physics with very intense muon beams

Physics with Very Intense Muon Beams

W. Molzon

U.S. Neutrino Factory and Muon Collider Collaboration MeetingUCLA

January 29 2007

Outline:−General considerations−Physics motivation for lepton flavor violation experiments−Experimental status of LFV−Prospects for improvement−Concept of -N→e-N experiment

• Physics of the conversion process• Background sources • Detector requirements

−MECO muon beam and detector concept−Possible -N→e-N experiment at Fermilab−Possible -N→e-N experiment at JPARC

January 29 2007William Molzon, UC Irvine Particle Physics with Intense Muon Beams 2

General Considerations

• Physics goals− Lepton flavor violation (-N e-N, + e++ e+e+e-)− Precision electroweak (g-2, decay spectrum, lifetime, capture on p, edm)− Very intense beams are useful for experiments in which the majority of the decays

do not cause rate in detectors and no accidental physics backgrounds− Cooling is generally useful, usually a tradeoff with available intensity, cost, etc.

• Muon beam properties − Charge – negative for conversion (use Coulomb bound muons) positive for others. Positive muons produced using stopping pions (surface muon beams).− Energy – many stopping experiments use low energy beams, some experiments

require specific energies− Energy spread – generally, reduced energy spread is beneficial− Time structure – depends on experiment, highly pulsed or DC generally preferred− Intensity – in most cases limited by instrumental or background effects


Existing and Possible Muon Sources

• Available muon beams at TRIUMF(500MeV, 0.3MW) and PSI (590MeV, 1MW)− DC beams with intensity of order 108 per second, higher for +

− Limitations of low energy machines• Muons per watt of beam power low due to low pion production cross section at low energy• Fewer negative muons• Fewer options to make pulsed beam• More difficulty with beam power on target (wrt higher energy accelerators)

• Potential muon beam sources− BNL proton synchrotron

• few to 24 GeV proton beam • low frequency RF for acceleration – relatively easy pulsing • available slow extraction• beam power 20-50 kW at 8 GeV

− JPARC• Few to 40 (50) GeV proton beam• Relatively low frequency RF• Slow extraction being developed• Beam power 1 MW at 50 GeV

− Fermilab• 8 to 120 GeV• No existing slow spill• Few 10s of kW at 8 GeV, few 100 kW at 120 GeV


Limitations: Detector Rates, Rate Induced Physics Backgrounds

For LFV experiments, stop muons and look at decay or conversion processes− Detector rates from Michel decays (e) decays

− Detector rates from other beam particles, muon capture processes, etc

− For most processes, physics backgrounds dominated by accidentals at useful rates• Example: + e+

sensitivity goal 10-14

running time 107 sdetection efficiency 0.1

macro duty cycle 1stop rate 108

background dominated by accidental coincidences of Michel positron, photon from radiative decay or positron annihilation in flight

− Without some care, detector rates are too high• Example: muon conversion on a nucleus -N e-N

sensitivity goal 10-17

running time 107 sdetection efficiency 0.2macro duty cycle 0.5stop rate 1011

decay rate of 1011 Hz, instantaneous intensity higher with pulsed beameven higher fluxes from neutrons, photons from muon capture

− Muon conversion experiment is unique in ability to use very high stopping rates


What Will Observation of →e or -Ne-N Teach Us?

Discovery of -N e-N or a similar charged lepton flavor violating (LFV) process will be unambiguous evidence for physics beyond the Standard Model. This process is completely free of background from Standard Model processes.

• For non-degenerate neutrino masses, oscillations can occur. Discovery of neutrino oscillations required changing the Standard Model to include massive .

• Charged LFV processes occur through intermediate states with mixing. Small mass differences and mixing angles expected rate is well below what is experimentally accessible:rate is proportional to [M)2/(MW)2]2

• Current limits on LFV in charged sector provides severe constraints on models for physics beyond the Standard Model

• Charged LFV processes occur in nearly all scenarios for physics beyond the SM, in many scenarios at a level that current generation experiments could detect.

• Effective mass reach of sensitivesearches is enormous, well beyondthat accessible with direct searches.

- -

-μ

16Γ(μ N e N)R 10

Γ(μ N ν N )e

2LM 3000 TeV/cd ed

-N e-N mediated by leptoquarks

e

W

- d

L

d e-

ed

d

Z,N


Sensitivity to Different Physics Processes

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


Supersymmetry Predictions for LFV Processes • From Hall and Barbieri

Large t quark Yukawa couplingsimply observable levels of LFV insupersymmetric grand unified models

• Extent of lepton flavor violation in grand unified supersymmetry related to quark mixing

• Original ideas extended by Hisano, et al.

ProcessCurrent

LimitSUSY level

10-12 10-15

10-11 10-13

10-6 10-9

+ e

- -N e N

100 200 300 100 200 300

Rem (GeV)

Rem (GeV)

N→eN single event sensitivity goal

10 -11

10 -13

10 -15

10 -19

10 -17

10 -21

Re

10 -10

10 -12

10 -14

10 -18

10 -16

10 -20

PSI →e single event sensitivity

Current SINDRUM2 boundCurrent MEGA

bound

B(

e

)


1

10-2

10-4

10-16

10-6

10-8

10-10

10-14

10-12

1940 1950 1960 1970 1980 1990 2000 2010

History of Lepton Flavor Violation Searches

K0 +e-

K+ + +e-

- N e-N + e+ + e+ e+ e-

MEGA

SINDRUM2

Bran

chin

g Fr

actio

n Up

per L

imit

MEG goal

-N→e-N goal


PSI-MEG +e+ Experiment

Search for e+ with sensitivity of 1 event for B(e ) = 10-13

• Italy− INFN and University of Genoa− INFN and University of Lecce− INFN and University Pavia − INFN Pisa− INFN and University of Roma

• Japan− ICEPP, University of Tokyo− KEK− Waseda University

• Russia− JINR, Dubna− BINP− Novosibirsk

• Switzerland− Paul Scherrer Institute

• United States− University of California, Irvine


The MEG Experiment at PSI

Ee / Emax

0.90 0.95 1.00

0.90

0.95

1.0

0

E

/ E

ma

x

• Experiment limited by accidental backgrounds: e+ from Michel decay, from radiative decay or annihilation in flight. S/N proportional to 1/Rate.

–Ee : 0.8% (FWHM) E : 4.5% (FWHM)–e : 18 mrad (FWHM) te : 141ps (FWHM)

• MEG uses the PSI cyclotron (1.8 mA at ~600 MeV) to produce 108 per second (surface muon beam)

• Partial engineering runs autumn 2006, spring 2007• First physics run 2007• Sensitivity of 10-13 with 2 years running (c.f. MEGA 1.2x10-11)• Possible improvements to reach 10-14 in later 2 year run• Cooled beam would reduce backgrounds


• Muons stop in matter and form a muonic atom.

• They cascade down to the 1S state in less than 10-16 s.

• They coherently interact with a nucleus (leaving the nucleus in its ground state) and convert to an electron, without emitting neutrinos Ee = MENR EB.Coherence gives extra factor of Z with respect to capture process, reduced for large Z by nuclear form factor.

• Experimental signature is an electron with Ee=105.1 MeV emerging from stopping target, with no incoming particle near in time: background/signal independent of rate.

• More often, they are captured on the nucleus: -(N,Z)→(N,Z-1) or decay in the Coulomb bound orbit: -(N,Z)→(N,Z)e

( = 2.2 s in vacuum, ~0.9 s in Al)

• Rate is normalized to the kinematically similar weak capture process:

Goal of new experiment is to detect -Ne-N if Re is at least 2 X 10-17

with one event providing compelling evidence of a discovery.

Coherent Conversion of Muon to Electrons (-Ne-N)

Re Ne-N-NN(Z-1))


What Drives the Design of the Next-Generation Conversion Experiment?

Considerations of potential sources of fake backgrounds specify much of the design of the beam and experimental apparatus.

SINDRUM2 currently has thebest limit on this process:

Expected signal

Cosmic raybackground

Prompt background

Experimental signature is105 MeV e- originating in a thin stopping target.

Muon decay


Potential Sources of Background

1. Muon decay in orbit –• Emax = Econversion when

neutrinos have zero energy

• dN/dEe (Emax – Ee)5

• Energy resolution of ~200 keV required

2. Radiative muon capture: - N N(Z-1) • For Al, E

max = 102.5 MeV/c2, P(E > 100.5 MeV/c2) = 4 10-9

• P( e+e-, Ee > 100.5 MeV/c2) = 2.5 10-5

• Restricts choice of stopping targets: Mz-1 > Mz

3. Radiative pion capture: - N N(Z-1) • Branching fraction ~ 1.2% for Eg > 105 MeV/c2

• P( e+e-, 103.5 < Ee< 100.5 MeV/c2) = 3.5 10-5

• Limits allowed pion contamination in beam during detection time

Other sources: beam electrons, muon decay, cosmic rays, anti-proton interactions

dN

/dE

10

-10

1

0-5

1

0 50 100 E [MeV]


Features of a New -N→e-N Experiment

• 1000 fold increase in muon intensity using an idea from MELC at MMF − High Z target for improved pion production− Graded solenoidal field to maximize pion capture− Produce 10-2 m-/p at 8 GeV (SINDRUM2 10-8, MELC 10-4, Muon Collider 0.3)− Muon transport in curved solenoid suppressing high momentum

negatives and all positives and neutrals

• Pulsed beam to eliminate prompt backgrounds following PSI method (A. Badertscher, et al. 1981)

− 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 for improved acceptance, rate handling, background

rejection following MELC concept− Spectrometer with nearly axial components and very high resolution


Pulsed Proton Beam Requirement for -N→e-N Experiment

• Subsequent discussion focuses on accelerator operating 8 GeV with 41013 protons per second and 50% duty cycle – 50 kW instantaneous beam power at 8 GeV

• Pulsed proton beam generated using RF structure of appropriate accelerator or storage ring

• To eliminate prompt backgrounds, we require < 10-9 protons between bunches for each proton in bunch. We call this the beam extinction.

• Gap between proton pulse and start of detection

time largely set by pion lifetime (~25


Secondary Extinction Device in Proton Beam• Improved extinction performance close in time to

the filled buckets (particularly just before the filled bucket is important)

• A means of continuously measuring secondary extinction at the target is essential

• Conceptual idea for a secondary extinction device− Time-modulated magnetic deflection synchronized

with filled buckets to deflect protons between buckets out of beamline

− Field integral required depends on beam optics – ~500 G-m

− Ideal time structure is rectangular wave – magnet pulsed at 731 kHz with rise and fall time < 50 ns

− Less ideal solution is a time structure approximating a rectangular wave using multiple harmonics

-1.5

-1

-0.5

0

0.5

1

1.5

-500 0 500 1000 1500

FOUR HARMONICS

"RECTANGULAR WAVE"

FIRST HARMONIC

TWO HARMONICS

• Conceptual design of technical implementation completed− Stripline magnets with 7x7 cm2 bore, 5 m long, with peak field ~75

Gauss• Provides ~1.5 mrad deflection• Low-loss ferrite return yoke decrease power and radiated energy

− Resonantly driven magnets, with one or more harmonics • Q of 100 reduces the required delivered power by ~100• Shape (and complexity) can be adjusted for conditions• Modular design suggested: peak currents of order kA and peak voltages of order kV • Commercial high-voltage, high current capacitors found• Commercial ferrite with low losses at high frequency found • 1st two harmonics are in AM radio band, where commercial power amplifiers in

needed power range (10 kW) are available Magnet cross-section


Production solenoid bore

calorimeter magnetcollimators

TOF counterssecondary

proton

Monitoring ExtinctionMeasure time of originating in the muon production target

− Dual magnetic spectrometer to measure 1-2 GeV secondary protons• Target, production solenoid fringe field, first collimator gives passive momentum selection • Two collimators with magnet between them gives second passive momentum selection• TOF counters reject soft background giving accidental coincidences• Calorimeter gives energy measurement• Trigger between bunches to get out-of-time beam rate• Periodically trigger in time with bunches to get normalization

– also a good monitor of beam macrostructure

incident proton beam

production target

Beam dump


A Model Muon Beam and Conversion Experiment

Straw Tracker

Crystal Calorimeter

Muon Stopping Target

Muon Beam Stop

Superconducting Production Solenoid

(5.0 T – 2.5 T)

Superconducting Detector Solenoid

(2.0 T – 1.0 T)

Superconducting Transport Solenoid

(2.5 T – 2.1 T)

Collimators


MIT PSFC – MECO Design of Magnet System for -N→e-N Experiment

5 T 2.5 T

1 T2 T 1 T•150 MJ stored energy•5T maximum field•Uses surplus SSC cable•Can be built in industry

•Very detailed CDR completed (300+ pages)•Complete 3D drawing package prepared•TS and SOW for commercial procurement developed

•Industrial studies contracts let and completed


Muons Production and Capture in Graded Magnetic Field

Pions produced in a target located in an axially graded magnetic field:• 50 kW beam incident on gold target

• Charged particles are trapped in 5 – 2.5 T, axial magnetic field

• Pions and muons moving away from the experiment are reflected

• Superconducting magnet is protected byCu and W heat and radiation shield

5T

2.5T

Productiontarget

Superconducting coil

Heat Shield

150 W load on cold mass15 W/g in superconductor

20 Mrad integrated dose

Axial positionA

zim

utha

l po

sitio

n

W/gm in coil


Production Target for Large Muon Yield Production target region designed for high yield of low energy muons:

• High Z target material• Little extraneous material in bore to absorb • Diameter 0.6 - 0.8 mm, length 160 mm• ~5 kW of deposited energy

Water cooling in 0.3 mm cylindrical shellsurrounding target

• Simulated with 2D and 3D thermal and turbulent fluid flow finite element analysis

• Target temperature well below 100 C• Pressure drop is acceptable ( ~10 Atm)• Prototype built, tested for pressure and flow

Fully developed turbulent flow in 300 m water channel

Preliminary cooling testsusing induction heating completed


Goals:

− Transport low energy -

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

Muon Beam Transport with Curved Solenoid

Features:

• 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

2.1T

2.1T2.0T

2.4T


Sign and Momentum Selection in the Curved Transport Solenoid

2 2s t

s

1+ p2

p1 sD= × × p0.3B R

Transport in a torus results in charge and momentum selection: positive particles and low momentum particles absorbed in collimators.

Rel

ativ

e pa

rtic

le r

ate

in

bunc

h

Rel

ativ

e pa

rtic

le fl

ux

Detection Time

3-15 MeV


Muon Beam Studies

• Muon flux estimated with Monte Carlo calculation including models of production and simulation of decays, interactions and magnetic transport.

• Estimates scaled to measured pion production on similar targets at similar energy• Expected yield is about 0.0025 - stops per proton• Cooling would increase stopping fraction

Pion Kinetic Energy [GeV] 0 0.5 1.0

Relative yield

Muon Momentum [MeV/c]0 50 100

Total flux at stopping

target

Stopping Flux

Data Model


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 downstream of target to reduce rates• Polyethylene with lithium/boron to absorb neutrons• Thin absorber to absorb protons

1T

1T

2T

Electron Calorimete

r

Tracking Detector

Stopping Target: 17 layers of 0.2 mm Al


Magnetic Spectrometer to Measure Electron Momentum

Measures electron momentum with precision of about 0.3% (RMS) – essential to eliminate muon decay in orbit background

• Must operate in vacuum and at high rates – 500 kHz rates in individual detector elements

• Energy resolution dominated by multiple scattering• Implemented in straw tube detectors –

• Vanes and octants, each nearly axial, and each with 3 close-packed layers of straws• 2800 detectors, 2.6-3.0 m long, 5 mm diameter, 0.025 mm wall thickness –

issues with straightness, wire supports, low mass end manifolds, mounting system• r-position resolution of 0.2 mm from drift time• axial resolution of 1.5 mm from induced charge on cathode pads –

requires resistive straws, typically carbon loaded polyester film•High resistivity to maximize induced signal •Low resistivity to carry cathode current in high rates

• Alternate implementation in straw tubes perpendicular to magnet axis has comparable performance

Electron starts upstream, reflects

in field gradient


Spectrometer Performance Calculations

• Performance calculated using Monte Carlo simulation of all physical effects

• Resolution dominated by multiple scattering in tracker and energy loss in target

• Resolution function of spectrometer convolved with theoretical calculation of muon decay in orbit to get expected background.

• Geometrical acceptance ~50% (60-120)• Cooling would reduce energy loss in target,

improving resolution ( ~factor of two on width, much reduced tails) – offset by need for thicker proton absorber

FWHM ~900 keV


Expected Signal and Background with 4x1020 Protons

Sources of background will be determined directly from data.

Background Source Events Comments decay in orbit (cooling) 0.25 S/N = 4 for Re = 2 10-17

Tracking errors < 0.006

Beam e- (cooling) < 0.04

decay in flight (cooling) < 0.03 No scattering in target

decay in flight (cooling) 0.04 Scattering in target

Radiative capture (?) 0.07 From out of time protons

Radiative capture (?) 0.001 From late arriving pions

Anti-proton induced (?) 0.007 Mostly from

Cosmic ray induced 0.004 10-4 CR veto inefficiencyTotal Background 0.45 With 10-9 inter-bunch extinction

Background calculated for 107 s running time at intensity giving 5 signal event for Re = 10-16.

Factors affecting the Signal Rate FactorRunning time (s) 107

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

entering transport solenoid / incident proton 0.0043

stopping probability 0.58

capture probability 0.60

Fraction of capture in detection time window 0.49

Electron trigger efficiency 0.90

Geometrical acceptance, fitting and selection criteria efficiency 0.19

Detected events for Re = 10-16 5.0

5 signal events with0.5 background events in 107 s running if Re = 10-16


Implementing a MECO Experiment at Fermilab

• Complex of accelerators underused after Tevatron run finished• Proton beam could be shared with neutrino program –

“15-20% loss of protons to neutrino experiments not a problemfor a second high quality physics program” (P. Odone)

• Encouraged by Fermilab director, especially in context of less than most optimistic NLC rampup

• Implementation builds on mostly available accelerators, concepts of the MECO experiment− Momentum stack 3-4 booster bunches in the accumulator− Transfer to the debuncher− Rebunch the beam with RF (bunch spacing 1.6 s)− Slow extract to new area− Very good macro duty cycle− Proton beamline, target station,

muon beamline, experiment following MECO


Required Fermilab Beam Modifications

•Increased booster throughput (approved)•Transfer line from booster→accumulator

symbiotic with neutrino program(unapproved)

•Transfer line accumulator→debuncher•RF for rebunching

•Slow extraction•Secondary beamline, experimental hall …•Concerns

− Impact on protons for neutrino program− Radiation impact (losses, shielding)− Resource availability (NLC)

AP4 Line

AP5 LineA-D Line

Beam Energy 8 GeVBunch Trains / sec: fTRAIN 0.682Bunch Spacing: TB 1.6 sNo. of bunches/train: NB

85×104

No. protons/bunch: nP 2.16107

Bunch Length (2.5s) : tB 150 ns (=60ns)Protons/train (4 batches)

1.841013

Protons/year (107 secs) 1.251020 (1.441020 MECO)

Conservative intensity projection


LOI to Implement a MECO Like Experiment at JPARC

• Uses JPARC high energy synchrotron running at 8 GeV− 4x1013 protons per second − 2x107

seconds running time

− 7x10-4 muon stops per proton − Pulsed, slow extracted proton beam with good extinction, 1.17 s pulse spacing


JPARC Muon Beamline and Detector Schematic

• Backward pion collection, production solenoid much smaller than MECO version

• Transport with constant direction bend, drift compensated with dipole field

• Converstion electrons transported to detectors in curved solenoid− Suppresses transport of low energy

electrons− Transverse drift compensated by

superimposed dipole field − Non-monotonic field gradient in

transport region – allows for gradient in electron transport to improve acceptance

• Expected sensitivity of 1 event for Re 4x10-17 for 2x107 s running with 0.34 expected background events


END


Summary

• A muon-to-electron conversion experiment at sensitivity below 10-16 has excellent capabilities to search for evidence of new physics and to study the flavor structure of new physics if it is discovered elsewhere first.

• A well studied, costed, and reviewed experimental design exists that could be the starting point for a new effort at Fermilab.

• A group of physicists is interested in exploring the possibility of doing this experiment at Fermilab and is eager to attract more interested physicists at the beginning of the effort.

• This experiment would complement the neutrino program at Fermilab in the decade following the end of the Tevatron program and the beginning of a major new program.

• An appropriate proton beam can probably be built and operated for such an experiment at Fermilab with net positive impact on the planned neutrino program: Dave McGinnis will discuss the beam and operational issues.


MECO Experiment Design as a Template for Fermilab Experiment

Boston University

I. Logashenko, J. Miller, B. L. Roberts

Brookhaven National Laboratory

K. Brown, M. Brennan, W. Marciano, W. Morse, P. Pile, Y. Semertzidis, P. Yamin

University of California, Berkeley

Y. Kolomensky

University of California, Irvine

M. Bachman, C. Chen, M. Hebert, T. J. Liu, W. Molzon, J. Popp, V. Tumakov

University of Houston

Y. Cui, E. V. Hungerford, N. Elkhayari, N. Klantarians, K. A. Lan

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

Syracuse University

R. Holmes, P. SouderUniversity of Virginia

C. Dukes, K. Nelson, A. Norman

College of William and Mary

M. Eckhause, J. Kane, R. Welsh

MECO collaborators at various stages in the experiment


Other Potential Sources of Backgrounds

4. Muon decay in flight + e- scattering in stopping target

5. Beam e- scattering in stopping target

• Limits allowed electron flux in beam

6. Antiproton induced e-

• Annihilation in stopping target or beamline

• Requires thin absorber to stop antiprotons in transport line

• Motivates proton energy not much above anti-proton production threshold

7. Cosmic ray induced e- – seen in earlier experiments

• Primarily muon decay and interactions

• Scales with running time, not beam luminosity

• Requires the addition of active and passive shielding

physics with very intense muon beams

Documents

muon conversion

muon beam properties

intense muon beamsexisting

intense muon beamslimitations

low energy beams

muon capture processes

pions surface muon beams

watt of beam power low