prospects for an energy-frontier muon collider

Feb 12, 2008 TJR Prospects for a Muon Collider 1

Prospects for an Energy-Frontier Muon Collider

Tom Roberts

Muons, Inc.Illinois Institute of Technology


Outline

• Background

• Why muons?

• The major challenges

• Surmounting the challenges

• Recent innovations that have improved the prospects for success

• Viewgraph-level design of a Muon Collider

• Current R&D Efforts

• Summary


Background Reminders

Historically, every significant increase in energyhas taught us something completely new.

Every new type of particle beam has alsotaught us something completely new.

The LHC is turning on later this year, so the “energy frontier” is above 14 TeV for protons,

or above ~1.5 TeV for leptons.


The Livingston Plot

X ILC

X 5 TeV MC

2025

Con

stitu

ent

Cen

ter-

of-M

ass

Ene

rgy

Panofsky and Breidenbach,Rev. Mod. Phys. 71, s121-s132

(1999)


Why Muons?

• Electrons have problems at the energy frontier– At the TeV scale, radiative processes limit both energy and

luminosity for electrons• Synchrotron radiation losses linear, large, and very expensive

• Beamstrahlung ~ E2, approaches the beam energy in one crossing low luminosity at peak energy, huge beam energy spread

– Remember those beautiful, narrow peaks for the J/Ψ? They won’t happen again because:

• The beam energy spread is very large

• Resonances above 2MW will have large weak-decay widths

• Protons have problems at the energy frontier– Without some tremendous breakthrough in high-field magnets,

the machine must be truly enormous (expensive)– As composite particles, beam energy must be considerably

higher than for leptons


Muons

• Clearly a whole new window into electroweak processes

• A path to the energy frontier– Radiative processes are far from limiting (as for

electrons)– Circular machine is possible, as are recirculating

linacs– Lepton, so beam energy and machine size are

significantly lower than for protons

• For S-channel Higgs production, cross-section~ m2 – 40,000 times larger than for e+e-.


Muons

[Ankenbrandt et al., PRST-AB 2, 081001 (1999)]

A 5 TeV muon collidercould fit on the existing

Fermilab site.


The Major Challenges

• Muons decay in 2.2 microseconds• Muons are created with a very large emittance, too

large for conventional accelerators, too large to give reasonable luminosity

• Muon production from 8-40 GeV protons scales roughly as proton beam power, independent of energy – A 1 to 4 Megawatt proton beam is required– The production target is also a challenge

• Muons decay into an electron plus neutrinos– Electron backgrounds in detector– Neutrino radiation problem (!)


Reducing the Phase Space – “Cooling”

• Loosely: the muons produced occupy the size of a beach ball (60 cm), the ILC accelerating cavities can accept a BB (4 mm) – take advantage of ILC R&D and optimization.

– overall reduction in phase space ~106.

• Luminosity ~ N2·ε┴-2 so lower transverse emittance

permits a reduction in N (which reduces other problems).

• Must select a process that avoids Liouville’s theorem.

• Must select a method consistent with the muon lifetime (2.2 μsec).

• Desirable to select a method consistent with the peak momentum of the produced muons (~300 MeV/c).


Muon Ionization Cooling

• Alternate absorbers and RF cavities• RF cavities restore the energy lost in the absorbers• A factor of 1/e reduction in transverse phase space occurs when the

total energy lost in absorbers equals the beam energy (both planes)• Optimal energy corresponds to a momentum of 100-250 MeV/c• Works only for muons (electrons shower, hadrons interact)• Transverse cooling only (small longitudinal heating due to straggling)

Absorberdp/dz || -p

RF Cavitydp/dz || +z

p┴ reduced,

p|| unchanged

(Skrinsky & Parkhomchuk, 1981)



Transverse Emittance change per unit length in the absorber:

Cooling term(energy loss)

Heating term(multiple scattering)

• Want:– Lower β┴ (stronger focusing at the absorber)– Minimize multiple scattering– Maximize energy loss

Here Here is the normalized emittance, Eis the normalized emittance, Eµµ is theis the muon energy, dEmuon energy, dEµµ/ds and X/ds and X00 are the are the

energy loss and radiation length of the absorber material, energy loss and radiation length of the absorber material, is the transverse beta- is the transverse beta-

function of the magnetic channel, and function of the magnetic channel, and is the particle velocity. is the particle velocity.

Lattice design

Absorber Material


Absorber Materials

Fcool ~ (Energy Loss) / (Multiple Scattering)


Emittance Exchange

Ionization cooling is only transverse. To get longitudinal cooling,use emittance exchange.


Innovation: Helical Cooling Channel

• Cools in all 6 dimensions – higher-energy particles have longer path length in the absorber

• A remarkable thing occurs: for specific values of the geometry, the solenoid, helical dipole, and helical quadrupole fields are all correct.

• With absorber and RF, parameters remain constant; with absorber only, parameters decrease with momentum.

• Acceptance is quite large compared to most accelerator structures.

These coils just surround the beam region.

All coils are normal to the Z axis; their centers are offset in X and Y

to form the helix.

The helical solenoid is filled with a continuous absorber, and perhaps with RF cavities.

BeamFollows

Helix


HCC Simulation

• Four sequential HCCs with decreasing diameter and period, increasing field (8 T max)

• Emittance reduction is 50,000 over 160 m(~15% decay)

• In the analogy of starting with a beach ball and needing a BB, this is a small marble (~1 cm dia.)


Related Innovation: Guggenheim Cooling Channel

• Helix with radius >> period• Also capable of emittance exchange• More like a ring cooler that has been “stretched” vertically

Figure is mine; concept is Palmer et al, BNL


Innovation: High Pressure Gas RF Cavities

• High-pressure hydrogen reduces breakdown via the Paschen effect• No decrease in maximum gradient with magnetic field• Need beam tests to show HPRF actually works for this application.

Paschen region

Electrode breakdown region

805MHz


Innovation: High Pressure Gas RF Cavities

• Copper plated, stainless-steel, 805 MHz test cellCopper plated, stainless-steel, 805 MHz test cell

• HH22 gas to 1600 psi and 77 K gas to 1600 psi and 77 K

• Paschen curve verified (at Fermilab’s Lab G and MuCool Test Area)Paschen curve verified (at Fermilab’s Lab G and MuCool Test Area)

• Maximum gradient limited by breakdown of metalMaximum gradient limited by breakdown of metal

• Fast conditioning seenFast conditioning seen

• Unlike vacuum cavities, there’s no measurable limitation for magnetic field!Unlike vacuum cavities, there’s no measurable limitation for magnetic field!


Understanding RF Breakdown

Scanning electron microscope images; Be (top) and Mo (bottom).


Innovation: Parametric Resonance Ionization Cooling

Clever method to greatly reduce Clever method to greatly reduce without increased magnetic fields. without increased magnetic fields.

Excite ½ integer parametric resonance (in Linac or ring)Excite ½ integer parametric resonance (in Linac or ring)

• Like vertical rigid pendulum or ½-integer extractionLike vertical rigid pendulum or ½-integer extraction

• Elliptical phase space motion becomes hyperbolicElliptical phase space motion becomes hyperbolic

• Use xx’=const to reduce x, increase x’ Use xx’=const to reduce x, increase x’

• Use IC to reduce x’Use IC to reduce x’

Detuning issues are being addressed (chromatic and spherical aberrations, Detuning issues are being addressed (chromatic and spherical aberrations, space-charge tune spread). Simulations are underway. space-charge tune spread). Simulations are underway.

Smaller beams from 6D HCC cooling are essential for this to work!Smaller beams from 6D HCC cooling are essential for this to work!

xX

X’

X

X’


Innovation: Reverse Emittance Exchange

• p(cooling)~200MeV/c, p(colliding)~2.5 TeV/c room in Δp/p space• After cooling and acceleration, the beam has much smaller

longitudinal emittance than necessary.• Reduce transverse emittance to increase luminosity, trading it for

increased longitudinal emittance (limited by accelerator acceptance and interaction point **).

EvacuatedDipole

Wedge Abs

Incident Muon Beam


Innovation: Bunch Coalescing

• Start with ~100 MeV/c cooled bunch train.• Accelerate to ~20 GeV/c with high-frequency RF.• Apply low-frequency RF to rotate the bunches longitudinally.• Permit them to drift together in time.• Avoids space charge problems at low energy.

p

t

1.3 GHz Bunch Coalescing at 20 GeV

RF

Drift

Cooled at 100 MeV/c

RF at 20 GeV

Coalesced in 20 GeV ring


Innovation: Dual-Use Linac

• Fermilab is considering “Project X”, a high-intensity 8 GeV superconducting linac

• Use it also to accelerate muons (after cooling)

~ 700m Active Length

Possible 8 GeV Project X Linac

Target and Muon Cooling Channel Recirculating

Linac for Neutrino Factory

Bunching Ring

NeutrinoFactoryaimed atSoudan, MN


Innovation: Pulsed Recirculating Linac

• Accelerating from 20 GeV to 2,500 GeV requires a lot of RF!• Muon decay dictates high ratio of RF/length.• A “dogbone” recirculating linac is a reasonable trade-off between

cost, size, and muon decay.• By pulsing the quadrupoles of the linac, more passes can be made

without losing transverse focusing.• This linac is several km long, so pulsing is feasible.• With careful design this can handle both μ+ and μ (time offset in RF

cavities, FODO vs DOFO lattice, travel opposite directions in arcs).

Injection

ExtractionLinac


Innovation: High-Field HTS Superconducting Magnets

• The high-temperature superconductors have a remarkable property: at low temperature (2-4 K) they sustain a high current density at large magnetic fields.

• Measured up to ~40 T, expected to hold to even higher fields.

• It is likely that solenoids in the range of 30 T to 50 T can be constructed.

• Higher field lower , so lower emittance can be achieved via ionization cooling.

• These materials are a challenge to work with…


Many New Arrows in the Quiver

• New Ionization Cooling TechniquesNew Ionization Cooling Techniques– Helical Cooling Channel Helical Cooling Channel – Momentum-dependent Helical Cooling ChannelMomentum-dependent Helical Cooling Channel– Guggenheim cooling channelGuggenheim cooling channel– Ionization cooling using a parametric resonanceIonization cooling using a parametric resonance

• Methods to manipulate phase space partitionsMethods to manipulate phase space partitions– Reverse emittance exchange using absorbersReverse emittance exchange using absorbers– Bunch coalescing (neutrino factory and muon collider share Bunch coalescing (neutrino factory and muon collider share

injector)injector)• Technology for better coolingTechnology for better cooling

– Pressurized RF cavities Pressurized RF cavities – High Temperature Superconductor for up to 50 T magnetsHigh Temperature Superconductor for up to 50 T magnets

• Acceleration TechniquesAcceleration Techniques– Dual-use Linac– Pulsed Recirculating Linac


Conceptual Block Diagram of a Muon Collider

Proton Driver(8-40 GeV)

ProductionTarget

Pion Capture, Decay Channel,Phase Rotation, and Pre-Cooling


Acceleration(0.2 to 20 GeV)

Reverse EmittanceExchange

Bunch Coalescing

Acceleration (20 to 2,500 GeV)

Storage Ring andInteraction Regions

Experiments

Must of course deal with both μ+ and μ-.


Fernow-Neuffer Plot

HCC 400 MHz

HCC 800 MHz

HCC 1600 MHzPIC

REMEX &Coalescing

Start Cooling: After Capture, Decay, Phase Rotation,

Pre-Cooling

AccelerationTo 20 GeV

End Cooling:Start

Acceleration to2.5 TeV


Viewgraph-level Design

Target, pion capture,Phase rotation

Helical cooling channel

Proton driver

2.5 km ILC-likelinacs

2.5 + 2.5 TeV muonstorage ring with

two IRs1 km radius

(= Fermilab Main Ring,but it’s not deep enough)

Final cooling, preacceleration

μ+

μ–

10 recirculating arcsIn one tunnel

L ~ 1035 cm-2 s-1


Related Facility: Neutrino Factory

• Muons in a storage ring with a long straight section aimed at the far neutrino detector

• Concept is more fleshed out that a muon collider– Cheaper, of striking current interest, perhaps more feasible

• Thousands of times more neutrino intensity than alternatives

• Higher energy neutrinos, with narrower energy spectrum• Essentially perfect purity (no π decays) – great for

wrong-sign appearance measurements of oscillation• Near detector looks a lot like old fixed-target hadron

experiments:• 30 cm liquid hydrogen target• Event rate ~ 1-100 Hz• Must be careful about material (spontaneous muons!)


Neutrino Factory


Current R&D Efforts

• Six different (but greatly overlapping) collaborations, more than 200 physicists:– Neutrino Factory and Muon Collider Collab.

• Umbrella U.S. collaboration– MERIT Collab.

• Mercury jet target in 15 Tesla solenoid• 24 GeV protons at CERN• Analyzing data

– MuCool Collab.• Engineering studies for individual components• ~4 years of studies so far, at Fermilab• Test beam (400 MeV H-) ~ SUMMER

– MICE Collab.• Single-particle demonstration of emittance reduction• First muon Beam (140-300 MeV/c μ) “Real Soon Now”

– MANX Collab.• Just forming

– Fermilab’s Muon Collider task Force

• Plus other Neutrino Factory organizations


Merit – Target Test

• High-power target test using a mercury jet in a 15 T solenoid, at CERN

• Data taking completed last fall, data analysis in progress• Preliminary conclusion: concept validated up to 4 MW at 50 Hz

1234

Syringe PumpSecondaryContainment

Jet Chamber

ProtonBeam

Solenoid


MuCool

Tests in progress at Fermilab MuCool Test Area (MTA)near Linac, with full-scale (201 MHz) and 1/4-scale (805 MHz) closed-cell (pillbox) cavities with novel Be windows for higher on-axis field


MICE(~10% 4d Cooling in 5.5 m)

• Installation in ISIS R5.2 is progressing• Beamline commissioning “Real soon now” (2-3 weeks)• A month or two until beamline is complete• Summer or fall until trackers are complete


The MANX Experiment(~500% 6d Cooling in 4 m)

• Purpose is to demonstrate the Helical Cooling Channel.• Could well become a “Phase III” of MICE (total is 2.5 m longer than MICE Stage VI – fits in hall).


Summary

• A number of clever innovations have made a Muon Collider much more feasible than previously thought.

• To make it possible to actually construct such a new facility, an ongoing program of research and development is essential.

• We are hosting a Low Emittance Muon Collider Workshop, at Fermilab in April.

• There is lots to do – come join us!

http://www.muonsinc.com

prospects for an energy-frontier muon collider

Documents

peak energy

energy frontierat

energy frontierwithout

independent of energy

muon colliderreducing

muon collidermuonsclearly

muon lifetime

muon collidermuonsankenbrandt