1. Prospects for an Energy-Frontier Muon Collider
Tom Roberts
Muons, Inc.
Illinois Institute of Technology
Feb 12, 2008 TJR
Prospects for a Muon Collider

2. Prospects for a Muon Collider
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
Feb 12, 2008 TJR
Prospects for a Muon Collider

3. Prospects for a Muon Collider
Background Reminders
Historically, every significant increase in energy has taught us something completely new.
Every new type of particle beam has also taught 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.
Feb 12, 2008 TJR
Prospects for a Muon Collider

4. The Livingston Plot Constituent Center-of-Mass Energy X 5 TeV MC X ILC
2025
Panofsky and Breidenbach,
Rev. Mod. Phys. 71, s121-s132 (1999)
Feb 12, 2008 TJR
Prospects for a Muon Collider

5. Prospects for a Muon Collider
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
Feb 12, 2008 TJR
Prospects for a Muon Collider

6. Prospects for a Muon Collider
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-.
Feb 12, 2008 TJR
Prospects for a Muon Collider

7. Muons A 5 TeV muon collider could fit on the existing Fermilab site.
[Ankenbrandt et al., PRST-AB 2, (1999)]
Feb 12, 2008 TJR
Prospects for a Muon Collider

8. Prospects for a Muon Collider
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 (!)
Feb 12, 2008 TJR
Prospects for a Muon Collider

9. 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).
Feb 12, 2008 TJR
Prospects for a Muon Collider

10. Muon Ionization Cooling
Absorber
dp/dz || -p
p┴ reduced,
p|| unchanged
RF Cavity
dp/dz || +z
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 MeV/c
Works only for muons (electrons shower, hadrons interact)
Transverse cooling only (small longitudinal heating due to straggling)
(Skrinsky & Parkhomchuk, 1981)
Feb 12, 2008 TJR
Prospects for a Muon Collider

11. Muon Ionization Cooling
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
Lattice design
Absorber Material
Here  is the normalized emittance, Eµ is the muon energy, dEµ/ds and X0 are the energy loss and radiation length of the absorber material,  is the transverse beta-function of the magnetic channel, and  is the particle velocity.
Feb 12, 2008 TJR
Prospects for a Muon Collider

12. Prospects for a Muon Collider
Absorber Materials
Fcool ~ (Energy Loss) / (Multiple Scattering)
Feb 12, 2008 TJR
Prospects for a Muon Collider

13. Prospects for a Muon Collider
Emittance Exchange
Ionization cooling is only transverse. To get longitudinal cooling, use emittance exchange.
Feb 12, 2008 TJR
Prospects for a Muon Collider

14. Innovation: Helical Cooling Channel
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.
Beam
Follows
Helix
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.
Feb 12, 2008 TJR
Prospects for a Muon Collider

15. Prospects for a Muon Collider
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.)
Feb 12, 2008 TJR
Prospects for a Muon Collider

16. 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
Feb 12, 2008 TJR
Prospects for a Muon Collider

17. 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
805 MHz
Feb 12, 2008 TJR
Prospects for a Muon Collider

18. Innovation: High Pressure Gas RF Cavities
Copper plated, stainless-steel, 805 MHz test cell
H2 gas to 1600 psi and 77 K
Paschen curve verified (at Fermilab’s Lab G and MuCool Test Area)
Maximum gradient limited by breakdown of metal
Fast conditioning seen
Unlike vacuum cavities, there’s no measurable limitation for magnetic field!
Feb 12, 2008 TJR
Prospects for a Muon Collider

19. Understanding RF Breakdown
Scanning electron microscope images; Be (top) and Mo (bottom).
Feb 12, 2008 TJR
Prospects for a Muon Collider

20. Innovation: Parametric Resonance Ionization Cooling
Clever method to greatly reduce  without increased magnetic fields.
Excite ½ integer parametric resonance (in Linac or ring)
Like vertical rigid pendulum or ½-integer extraction
Elliptical phase space motion becomes hyperbolic
Use xx’=const to reduce x, increase x’
Use IC to reduce x’
Detuning issues are being addressed (chromatic and spherical aberrations, space-charge tune spread). Simulations are underway.
Smaller beams from 6D HCC cooling are essential for this to work! x X’ X’ X X
Feb 12, 2008 TJR
Prospects for a Muon Collider

21. 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 *).
Evacuated
Dipole
Wedge Abs
Incident Muon Beam
Feb 12, 2008 TJR
Prospects for a Muon Collider

22. 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
Feb 12, 2008 TJR
Prospects for a Muon Collider

23. 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
Bunching
Ring
Recirculating
Linac for Neutrino Factory
Neutrino Factory aimed at Soudan, MN
Target and Muon Cooling Channel
Feb 12, 2008 TJR
Prospects for a Muon Collider

24. 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
Extraction
Linac
Feb 12, 2008 TJR
Prospects for a Muon Collider

25. 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…
Feb 12, 2008 TJR
Prospects for a Muon Collider

26. Many New Arrows in the Quiver
New Ionization Cooling Techniques
Helical Cooling Channel
Momentum-dependent Helical Cooling Channel
Guggenheim cooling channel
Ionization cooling using a parametric resonance
Methods to manipulate phase space partitions
Reverse emittance exchange using absorbers
Bunch coalescing (neutrino factory and muon collider share injector)
Technology for better cooling
Pressurized RF cavities
High Temperature Superconductor for up to 50 T magnets
Acceleration Techniques
Dual-use Linac
Pulsed Recirculating Linac
Feb 12, 2008 TJR
Prospects for a Muon Collider

27. Conceptual Block Diagram of a Muon Collider
Proton Driver (8-40 GeV)
Production Target
Pion Capture, Decay Channel, Phase Rotation, and Pre-Cooling
Muon Ionization Cooling
Acceleration (0.2 to 20 GeV)
Reverse Emittance Exchange
Bunch Coalescing
Acceleration (20 to 2,500 GeV)
Storage Ring and Interaction Regions
Experiments
Must of course deal with both μ+ and μ-.
Feb 12, 2008 TJR
Prospects for a Muon Collider

28. Fernow-Neuffer Plot
Start Cooling: After Capture, Decay, Phase Rotation, Pre-Cooling
End Cooling: Start Acceleration to 2.5 TeV
HCC 400 MHz
REMEX & Coalescing
HCC 800 MHz
PIC
AccelerationTo 20 GeV
HCC 1600 MHz
Feb 12, 2008 TJR
Prospects for a Muon Collider

29. Viewgraph-level Design
TeV muon
storage ring with
two IRs
1 km radius
(= Fermilab Main Ring, but it’s not deep enough)
L ~ 1035 cm-2 s-1 μ+
2.5 km ILC-like
linacs
10 recirculating arcs
In one tunnel μ–
Final cooling, preacceleration
Helical cooling channel
Target, pion capture,
Phase rotation
Proton driver
Feb 12, 2008 TJR
Prospects for a Muon Collider

30. 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 ~ Hz
Must be careful about material (spontaneous muons!)
Feb 12, 2008 TJR
Prospects for a Muon Collider

31. Prospects for a Muon Collider
Neutrino Factory
Feb 12, 2008 TJR
Prospects for a Muon Collider

32. Prospects for a Muon Collider
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 ( MeV/c μ) “Real Soon Now”
MANX Collab.
Just forming
Fermilab’s Muon Collider task Force
Plus other Neutrino Factory organizations
Feb 12, 2008 TJR
Prospects for a Muon Collider

33. Prospects for a Muon Collider
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 1 2 3 4
Syringe Pump
Secondary
Containment
Jet Chamber
Proton
Beam
Solenoid
Feb 12, 2008 TJR
Prospects for a Muon Collider

34. Prospects for a Muon Collider
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
Feb 12, 2008 TJR
Prospects for a Muon Collider

35. Prospects for a Muon Collider
MICE (~10% 4d Cooling in 5.5 m)
Installation in ISIS R 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
Feb 12, 2008 TJR
Prospects for a Muon Collider

36. 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).
Feb 12, 2008 TJR
Prospects for a Muon Collider

37. Prospects for a Muon Collider
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!
Feb 12, 2008 TJR
Prospects for a Muon Collider