MECO the Experiment Searching for µ-e Conversion with 10...

MECOthe Experiment Searching

for µ-e Conversionwith 10-17 Sensitivity

Masaharu AokiOsaka University

7th International Workshop on Neutrino Factoriesand Superbeams

Laboratori Nazionali di Frascati, Frascati (Rome)June 21 - 26, 2005

June 21 - 26, 2005Masaharu Aoki, Osaka University 7th NuFact '05 Workshop 2

Outline

• Introduction– Lepton Flavor Violation and SUSY-GUT– Muon-Electron Conversion (µ N → e N)– µ N → e N vs. µ → e γ

• MECO– Learning from SINDRUM II– MECO features

• MECO muon beam line– Pulsed proton beam from AGS– Pion Production Target– Production Solenoid– Transport Solenoid– Detector Solenoid

• MECO Detector– Electron Tracker– Electron Calorimeter

• MECO Sensitivity and Backgrounds

ProtonBeam

Heat & RadiationShield


Lepton Flavor Violation

• Lepton Flavor (Le, Lµ and Lτ) Conservation is definitely violated atleast for neutral lepton sector (neutrino).

Extension of the Standard Model to include massive ν.

• Lepton Flavor Violation (LFV) in charged lepton sector could occurthrough intermediate states withν mixing. However, the expectedrate is far below the experimentallyaccessible range; (mν

2/MW2)2~10-60

• Many scenarios for physics beyond the SM predict sizable LFVprocesses in charged lepton sector.

Charged LFV Physics beyond the SM

LFV diagram in SUSY-GUT

LFV diagram in Standard Model

mixing in massive neutrinos

! e

! e˜ ˜

B̃

mixing

! e

mixing

!µ !e

W

large top Yukawa coupling

" (m! / mW)4

# 10$26


SUSY-GUT and LFV

•Large top Yukawa couplings result insizable off-diagonal components in aslepton mass matrix through radiativecorrections, and that impliesobservable levels of LFV in somemodels of SUSY-GUT

Barbieri and Hall, 1994

LFV diagram in SUSY-GUT

LFV diagram in Standard Model

mixing in massive neutrinos

! e

! e˜ ˜

B̃

mixing

! e

mixing

!µ !e

W

large top Yukawa coupling

" (m! / mW)4

# 10$26

md

ms

mb

!

"

# # #

$

%

& & &

m!e

m!µ

m! "

#

$

% % %

&

'

( ( (

m˜ d

m˜ s

m˜ b

!

"

# # #

$

%

& & &

m˜ e ̃ e

2 !m˜ e ̃ µ

2 !m˜ e ̃ "

2

!m˜ µ ̃ e

2

m˜ µ ̃ µ

2 !m˜ µ ̃ "

2

!m˜ " ̃ e

2 !m˜ " ̃ µ

2

m˜ " ˜ "

2

#

$

% % %

&

'

( ( (

quark

lepton (neutrino) slepton

squarknormal particles SUSY particles

ex. K-decays, B-decays

ex. neutrino oscillation ex. charged lepton LFV

•Study of Charged LFV is almostequivalent to the study of sleptonmass matrix in a framework ofSUSY-GUT.


• SU(5) SUSY-GUT Predictiononly a few orders of magnitude below the current experimental limit.

• SO(10) SUSY-GUT Predictionenhanced by (mτ/mµ)2(~100) from SU(5) prediction.

SUSY-GUT Prediction

Courtesy Hisano

10-910-6τ → µ γ

10-1410-11µ → e γ10-1610-13µ N → e N

SUSY-GUTlevel

CurrentLimit

Process

MECO single event sensitivity


MECO single event sensitivity

SUSY with RH Majorana neutrino

• SUSY + See-Saw• Solar Neutrino

MSW Large Angle


Muon Electron Conversion

• Muonic atom (1s state)

• Neutrinoless muon nuclear capture

– Single mono-energetic electron of Emax = (Mµ - Bµ) MeV (~105 MeV)– Rate is normalized to the kinematically similar weak capture process:

Rµe ≡ Γ(µ-N→e-N) / Γ(µ-N→νµN(Z-1))

muon decay in orbit

nucleus µ−

nuclear muon capture

lepton flavors change by one unit.

coherent process


µ-N → e-N vs. µ→e γ

µ-N → e-N

•Sensitive to non-photonic process

• µ− available for stopped experimenthas been much less than that of µ+

•No accidental background.•Rate-limited only in the sense thathigh detector rates will contaminateevents and cause measurementerrors.

µ→e γ•B(µ→e γ) = 200 × B(µ-N → e-N)

for photonic process

•Abundance of high intensitysurface muons

•Rate-limited due to accidentalbackground.

•Requires heroic efforts below 10-14

level.

The physics motivation is sufficiently strong for both

Possibly different systematics, thus complementary each others.Both should be done to maximize discovery potential


MECO Collaboration

Institute for Nuclear Research, MoscowV. M. Lobashev, V. Matushka,

New York UniversityR. M. Djilkibaev, A. Mincer,P. Nemethy, J. Sculli

Osaka UniversityM. Aoki, Y. Kuno, A. Sato

Syracuse UniversityR. Holmes, P. Souder

University of VirginiaC. Dukes, K. Nelson, A. Norman

College of William and MaryM. Eckhause, J. Kane, R. Welsh

Boston UniversityR. Carey, I. Logashenko,J. Miller, B. L. Roberts,

Brookhaven National LaboratoryK. Brown, M. Brennan, G. Greene,L. Jia, W. Marciano, W. Morse,P. Pile, Y. Semertzidis, P. Yamin

BerkeleyY. Kolomensky

University of California, IrvineC. Chen, M. Hebert, P. Huwe,W. Molzon, J. Popp, V. Tumakov

University of HoustonY. Cui, N. Elkhayari,E. V. Hungerford, N. Klantarians,K. A. Lan

University of Massachusetts, AmherstK. Kumar


Learning from SINDRUM II

• BR = 2 × 10-13 (2000 Gold run)• Major background sources were

– Muon decay in orbit• Requires much better resolution

– Prompt π− background• Requires pulsed proton beam

– Cosmic ray background


MECO Features

• Muon Intensity– 1000 fold increase by using an idea from MELC at MMF

10-2 µ−/P at 8 GeV (SINDRUM II = 10-8, MELC = 10-4, Muon Collider = 0.3)•High Z target for improved pion production•Graded solenoid field to maximize pion capture

• Muon Beam Quality Control– Muon transport in curved solenoid suppressing high momentum

negatives and all positives and neutrals (new for MECO)• Pulsed Beam A. Baertscher et al., NPA377(1982)406

– Beam pulse duration << τµ

– Pulse separation = τµ

– Large duty cycle (50%)– Extinction between pulses < 10-9

• Detector– Graded solenoid field for improved acceptance, rate handling,

background rejection (following MELC concept)– Spectrometer with nearly axial components and good resolution (new for

MECO)


The MECO Apparatus

Proton Beam

Straw Tracker

CrystalCalorimeter

MuonStoppingTarget

Muon BeamStop

SuperconductingProduction Solenoid

(5.0 T – 2.5 T)

SuperconductingDetector Solenoid

(2.0 T – 1.0 T)

SuperconductingTransport Solenoid

(2.5 T – 2.1 T)

Collimators

Heat & Radiation Shield


Pulsed Proton Beam from AGS BNL• AGS at BNL

– One of the most powerful pulsed protondrivers in the world today.

• 8 GeV with 4 × 1013 protons persecond - 50kW beam power– Below the transition energy of AGS– Suppress the antiproton production

• Cycle time of 1.0 sec with 50% dutyfactor

• Revolution time = 2.7 µs with 6 RFbuckets in which protons can betrapped and accelerated.

• Fill only 2 RF buckets for 1.35 µs pulsespacing

• 2 × 1013 protons/RF bucket– twice the current bunch intensity

• Requirement:the excellent beam extinction<10-9 protons between bunches


The extinction = 10-9 at AGS

• Quick test measurements– 1st test

24 GeV with one RF bucketextinction < 10-6 between buckets

< 10-3 in unfilled buckets– 2nd test

7.4 GeV with one filled bucketextinction < 10-7

• How to improve extinction– 40 kHz AC dipole and 740 kHz fast

kicker magnets inside AGS ring

and

– RF modulated magnet andLambertson septum magnets inprimary proton transport beam line.

Filled bucket

Unfilled bucket


Pion Production Target• High Z target material• Minimal material in bore to absorb π/µ

• Cylinder with diameter 0.6-0.8 cm, length16 cm

• About 5 kW of deposited energy

• Water cooling in 0.3 mm sylindrical shellsurrounding target

– Simulated with 2D and 3D thermal andturbulent fluid flow finite element analysis.

– Temperature rise is below 100K– Pressure drop is acceptable (~10 atm)– Prototype built, tested for pressure and flow.

Measured vs. Theoretical

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Flow (gpm)P

res

su

re D

rop

(p

si)

Prototype 02 (measured)

Prototype 03 (measured)

Single Annular Channel (theory)

Two Right Angle Turns (theory)

Fully developed turbulent flow in 300 µm water channel

(F05) Target 01 - High Permeability Alloy - 100% power

-5

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800 900

Time (sec)

Tem

pera

ture

(C

)

Target - Inlet


The MECO Superconducting Magnets

• Magnet Conceptual Design Report (CDR)from MIT Plasma Science and Fusion Center


Production Solenoid

•Requirements– Capture most of pions and muons, and transport them toward detector– Stand up under high radiation and heat environment

– 50kW primary proton

Proton beam

Production target

Toward Detector

Superconducting coil

Radiation and heat sheild

•109.20 MJ stored energy

•150 W load on cold mass•15 µW/g in superconductor

•20 Mrad integrated dose

•Implementation– Axially graded magnetic field

• 5 - 2.5 T– Cu and W heat and radiation shield


Transport Solenoid

2.5T

2.5T

2.0T

2.2T2.0T

2.3T

Goals:•Transport low energy µ− to the detectorsolenoid•Minimize transport ofpositive particles andhigh energy particles•Minimize transport ofneutral particles•Absorb anti-protons ina thin window•Minimize long transittime trajectories

•Implementation– Curved solenoid

eliminate line-of-sighttransport of photons andneutrons.

– Toroidal sections causesparticles to drift out ofplane; used to sign andmomentum select beam.

– dB/dS < 0 for straightsection to avoidreflections which wouldcause long transit timetrajectories.


Function of the Curved Solenoid

• Particle drifts in torus magnetic fieldsJD Jackson, Classical Electrodynamics

Curvature drift

Gradient drift

Charge and momentum selection


Muon Yield

• Monte Carlo calculation– GEANT3– Including original data models based on a measured pion production on

similar targets at similar energy.– Decays, interactions, and magnetic transport included.

Rel

ativ

e yi

eld

Muon Momentum [MeV/c]0 50 100

Total flux atstopping

targetStopping

Flux

0.0025 µ− stops/proton


Detector Solenoid

Electron startsupstream, reflects

in field gradient

• Graded field in front section to increase acceptance and reduce cosmic raybackground.

• Uniform field in spectrometer region to minimize correctionsin momentum analysis.

• Tracking detector downstream of target toreduce rates from photons

and neutrons.

ElectronCalorimeter

TrackingDetector Stopping Target:

17 layers of 0.2mm Al

Tracker Goal• Excellent momentum resolution;

900 keV FWHM (Δp/p=0.3%), Essential to eliminatemuon decay in orbit background.

• High rate capability; 500kHz in individualdetector elements

1T1T

2TCalorimeter Goal•Used for on-line Trigger•Energy resolution ~ 5%


Spectrometer Performance• Tracker : Axially aligned straw tubes in vacuum

– 5mmφ, 2.5mL, 25µmt

• Transverse direction: 0.2mm(rms) by drift time measurement

• Axial direction: 1.5mm(rms) by cathode pads on the exteriorof the straws

GEANT3 Monte Carlo– Resolution dominated by

• Energy loss in muon stopping target; 636 keV FWHM• Tracker intrinsic resolution; 353 keV FWHM

– Geometrical acceptance ~50% (60°-120°)• Alternate transverse geometry has similarly good tracking

performance with sophisticated fitting.

900keV

FWMH

Background withDetector Response

Full GEANTSimulation

Signal


Electron Calorimeter• Provides prompt signal proportional to

electron energy for use in online eventselection

• Provides position measurement toconfirm electron trajectory

• Energy resolution ~ 5% and spatialresolution ~1 cm

• Consists of 4 vanes extending radiallyfrom 0.39 m to 0.69 m and 1.5 m along thebeam axis

• ~20003 cm x 3 cm x 12 cm(PbWO4 or BGO) crystals with APDreadout

• Small arrays currently being studied forlight yield, APD evaluation, electronicsdevelopment


MECO Expected Sensitivity & Background

0.60µ capture probability

Rµe = 2 × 10-17Single Event Sensitivity

0.19Fitting and selection criteria efficiency

0.90Electron trigger efficiency

0.49Fraction of µ capture in detection time window

0.58µ stopping probability

0.0043µ entering transport solenoid / incident proton

4 ×1013Proton flux (Hz) (50% duty factor, 740 kHz µpulse)

107Running time (s)

FactorContributions to the Signal Rate

No scattering in target< 0.03µ decay in flight

10-4 CR veto inefficiency0.004Cosmic ray inducedMostly from π−0.007Anti-proton induced

With 10-9 inter-bunch extinction0.45Total Background

From late arriving pions0.001Radiative π captureFrom out of time protons0.07Radiative π capture

< 0.001π decay in flightScattering in target 0.04µ decay in flight

< 0.04Beam e-< 0.005Radiative µ decay< 0.006Tracking errors

S/N = 4 for Rµe = 2 × 10-170.25µ decay in orbitCommentsEventsSource

•Background calculated for107 s running withsensitivity of a single eventfor Rµe= 2 × 10-17

•Sources of backgroundwill be determined directlyfrom data.

5 signal events with0.5 background events in 107 s runningif Rµe = 10-16


Summary

• Muon-electron conversion is definitely a “must do” experiment. Thediscovery will be right down there only a few orders of magnitudebelow the current experimental limit.

• The physics output from the muon-electron conversion is veryrobust. Many different types of theoretical models beyond SM canbe studied with this process.

• The discovery potential of MECO is MAXIMUM:– 1000 fold increase of the µ- stops in the target; 0.0025 µ- stops/proton.– Well designed beam line, which minimizes the beam contamination while

maintains maximum muon yield.– Large detector acceptance with good resolution.

• Waiting for budget approval. After 65 weeks of the magnetconstruction, the experiment will run.

Rµe = 2 × 10-17

MECO the Experiment Searching for µ-e Conversion with 10...

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