A Muon to Electron Experiment at Fermilab

Eric PrebysFermilab

For the Mu2e Collaboration

March 10, 2010

R.M. Carey, K.R. Lynch, J.P. Miller*, B.L. Roberts - Boston University

W.J. Marciano, Y. Semertzidis, P. Yamin - Brookhaven National Laboratory

Yu.G. Kolomensky - University of California, Berkeley

W. Molzon - University of California, Irvine

C.M. Ankenbrandt , R.H. Bernstein*, D. Bogert, S.J. Brice, D.R. Broemmelsiek, R.M. Coleman, D.F. DeJongh, S. Geer, D.A. Glezinski, D.F. Johnson, R.K. Kutsche, M.A. Martens, S. Nagaitsev, D.V. Neuffer, M. Popovic, E.J. Prebys, R.E. Ray,

V.L. Rusu, P. Shanahan, M.J. Syphers, R.S. Tschirhart, H.B. White Jr., K. Yonehara, C.Y. Yoshikawa – Fermi National Accelerator Laboratory

K.J. Keeter, E. Tatar - Idaho State University

P.T. Debevec, G.D. Gollin, D.W. Hertzog, P. Kammel - University of Illinois, Urbana-Champaign

V. Lobashev - Institute for Nuclear Research, Moscow, Russia

D.M. Kawall, K.S. Kumar - University of Massachusetts, Amherst

R.J. Abrams, M.A.C. Cummings, R.P. Johnson, S.A. Kahn, S.A. Korenev, T.J. Roberts, R.C. Sah - Muons, Inc.

A.L. de Gouvea - Northwestern University

F. Cervelli, R. Carosi, M. Incagli, T. Lomtadze, L. Ristori, F. Scuri, C. Vannini - Instituto Nazionale di Fisica Nucleare Pisa

M.D. Cororan - Rice

R.S. Holmes, P.A. Souder - Syracuse University

M.A. Bychkov, E.C. Dukes, E. Frlez, R.J. Hirosky, A.J. Norman, K.D. Paschke, D. Pocanic - University of Virginia

J. Kane – College of William and Mary

2March 10, 2010University of Maryland HEP Seminar

This effort has benefited greatly from over a decade of voluminous work done by the MECO collaboration, not all of whom have chosen to join the current collaboration.


Theoretical Motivation Experimental Technique Making Mu2e work at Fermilab Sensitivities Future Upgrades Conclusion


The study or rare particle decays allows us to probe mass scales far beyond those amenable to direct searches.

Among these decays, rare muon decays offer the possibility of experimentally clean and unambiguous evidence of physics beyond the current Standard Model.

Such searches are a natural part of the “Intensity Frontier”, which is being proposed for Fermilab after the end of the current collider program.

In the case of muon conversion, we can take advantage of a great deal of work that has already been done in the planning of the Muon to Electron Conversion Experiment (MECO), which was proposed at Brookhaven.


Forbidden in Standard Model Observation of neutrino mixing shows this can occur at a very small rate

Photon can be real (->e) or virtual (N->eN)

Standard model B.R. ~O(10-50)

e

0Z

First Order FCNC: Higher order dipole “penguin”:

e

Virtual mixing

W


Because extensions to the Standard Model couple the lepton and quark sectors, lepton number violation is virtually inevitable.

Charged Lepton Flavor Violation (CLFV) is a nearly universal feature of such models, and the fact that it has not yet been observed already places strong constraints on these models.

CLFV is a powerful probe of multi-TeV scale dynamics: complementary to direct collider searches

Among various possible CLFV modes, rare muon processes offer the best combination of new physics reach and experimental sensitivity


?

?

?

Flavor Changing Neutral Current

e

?N N

Mediated by massive neutral Boson, e.g. Leptoquark Z’ Composite

Approximated by “four fermi interaction”

Dipole (penguin)

Can involve a real photon

Or a virtual photon

?

?

?


Similar to ewith important advantages: No combinatorial background Because the virtual particle can be a photon or heavy neutral

boson, this reaction is sensitive to a broader range of BSM physics Relative rate of eand NeNis the most important clue

regarding the details of the physics

105 MeV e-

• When captured by a nucleus, a muon will have an enhanced probability of exchanging a virtual particle with the nucleus.

• This reaction recoils against the entire nucleus, producing the striking signature of a mono-energetic electron carrying most of the muon rest energy


Courtesy: A. de Gouvea

?

?

?

Sindrum IIMEGA

MEG proposal

We can parameterize the relative strength of the dipole and four fermi interactions.

This is useful for comparing relative rates for NeN and e


1

10-2

10-16

10-6

10-8

10-10

10-14

10-12

1940 1950 1960 1970 1980 1990 2000 2010

Initial mu2e Goal

- N e-N

+ e+ + e+ e+ e-

K0 +e-

K+ + +e-

SINDRUM II

Initial MEG Goal

10-4

10-16


Example Sensitivities*

CΛ = 3000 TeV

-4HH μμμeg =10 ×g

Compositeness

Second Higgs doublet

2Z

-17

M = 3000 TeV/c

B(Z μe) <10

Heavy Z’, Anomalous Z

coupling

Predictions at 10-15

Supersymmetry

2* -13μN eNU U = 8×10

Heavy Neutrinos

L

2μd ed

M =

3000 λ λ TeV/c

Leptoquarks

*After W. Marciano


Examples with >>1 (no e signal):LeptoquarksZ-primeCompositeness

SU(5) GUT Supersymmetry: << 1

Littlest Higgs: 1

Br(e)

Randall-Sundrum: 1

MEG

mu2e

10-12

10-14

10-16

10-1110-1310-15

R(TieTi)

10-13 10-11 10-9

Br(e)

10-16

10-10

10-14

10-12

10-10

R(TieTi)


Very high rate Peak energy 52 MeV Must design detector to be

very insensitive to these.

Nucleus coherently balances momentum

Rate approaches conversion (endpoint) energy as (Es-E)5

Drives resolution requirement.

N

e

e

e

e

Ordinary: Coherent:


Rate limited by need to veto prompt backgrounds!

>e Conversion: Sindrum II

12103.4capture

Ti

TieTiR e

11

12

11

2

102.72100.1102.1102.1

eeee

ee e

LFV Decay:

High energy tail of coherent Decay-in-orbit (DIO)


Eliminate prompt beam backgrounds by using a primary beam with short proton pulses with separation on the order of a muon life time

Design a transport channel to optimize the transport of right-sign, low momentum muons from the production target to the muon capture target.

Design a detector to strongly suppress electrons from ordinary muon decays

~100 ns ~1.5 s

Prompt backgrounds

live window


Single, monoenergetic electron with E=105 MeV, coming from the target, produced ~1 s (Al ~ 880ns) after the “” bunch hits the target foils

• Need good energy resolution: ≲0.200 MeV

• Need particle ID• Need a bunched beam with less

than 10-9 contamination between bunches


negligible 95.56 MeV10.08 MeV.0726 s~0.8-1.5Au(79,~197)

0.16

0.45

Prob decay >700 ns

104.18 MeV

104.97 MeV

Conversion Electron Energy

1.36 MeV.328 s1.7Ti(22,~48)

0.47 MeV.88 s1.0Al(13,27)

Atomic Bind. Energy(1s)

Bound lifetime

Re(Z) / Re(Al)

Nucleus

Aluminum is nominal choice for Mu2e

Dipole rates are enhanced for high-Z, but Lifetime is shorter for high-Z

Decreases useful live window Also, need to avoid background from radiative muon capture

ee

NN Want M(Z)-M(Z-1) < signal energy


MECO spectrometer design

for every incident proton 0.0025 ’s are stopped in the 17 0.2 mm Al target

foils


Production RegionProduction Region

• Axially graded 5 T solenoid captures low energy backward and reflected pions and muons, transporting them toward the stopping target

• Cu and W heat and radiation shield protects superconducting coils from effects of 50kW primary proton beam

Superconducting coils

Production TargetHeat & Radiation Shield

Proton Beam

5 T2.5 T


Transport SolenoidTransport Solenoid

• Curved solenoid eliminates line-of-sight transport of photons and neutrons

• Curvature drift and collimators sign and momentum select beam

• dB/ds < 0 in the straight sections to avoid trapping which would result in long transit times

Collimators and pBar Window

2.5 T

2.1 T


Detector RegionDetector Region

1 T1 T

2 T

• Axially-graded field near stopping target to sharpen acceptance cutoff.

• Uniform field in spectrometer region to simplify momentum analysis

• Electron detectors downstream of target to reduce rates from and neutrons

Stopping Target Foils Straw Tracking Detector

Electron Calorimeter


Production

Solenoid

Transport Solenoid

Detector Solenoid


E~3-15 MeV

Vital that e- momentum < signal momentum


3000 2.6 m straws (r,) ~ 0.2 mm

17000 Cathode strips z) ~ 1.5 mm

1200 PBOW4 cyrstals in electron calorimeter E/E ~ 3.5%

Resolution: .19 MeV/c


Re = 10-16 gives 5 events for 4x1020 protons on target

0.4 events background, half from out of time beam, assuming 10-9 extinction Half from tail of coherent

decay in orbit Half from prompt

Coherent Decay-in-orbit, falls as (Ee-E)5


1992 MELC proposed at Moscow Meson Factory

1997MECO proposed for the AGS at Brookhaven as part of RSVP (at this time, experiment incompatible with Fermilab)

1998-2005 intensive work on MECO technical design: magnet system costed at $58M, detector at $27M

July 2005 RSVP cancelled for financial reasons

2006 MECO subgroup + Fermilab physicists work out means to mount experiment at Fermilab

June 2007 mu2e EOI submitted to FermilabOctober 2007 LOI submitted to Fermilab

Fall 2008 mu2e submits proposal to FermilabNovember

2008 Stage 1 approval. Formal Project Planning begins

2010 technical design approval: start of construction2014? first beam

27March 10, 2010 University of Maryland HEP Seminar

Fermilab Built ~1970

200 GeV proton beams Eventually 400 GeV

Upgraded in 1985 900GeV x 900 GeV p-pBar collisions Most energetic in the world ever since

Upgraded in 1997 Main Injector-> more intensity 980 GeV x 980 GeV p-pBar collisions Intense neutrino program

Will become second most energetic accelerator (by a factor of seven) when LHC comes on line ~2009 What next???


MiniBo

oNE/

BNB

NUM

I


“Preac” - Static Cockroft-Walton generator

accelerates H- ions from 0 to 750 KeV.

“Old linac”(LEL)- accelerate H- ions from 750 keV to 116

MeV

“New linac” (HEL)- Accelerate H- ions from 116 MeV to 400 MeV


• Accelerates the 400 MeV beam from the Linac to 8 GeV

• Operates in a 15 Hz offset resonant circuit

• Sets fundamental clock of accelerator complex

•From the Booster, 8 GeV beam can be directed to

• The Main Injector

• The Booster Neutrino Beam (MiniBooNE)

• A dump.

•More or less original equipment


• The Main Injector can accept 8 GeV protons OR antiprotons from

• Booster

• The anti-proton accumulator

• The Recycler (which shares the same tunnel and stores antiprotons)

• It can accelerate protons to 120 GeV (in a minimum of 1.4 s) and deliver them to

• The antiproton production target.

• The fixed target area.

• The NUMI beamline.

• It can accelerate protons OR antiprotons to 150 GeV and inject them into the Tevatron.


Protons are accelerated to 120 GeV in Main Injector and extracted to pBar target

pBars are collected and phase rotated in the “Debuncher”

Transferred to the “Accumulator”, where they are cooled and stacked

Not used for NOvA


Roughly 6*(4x1012 batch)/(1.33 s)*(2x107 s/year)=3.6x1020 protons/year available

MI uses 12 of 20 available Booster Batches per 1.33 second cycle

Preloading for NOvA

Available for 8 GeV program

Recycler

Recycler MI transfer

15 Hz Booster cyclesMI NuMI cycle (20/15 s)


Deliver beam to Accumulator/Debuncher enclosure with minimal beam line modifications and no civil construction.

Recycler(Main Injector

Tunnel)

MI-8 -> Recycler done for NOvA

New switch magnet extraction to P150 (no need for kicker)


Inject a newly accelerated Booster batch every 67 mS onto the low momentum orbit of the Accumulator

The freshly injected batch is accelerated towards the core orbit where it is merged and debunched into the core orbit

Momentum stack 3-6 Booster batches

T<133ms

T=134ms

T=0

Energy

1st batch is injected onto the injection orbit

1st batch is accelerated to the core orbit

T<66ms

2nd Batch is injected

T=67ms

2nd Batch is accelerated

3rd Batch is injected


March 10, 201037 37

Booster-Era Beam Timelines for Mu2E Experiment

Base line scenario. Numerous other options being discussed.

University of Maryland HEP Seminar

Momentum stack 6 Booster batches directly in Accumulator (i.e. reverse direction)

Capture in 4 kV h=1 RF System.

Transfer to Debuncher

Phase Rotate with 40 kV h=1 RF in Debuncher

Recapture with 200 kV h=4 RF system t~40 ns


Exploit 29/3 resonance Extraction hardware similar to

Main Injector Septum: 80 kV/1cm x 3m Lambertson+C magnet ~.8T x

3m


RF noise, gas interaction, and intrabeam scattering cauase beam to “wander out” of the RF bucket.

D is the dispersion function: Transverse Offset = ΔE/E

× D

Anticipate installation of collimator in region with dispersion, removing off-momentum particles: Momentum scraping

University of Maryland HEP Seminar40March 10, 2010

Possible change from baseline in proposal: two stage collimation Dipoles at 0 and 360 Collimators at 90 and 180

University of Maryland HEP Seminar41

Baseline design, single collimator

March 10, 2010

Requires new building.

Minimal wetland issues.

Can tie into facilities at existing experimental hall.


Proton flux 1.8x1013 p/sRunning time 2x107 sTotal protons 3.6x1020 p/yr stops/incident proton 0.0025 capture probability 0.60Time window fraction 0.49Electron trigger efficiency 0.90Reconstruction and selection efficiency

0.19

Detected events for Re = 10-16 4.5

43March 10, 2010 University of Maryland HEP Seminar

Muon decay in orbit: → e

• Ee < mc2 – ENR – EB

• N (E0 - Ee)5

• Fraction within 3 MeV of endpoint 5x10-15

• Defeated by good energy resolution

Radiative muon capture: Al → Mg

• endpoint 102.5 MeV• 10-13 produce e- above 100 MeV

1. Stopped Muon Induced Backgrounds


2. Beam Related Backgrounds• Suppressed by minimizing beam

between bunches– Need ≲ 10-9 extinction– (see previous slides)

• Muon decay in flight: → e

• Since Ee < mc2/2, p > 77 GeV/c• Radiative capture:N →N*, Z → ee

• Beam electrons• Pion decay in flight: → ee

3. Asynchronous Backgrounds• Cosmic rays

• suppressed by active and passive shielding


Roughly half of background is beam related, and half interbunch contamination related

Total background per 4x1020 protons, 2x107 s: 0.43 events

Signal for Re = 10-16: 5 eventsSingle even sensitivity: 2x10-17

90% C.L. upper limit if no signal: 6x10-17

Blue text: beam related.


Beam delivery schemes Try to minimize charge in Accumulator at one time. Generally a trade-off that increases instantaneous rate.

Recalculating rates and backgrounds Models and data on low energy pion production have come a long way in

recent years. Optimizing magnet design

Original design based on SSC superconductor, which has since mysteriously vanished.

Is magnetic mirror worth it? New detector options

Investigating low pressure drift chamber Similar mass and less probability of fakes

Calibration schemes How can we convince the world we can measure something at a < 10-16

BR? Siting optimization and synergy with other programs

g-2 Muon collider R&D


One 5 Hz pulses every 1.4 s Main Injector cycle = 2.1MW at 120 GeV

This leaves six pulses (~860 kW) available for 8 GeV physics These will be automatically stripped and stored in the Recycler,

and can also be rebunched there.


Beam delivery Accumulator/Debuncher (like initial operation)?

How much beam can we put into the Accumulator and Debuncher and keep the beam stable?

Radiation issues (already a problem at initial intensities). Directly from Recycler?

Not enough aperture for conventional resonant extraction.

Investigating more clever ideas


Achieve sufficient extinction of proton beam. Current extinction goal directly driven by total protons

Upgrade target and capture solenoid to handle higher proton rate Target heating Quenching or radiation damage to production solenoid

Improve momentum resolution for the ~100 MeV electrons to reject high energy tails from ordinary DIO electrons. Limited by multiple scattering in target and detector planes Requirements at or beyond current state of the art.

Operate with higher background levels. High rate detector

Manage high trigger rates All of these efforts will benefit immensely from the knowledge and

experience gained during the initial phase of the experiment. If we see a signal a lower flux, can use increased flux to study in detail

Precise measurement of Re Target dependence Comparison with e rate


(from Dep. Director Y-K Kim)


We have proposed a realistic experiment to measure

Single event sensitivity of Re=2x10-17

90% C.L. limit of Re<6x10-17

This represents an improvement of more than four orders of magnitude compared to the existing limit, or over a factor of ten in effective mass reach. For comparison TeV -> LHC = factor of 7 LEP 200 -> ILC = factor of 2.5

Potential future upgrades could increase this sensitivity by one or two orders of magnitude

ANY signal would be unambiguous proof of physics beyond the Standard Model

The absence of a signal would be a very important constraint on proposed new models.

capture Al

AlAl

eR e


Eliminate prompt beam backgrounds by using a primary beam with short proton pulses with separation on the order of a muon life time

Design a transport channel to optimize the transport of right-sign, low momentum muons from the production target to the muon capture target.

Design a detector to strongly suppress electrons from ordinary muon decays

~100 ns ~1.5 s

Prompt backgrounds

live window

March 10, 2010 53University of Maryland HEP Seminar

Goal: make total backgrounds related to inter-bunch beam roughly equal to other backgrounds.

Need extinction at a level of 10-9 or better!

Blue text: beam related.


In ring Momentum scraping Gap-clearing kicker 10-4 to 10-5?

In beam line System of AC dipoles and collimators

Think minature golf 10-5 to 10-6 (at least)

Monitoring Very important to measure extinction Big question

Can we measure inter-bunch contamination bunch by bunch, or only statistically?


During h=4 capture, some beam may be captured in wrong bucket. Install gap cleaning kicker. Fire once per cycle, just prior to

extraction. RF noise or gas interactions

can cause beam to “wander” out of bucket, but tends to be driven well off momentum, as shown at right Noise set to 1% to exaggerate

effect.


Animations courtesy of Mike Syphers

Momentum scraping in high dispersion sections can capture particles lost from bunches.

Still working to understand efficiency. In principle can be very high.


Animations courtesy of Mike Syphers

Two matched dipoles at 180 phase separation Collimation channel at 90 Beam is transmitted at node

System resonant at half bunch frequency (~300 kHz)

Parameter Value CommentKinetic Energy 8 GeV

Emittance (95%) 20 -mm-mrErms 71 MeV

Beam line admittance 50 -mm-mr Set by collimators


Consider it axiomatic that some beam may be present anywhere in the admittance of the beam line Historically very hard to predict or model.

Therefore, it’s important to have the beam admittance well defined by a collimation system, rather than rely on the limiting aperture of magnets, beam pipes, etc.

For the moment, assume that the defining admittance of the beam line is equal to the defining admittance of the collimation channel.

March 10, 2010University of Maryland HEP Seminar 59

*al la FNAL-BEAM-DOC-2925

Beam fully extinguished when deflection equals twice full admittance (A) amplitude

At collimator:

x

Af 2

At kicker:Full scale deflection

Fraction of FS to extinguish


Phase space (live window ): Full amplitude:

Short live window -> large “extra” amplitude


TLL

TAB

LgwBgwLBU

xx2/3

2/1

2/12/1

222

0

22

0

20

0

181

21

21

Falls with x

For a particular x, there is an optimum length L0: xx TL

0

For which the optimized parameters are:

2/5122/52

2

0min

22/32/12

3

2/12/12/1

2/12/1

1161

2

2

4

xx

xx

opt

xx

opt

xx

opt

ATABU

ATABB

AT

Ag

AA

w


Parameter Value Commentx 50 m Typical beam line beta max

Effective length (L) 2 m

Full width (w) 5 cm

Vertical gap (g) 1 cm Scaled up for practicalityPeak field (B0) 600 Gauss

Peak stored energy (U) 1.43 J A little over twice the minimum

Recent analyses show that the pararameters are challenging Will probably go to larger , and longer magnets


Symmetric about 2m collimator with x = 50m, y= 1m, x = .25 (at collimator center)

Shortest line which fits constraints (32 m) Small x (7.9 m) means small hole (x/y = 1.29 x 2.54 cm)


Specified field and frequency leads to high voltages (few kV)


The amount of beam transmitted (or which hits the target) is given by

This can be expressed in a generic way as

Where

dx

A95

Lateral displacement

Half-aperture

emittance

admittance



3 harmonic design of MECO 3 harmonics (1x, 2x, and 3x bunch rate) generate ~square

wave. Transmits at peak

Single harmonic designas in proposal Runs at half of bunch rate Transmits on the null

Modified sine wave Add high harmonic to reduce

slewing in transmission window. Important questions

Transmission during 200 ns live window

Magnet design Is second magnet necessary?

200 ns transmission window


Normalized all waveforms to complete extinction at ±100 ns



Our baseline design has significant issues with transmission efficiency unless bunches are very short (~10ns).

The MECO design is markedly superior in this regard.

A new proposal involving a small amount of 4.8MHz harmonic looks very promising.

In comparing the two proposals, consideration will be given to Higher harmonic rate vs Reduced number of harmonics and lower magnetic field.


It’s clear the original proposal parameters raise challenges for magnet and power supply design.

Analyzing switching to a lower field, longer magnet MECO design, for example was 6 m, 80 G Would required 250m

Working to balance practicalities of magnet and beam line design.

Also clear single harmonic is impractical unless pulse is extremely short (<10 ns)

Comparing MECO 3 harmonic design to modified sine wave design. Lower frequency vs. less harmonics and lower field.

In either case, is compensating dipole needed? Perhaps not.


Challenge Measuring inter-bunch extinction requires a dynamic range (or

effective dynamic range) of at least 109. Options being considered

Statistical: use either a thin scatterer, or small acceptance target monitor

to count a small (10-7 or 10-8?) fraction of beam particles. Statistically measure inter-bunch beam. Pros: straightforward Cons: limited sensitivity to fluctuations in extinction (is that

important?) Single Particle

Measure inter-bunch beam at the single particle level Need something very fast (Cerenkov?) Probably have to “blind” detector at bunch time Pros: best picture of out of bunch beam Cons: hard


Example Design to count ~10 protons/nominal bunch

~1 in 107

Can build up a 3s 10-9 measurement in 109 bunches ~30 minutes


Primary beam

Scattered protons

target

Small acceptance

proton counter

Background rejection Need energy threshold

Sweeping magnet Calorimetric Cerenkov based

Rad hardness If placed after target, access could be difficult.


Pros: Rad hard Variable light yield (pressure)

Cons: High pressure -> thick windows Scintillation? Difficult to gate


Pros: Lots of light Coincidence to suppress scintillation Potentially gate light with Pockels cell during bunch

Cons: Beam scattering? Rad harness an issue (Grad ~ few days)


Mu2e is working on all aspects of extinction and extinction measurement.

Still more questions than answers at this point.


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A Muon to Electron Experiment at Fermilab

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