A New Charged Lepton Flavor Violation Experiment: Muon-Electron Conversion at FNAL

muon converts to electron in the presence of a nucleus What is μe Conversion? muon converts to electron in the presence of a nucleus Charged Lepton Flavor Violation (CLFV) Related Processes: μ or τ → eγ, e+e-e, KL→μe, and more 3

Experimental Signal A Single Monoenergetic Electron If N = Al, Ee = 105. MeV energy depends on Z will use this many times, so take a moment

Overview Of Processes μ- stops in thin Al foil μ- in 1s state Al Nucleus ~4 fm μ- in 1s state the Bohr radius is ~ 10 fm, so the μ- sees the nucleus total disappearance rate =0.864 μ sec NORMALIZATION 60% captured μ- emits ν Al turns into Mg 40% decay-in-orbit decays by normal process but can recoil off nucleus BACKGROUND 2.2 μ sec 1.4

Why Normalize to Capture? Al turns into Mg Nuclear wavefunctions “cancel,” calculation simpler As muon cascades to 1s, X-rays give stop rate and Mg →Al yields a 2.6 MeV β followed by γ that can be used to measure capture rate μ- emits ν Al turns into Mg NORMALIZATION

Neutrino Oscillations and LFV

“Model-Independent” Picture “Loops” “Contact Terms” how can we use both experiments? Exchange of a new, massive particle Supersymmetry and Heavy Neutrinos Contributes to μ→eγ Does not produce μ→eγ

μe Conversion and μ→eγ κ Λ (TeV) 1) Mass Reach to 104 TeV 2) x10 beyond MEG in loop-dominated physics Project X Mu2e Mu2e higher mass scale generally acknowledge for experimental reasons mu e gamma will not get much better MEG MEGA SINDRUM κ

Supersymmetry and Mu2e in Minimal SU(5) J. Hisano, T. Moroi, K. Tobe and M. Yamaguchi, Phys. Lett. B 391, 341 (1997). [Erratum-ibid. B397, 357 (1997).]

Supersymmetry in Tau LFV L. Calibbi, A. Faccia, A. Masiero, S. Vempati hep-ph/0605139 Neutrino-Matrix Like (PMNS) Minimal Flavor Violation(CKM) neutrino mass via the see-saw mechanism, analysis is performed in an SO(10) framework

….and Muon-Electron Conversion Neutrino-Matrix Like (PMNS) Minimal Flavor Violation(CKM) Mu2e Phase I complementarity between Lepton Flavor Violation (LFV) and LHC experiments!

Contributions to μe Conversion note both loop processes and new particle exchange, will come back to this next slide

Design of Mu2e How can we do better? Examine previous best experiment What were the limitations? limitations from prompts limitations from Decay-in- Orbit How can we do better?

Previous Best Experiment SINDRUM-II Rμe < 6.1 x 10-13 in Au Want to probe to 10-16 or better ≈104 improvement

SINDRUM II Results Final SINDRUM-II on Au Note Two Background Events past Signal Region signal region radiative π’s? W. Bertl et al, Eur. Phys. J. C 47, 337-346 (2006)

What Limited SINDRUM-II? DC Beam no time separation between signal and prompt background point to radiative pi capture in b), explain degrader and veto. We use a different method, beam extinction, radiative π capture

How Can We Do Better? Requiring >103 increase in muon intensity from SINDRUM Requiring Pulsed Beam to Eliminate prompt backgrounds like radiative π capture protons out of beam pulse/ protons in beam-pulse < 10-9 and we must measure it

Advantage of Pulsed Beam target foils: muon converts here Recall: Muon-electron conversion signal is a single,monoenergetic electron pulsed beam lets us wait until after prompt backgrounds disappear delayed 105 MeV electron

Pulsed Beam Structure Tied to prompt rate and machine: FNAL “perfect” Want pulse duration << τμ, pulse separation ≈ τμ FNAL Accumulator has circumference 1.7μsec ! Extinction between pulses < 10-9 needed = # protons out of pulse/# protons in pulse 10-9 based on simulation of prompt backgrounds

achieving 10-9 is hard; normally get 10-2 – 10-3 Extinction Scheme Eliminate protons in beam in-between pulses: “Switch” dipole timing to switch signal and background: accept only out-of-time protons for direct measurement of extinction Other schemes under investigation Measurement: off-angle collimators and telescope? achieving 10-9 is hard; normally get 10-2 – 10-3

FNAL Beam Delivery FNAL has unique, major strength: Multiple Rings no interference with NOvA neutrino oscillation experiment reuse existing rings with only minor modifications big strength of FNAL: can run without interference

8 GeV Proton Beam MI/Recycler ¯ were used for p After TeVatron shut-down, Accumulator, Debuncher, and Recycler no longer needed for antiprotons were used for p ¯ 27

Detector and Solenoid Tracking and Calorimeter Decay into muons and transport to stopping target S-curve eliminates backgrounds and sign-selects Production: Magnetic bottle traps backward-going π that can decay into accepted μ’s

Production Solenoid: Protons enter opposite to outgoing muons – this is a central idea to remove prompt background Protons leave through thin window π’s are captured, spiral around and decay muons exit to right Target Shielding Proton Target Pions Protons enter here 4 m X 0.75 m

Transport Solenoid Curved solenoid eliminates line-of-sight transport of photons and neutrons Curvature drift and collimators sign and momentum select beam occasional μ+

Detector Solenoid octagonal tracker surrounding central region: radius of helix proportional to momentum low momentum particles and almost all DIO background passes down center so background goes down center and signal spirals out to tracker Al foil stopping target signal events pass through octagon of tracker and produce hits

Graded Fields Production Solenoid: graded from ~5.0 to 2.5T to (a) capture backwards-going pions and allow them to decay and (b) “reflect” backward-going muons Transport Solenoid: graded from ~2.5 to 2.0T to accelerate muons along beamline reflection nearly 100% efficient near detector, doubling statistics Detector Solenoid: graded from ~2.0 to 1T to “reflect” backwards-going electrons and send them into detector

Choice of Stopping Material: rate vs wait Stop muons in target (Z,A) Physics sensitive to Z: with signal, can switch target to probe source of new physics Why start with Al? rate normalized to Al 2.5 Rate 1.0 Z Kitano, et al., PRD 66, 096002 (2002) shape governed by relative conversion/capture rate, form factors, ...

Prompt Background and Choice of Z choose Z based on tradeoff between rate and lifetime: longer lived reduces prompt backgrounds Nucleus Rμe(Z) / Rμe(Al) Bound Lifetime Conversion Energy Fraction >700 ns Al(13,27) 1.0 864 nsec 104.96 MeV 0.45 Ti(22,~48) 1.7 328 nsec 104.18 MeV 0.16 Au (79,~197) ~0.8-1.5 72.6 nsec 95.56 MeV negligible

Detector σ = 200 μ transverse, 1.5 mm axially Octagon and Vanes of Straw Tubes Immersed in solenoidal field, so particle follows near-helical path up to dE/dx, scattering, small variations in field Particles with pT < 55 MeV do not pass through detector, but down the center Followed by Calorimeter 2800 axial straw tubes, 2.6 m by 5 mm, 25μ thick use return yoke as CR shield Calorimeter/Trigger: σ /E = 5%, 1024 3.5 × 3.5 × 12 cm PbWO4

Beam’s Eye View of Tracker Octagon and Vanes of Straw Tubes Immersed in solenoidal field Below pT = 55 MeV, electron stays inside tracker and is not seen; about 60° at 103.5 MeV Looking for helix as particle propagates downstream target target Only ~ 0.3% of DIO’s are even accepted

Signal and Background Rμe = 10-16

Backgrounds... bd = albedo from beam stop (after calorimeter): splashback, extra hits confusing pattern recognition

Two Classes of Backgrounds Prompt Decay-In-Orbit Source Mostly π’s produced in target Physics Background nearly indistinguishable from signal Solution Design of Muon Beam, formation, transport, and time structure Spectrometer Design: resolution and pattern recognition

Decay-in-Orbit Background High Rate Peak 52.8 MeV Detector insensitive to these Fraction within 3 MeV of signal is 5 x 10-15 Rate falls as (Emax- E)5Drives Resolution Requirement Zero energy neutrinos and coherent scatter off nucleus put DIO’s at conversion energy Rate falls as (Emax- E)5 Fraction within 2 MeV of signal is 1.2 x 10-15 fifth power expression approximate, see Shanker

Final Backgrounds For Rμe = 10-16 expect ~5 events / 0.5 bkg Extinction factor of 10-9 Source Number/ 4 x 1020 DIO 0.25 Radiative π capture 0.08 μ decay-in-flight Scattered e- 0.04 π decay in flight <0.004

Time Schedule

Cost Estimate

INFN-Pisa and Mu2e Experiment

R. Carosi,F. Cervelli,M.Incagli, T. Lomtadze, L.Ristori, F. Scuri Participants: R. Carosi,F. Cervelli,M.Incagli, T. Lomtadze, L.Ristori, F. Scuri and C. Vannini (staff members only)

Medium Term Milestone: TDR

A light supporting structure Mechanical Project A light supporting structure

Choice: LSO vs PbWO4 vs Kloe (?) Calorimeter Proposal: 1024 3.5 × 3.5 × 12 cm PbWO4 Choice: LSO vs PbWO4 vs Kloe (?)

Choice: Long vs Transv vs 4p (?) Tracker target Octagon and Vanes of Straw Tubes Immersed in solenoidal field Below pT = 55 MeV, electron stays inside tracker and is not seen; about 60° at 103.5 MeV Looking for helix as particle propagates downstream Only ~ 0.3% of DIO’s are even accepted target Choice: Long vs Transv vs 4p (?)

Alternative Tracker T-tracker (T for transverse): 260 sub-planes sixty 5 mm diameter conducting straws length from 70-130 cm total of 13,000 channels T-Tracker Pattern Recognition Difficult

Trigger Design and related Electronics Trigger: Calorimeter + (Tracker) + (TOF)

Backgrounds Studies bd = albedo from beam stop (after calorimeter): splashback, extra hits confusing pattern recognition

A contribution to Solenoid (?) Production Transportation Detection

Conclusion A difficult experiment looking for NP cLFV experiment at the High Intensity Frontier: A difficult experiment looking for NP Enforcing collaboration between INFN and Fermilab Nothing is frozen: other participants are welcome (expecially from Meg and other Flavour Physics…)

BACKUPS R. Bernstein, FNAL

Better than MECO because of better beam structure if MECO could handle rates, Mu2e at FNAL can as well: pre-project X or with Project X MECO Mu2e Booster Mu2e Project X, no expt. upgrade Mu2e Project X, expt. upgrade protons/sec 40x1012 (design) 18x1012 70x1012 160x1012 average beam power 50 kW (design) 23 kW 90 kW 200 kW duty factor 0.5 s on, 0.5 s off, 50% 75-90% instantaneous rate 80x1012 (design) 20x1012 77x1012 220x1012 short term beam power 100 kW (design) 25 kW 100 kW 220 kW Beam pulse period, msec 1.35 1.65 Data collection time interval msec 0.7-1.35 0.7-1.65

André de Gouvêa, Project X Workshop Golden Book μe Conversion and μ→eγ Λ (TeV) 1) Mass Reach to 104 TeV 2) x10 beyond MEG in loop-dominated physics Project X Mu2e Mu2e higher mass scale generally acknowledge for experimental reasons mu e gamma will not get much better MEG MEGA SINDRUM κ André de Gouvêa, Project X Workshop Golden Book

Prompt Background and Choice of Z choose Z based on tradeoff between rate and lifetime: longer lived reduces prompt backgrounds Prompt Background and Choice of Z Nucleus Rμe(Z) / Rμe(Al) Bound Lifetime Conversion Energy Fraction >700 ns Al(13,27) 1.0 864 nsec 104.96 MeV 0.45 Ti(22,~48) 1.7 328 nsec 104.18 MeV 0.16 Au (79,~197) ~0.8-1.5 72.6 nsec 95.56 MeV negligible

Quick Fermilab Glossary Booster: The Booster accelerates protons from the 400 MeV Linac to 8 GeV Accumulator: momentum stacking successive pulses of antiprotons now, 8 GeV protons later Debuncher: smooths out bunch structure to stack more p now; rebunch for mu2e Recycler: holds more p than Accumulator can manage, “store” here _ _

Cost and Schedule A detailed cost estimate of the MECO experiment performed just before RSVP was cancelled: (in Actual Year $, including inflation) Solenoids and cryogenics: $59M Remainder of experimental apparatus: $21M Additional Fermilab costs have not been worked out in detail accelerator and civil construction costs are being worked out Estimate for contingency, overhead, etc then yields $120M before beamline and civil costs

Schedule: 2016 for commissioning Based on the original MECO proposal, we believe the experiment could be operational within 3-4 years of “CD-2/3a” = begin large, long-lead time purchases Use NOνA experience for time for DOE Approval Process Use MECO schedule for Technical Issues, especially solenoid construction Aggressive but possible

Rates at Beginning of > 700 nsec Live Window, so these are highest ≈ 2 hits per straw during beam flash Rates are manageable: (1/4 of MECO instantaneous) Rates In Tracker

Final Backgrounds For Rμe = 10-16 expect ~5 events / 0.5 bkg Extinction factor of 10-9 Source Number/ 4 x 1020 DIO 0.25 Radiative π capture 0.08 μ decay-in-flight Scattered e- 0.04 π decay in flight <0.004

Mu2e Phase II Signal? Yes No 1. Change Z of Target to determine source of new physics 2. Need Project X to provide statistics 1. Probe additional two orders of magnitude made possible by Project X 2. Need upgrades to muon transport and detector

available 8 GeV Power for intensity frontier 20 kW (current) 200 kW (Project X) 2000 kW (Project X Upgrades) Mu2e and Project X Project X is required for the next step Needed whether first phase sees a signal or sets a limit Well timed for Mu2e first phase, late this decade or early next

Conclusions With Mu2e we would either: Reduce the limit for Rμe by more than four orders of magnitude (Rμe <6x10-17 @ 90% C.L.) Discover unambiguous proof of Beyond Standard Model physics In a second phase (Project X) Extend the limit by up to two orders of magnitude Study the details of new physics

Z-Dependence For a small nucleus compared to the extent of the muon wf, the Schroedinger equation gives hydrogenic wavefunctions which scale like Z**3 at small radius. There are Z protons in the nucleus, so the probability of ordinary capture goes like Z**4. For a conversion process, the cross section is coherent and therefore goes like Z**2 (or A**2) rather than Z in the case of ordinary capture. So overall the process goes like Z**5. Strictly speaking this overlap argument only works well for short-range forces like the weak force, since we assume that the probability of reaction is proportional to the overlap between the nuclear and the muon wavefunctions. For the EM force it will not work as well. Therefore Rμe is proportional to Z4/Z3 = Z for small Z. As Z increases, the finite size of the nucleus becomes important. The muon wavefunction is inside the nucleus and does not see the full Z, reducing the wavefunction overlap. Also, there are relativistic effects which reduce the Z dependence, and a few other effects.

Outline The search for muon-electron conversion Experimental Technique Fermilab Accelerator Project X Upgrades and Mu2e

Outline The search for muon-electron conversion Experimental Technique Fermilab Accelerator Project X Upgrades and Mu2e

Outline The search for muon-electron conversion Experimental Technique Fermilab Accelerator Project X Upgrades and Mu2e

History of CLFV Searches supersymmetry may turn on around here in many models, and this plot does not include tau channels, discussed later supersymmetry ~ 10-15

Current and Planned Lepton Flavor Violation Searches Neutrino Oscillations! τ LFV current limits at 10-7 for τ→μγ MEG and μ→eγ Mu2e: Strengths of muon-electron conversion Complementarity to other processes what’s going on in the world now?

“Model-Independent” Picture “Loops” “Contact Terms” how can we use both experiments? Exchange of a new, massive particle Supersymmetry and Heavy Neutrinos Contributes to μ→eγ Does not produce μ→eγ Quantitative Comparison?

Alternative Tracker T-Tracker Pattern Recognition Difficult but T-tracker (T for transverse): 260 sub-planes sixty 5 mm diameter conducting straws length from 70-130 cm total of 13,000 channels T-Tracker Pattern Recognition Difficult but Kalman Filter is promising

L-Tracker vs. T-Tracker T-Tracker: straws perp to beam More prone to pattern recognition errors? L-Tracker vs. T-Tracker L-Tracker: straws along beam Conceptually simpler tracking Basis of MECO Where does support/infrastructure go? Material in electron path Can anyone build straws 0.5 cm × 2.6m in vacuum? Active Investigation: kalman filter, applied to both on same events work just beginning help welcome!

Background Rates vs. Time 0-1400 nsec 700 nsec beam e- Rate (15 MHz/wire) μ DIF divide by 4 FNAL/BNL this is why we wait 700 nsec Protons in stopping tgt 700-1400 nsec Rate (560 kHz/wire)

Expected Resolution We must understand resolution this side lowers signal acceptance We must understand resolution Measure resolution with special runs varying target foils, field, location of stopping target this side smears background into signal relevant tail for DIO σ ~110 keV

Proposed Site

History of Lepton Flavor Violation Searches 1 10-2 K0 +e- K+ + +e- - 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 1940 1950 1960 1970 1980 1990 2000 2010

L-Tracker vs. T-Tracker T-Tracker: straws perp to beam More prone to pattern recognition errors? L-Tracker vs. T-Tracker L-Tracker: straws along beam Conceptually simpler tracking Basis of MECO Where does support/infrastructure go? Material in electron th Can anyone build straws 0.5 cm × 2.6m in vacuum? Active Investigation: kalman filter, applied to both on same events work just beginning help welcome!

Rates In Tracker Rates at Beginning of > 700 nsec Live Window, so these are highest ≈ 2 hits per straw during beam flash Rates are manageable: (1/4 of MECO instantaneous)

Expected Resolution We must understand resolution this side lowers signal acceptance We must understand resolution Measure resolution with special runs varying target foils, field, location of stopping target this side smears background into signal relevant tail for DIO σ ~110 keV

Background Studies Rμe = 10-16

Two Classes of Backgrounds Prompt Decay-In-Orbit Source Mostly π’s produced in target Physics Background nearly indistinguishable from signal Solution Design of Muon Beam, formation, transport, and time structure Spectrometer Design: resolution and pattern recognition

Decay-in-Orbit Background High Rate Peak 52.8 MeV Detector insensitive to these Fraction within 3 MeV of signal is 5 x 10-15 Rate falls as (Emax- E)5Drives Resolution Requirement Zero energy neutrinos and coherent scatter off nucleus put DIO’s at conversion energy Rate falls as (Emax- E)5 Fraction within 2 MeV of signal is 1.2 x 10-15 fifth power expression approximate, see Shanker

A Contribution to Solenoid Construction (?) Decay into muons and transport to stopping target S-curve eliminates backgrounds and sign-selects Production: Magnetic bottle traps backward-going π that can decay into accepted μ’s

Production Solenoid: Protons enter opposite to outgoing muons – this is a central idea to remove prompt background Protons leave through thin window π’s are captured, spiral around and decay muons exit to right Target Shielding Proton Target Pions Protons enter here 4 m X 0.75 m

Transport Solenoid Curved solenoid eliminates line-of-sight transport of photons and neutrons Curvature drift and collimators sign and momentum select beam occasional μ+

Detector Solenoid octagonal tracker surrounding central region: radius of helix proportional to momentum low momentum particles and almost all DIO background passes down center so background goes down center and signal spirals out to tracker Al foil stopping target signal events pass through octagon of tracker and produce hits

Contributions from Pisa

Better than MECO because of better beam structure if MECO could handle rates, Mu2e at FNAL can as well: pre-project X or with Project X MECO Mu2e Booster Mu2e Project X, no expt. upgrade Mu2e Project X, expt. upgrade protons/sec 40x1012 (design) 18x1012 70x1012 160x1012 average beam power 50 kW (design) 23 kW 90 kW 200 kW duty factor 0.5 s on, 0.5 s off, 50% 75-90% instantaneous rate 80x1012 (design) 20x1012 77x1012 220x1012 short term beam power 100 kW (design) 25 kW 100 kW 220 kW Beam pulse period, msec 1.35 1.65 Data collection time interval msec 0.7-1.35 0.7-1.65

Rates at Beginning of > 700 nsec Live Window, so these are highest ≈ 2 hits per straw during beam flash Rates are manageable: (1/4 of MECO instantaneous) Rates In Tracker