or better appears feasible J. Miller, BU PAVI 09 June, 2009 70 What Does This Mean? design, prototyping, test beams... busy measuring beam, detector properties, ... background studies J. Miller, BU PAVI 09 data-taking June, 2009

71 Production Solenoid 4m long x 0.75 m radius, 0.30 m clear bore Protons enter solenoid in upstream direction Field holds many pions and their muon decay products in spirals Uniformly graded field: 5 T 2.5 T upstream downstream, target in center Pions and muons are pushed downstream by the field gradient some upstream-going particles are reflected back downstream (mirror effect), increasing the acceptance of particles into the transport solenoid. particles born with small pitches acquire larger pitches as they move downstream Bz B0 | Gz | z Proton beam 1 Br | Gz | r 2 Case: B field decreasing out of paper, Gz<0. Note that net qp t xB r

points downstream regardless of q (if q flips sign, pt reverses direction) J. Miller, BU y z Br x PAVI 09 pt June, 2009 72 Detector Solenoid: Tracker Octagon shape with 8 panels and vanes, each with 3 layers of straws Each straw 2.4-2.9 m long ~parallel to solenoid axis, 0.5 cm diameter The longitudinal coordinate is given by cathode pads.

Specsz~1.5 mm, r~~200 m Tracker measures energy of electrons to <1MeV FWHM, high-side tail keV (mainly due to energy straggling in target and multiple scattering in tracker) Uniform, B=1 T Graded B B=2 T pT<55 MeV/c misses tracker 73 J. Miller, BU PAVI 09 June, 2009 73 Quick Fermilab Glossary Booster: The Booster accelerates protons from the 400 MeV Linac up to 8 GeV Recycler Ring: Now: stacks beam for Tevatron; transport line for Mu2e Accumulator Ring: Now: momentum stacking successive pulses of antiprotons; 8 GeV protons for Mu2e Debuncher Ring (parallel to Accumulator):

Now: smooths out bunch structure to stack more pbars; rebunch and extraction for Mu2e J. Miller, BU PAVI 09 June, 2009 74 Muonic Atom Formation and Nuclear Capture Ordinary Muon Capture Rate, A( N , Z ) A( N 1, Z 1) ... Proportionalto theprobability of overlap between nucleus and muon times number of protons Hydrogenic Radial wavefunction: R(r) r Z3/2 Proportional to: # protons x Prob. nuclear overlap ~ Z4 Conversion process is coherent proportional to (# protons)2 x Prob. nuclear overlap ~Z5 Re High Z preferred. Max is at Z~50-60, slowly decreases after that. Nucleus

J. Miller, BU PAVI 09 June, 2009 - 75 Model-Independent Picture Add CLFV effective operators sets the energy scale, controls relative weights of terms Contact Terms Loops ? ? ? 0 Exchange of a massive new particle Supersymmetry and Heavy Neutrinos

Does not produce eee Also contributes to ee Quantitative Comparison? J. Miller, BU PAVI 09 June, 2009 76 Physics Reach of ee and ee Conversion Andr de Gouva, Project X Workshop Golden Book small: magneticmoment term large: contact term (TeV)) Project X Mu2e Mu2e 2) Mu2e ~2 times

more sensitivity than MEG in loopdominated physics MEG 3) Mu2e has far greater sensitivity to contact terms J. Miller, BU MEGA(>e) PAVI 09 higher mass scale 1) Mu2e scale extends to several x 103 TeV SINDRUM(e) June, 2009 77

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