1. Feb 12, 2008 TJRProspects for a Muon Collider1 Prospects for an Energy- Frontier Muon Collider Tom Roberts Muons, Inc. Illinois Institute of Technology

2. Feb 12, 2008 TJRProspects for a Muon Collider2 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

3. Feb 12, 2008 TJRProspects for a Muon Collider3 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.

4. Feb 12, 2008 TJRProspects for a Muon Collider4 The Livingston Plot XILC X5 TeV MC 2025 Constituent Center-of-Mass Energy Panofsky and Breidenbach, Rev. Mod. Phys. 71, s121-s132 (1999)

5. Feb 12, 2008 TJRProspects for a Muon Collider5 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 ~ E 2, 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 2M W 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

6. Feb 12, 2008 TJRProspects for a Muon Collider6 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 ~ m 2 – 40,000 times larger than for e + e -.

7. Feb 12, 2008 TJRProspects for a Muon Collider7 Muons [Ankenbrandt et al., PRST-AB 2, 081001 (1999)] A 5 TeV muon collider could fit on the existing Fermilab site.

8. Feb 12, 2008 TJRProspects for a Muon Collider8 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 (!)

9. Feb 12, 2008 TJRProspects for a Muon Collider9 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 ~10 6. Luminosity ~ N 2 ·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).

10. Feb 12, 2008 TJRProspects for a Muon Collider10 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) Absorber dp/dz || -p RF Cavity dp/dz || +z p ┴ reduced, p || unchanged (Skrinsky & Parkhomchuk, 1981)

11. Feb 12, 2008 TJRProspects for a Muon Collider11 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 Here  is the normalized emittance, E µ is the muon energy, dE µ /ds and X 0 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. Lattice design Absorber Material

12. Feb 12, 2008 TJRProspects for a Muon Collider12 Absorber Materials F cool ~ (Energy Loss) / (Multiple Scattering)

13. Feb 12, 2008 TJRProspects for a Muon Collider13 Emittance Exchange Ionization cooling is only transverse. To get longitudinal cooling, use emittance exchange.

14. Feb 12, 2008 TJRProspects for a Muon Collider14 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. Beam Follows Helix

15. Feb 12, 2008 TJRProspects for a Muon Collider15 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.)

16. Feb 12, 2008 TJRProspects for a Muon Collider16 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

17. Feb 12, 2008 TJRProspects for a Muon Collider17 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

18. Feb 12, 2008 TJRProspects for a Muon Collider18 Innovation: High Pressure Gas RF Cavities Copper plated, stainless-steel, 805 MHz test cellCopper plated, stainless-steel, 805 MHz test cell H 2 gas to 1600 psi and 77 KH 2 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!

19. Feb 12, 2008 TJRProspects for a Muon Collider19 Understanding RF Breakdown Scanning electron microscope images; Be (top) and Mo (bottom).

20. Feb 12, 2008 TJRProspects for a Muon Collider20 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 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, space-charge tune spread). Simulations are underway. Smaller beams from 6D HCC cooling are essential for this to work! x X X’ X

21. Feb 12, 2008 TJRProspects for a Muon Collider21 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

22. Feb 12, 2008 TJRProspects for a Muon Collider22 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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

23. Feb 12, 2008 TJRProspects for a Muon Collider23 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 Neutrino Factory aimed at Soudan, MN

24. Feb 12, 2008 TJRProspects for a Muon Collider24 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

25. Feb 12, 2008 TJRProspects for a Muon Collider25 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…

26. Feb 12, 2008 TJRProspects for a Muon Collider26 Many New Arrows in the Quiver New Ionization Cooling TechniquesNew 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 partitionsMethods to manipulate phase space partitions –Reverse emittance exchange using absorbers –Bunch coalescing (neutrino factory and muon collider share injector) Technology for better coolingTechnology for better cooling –Pressurized RF cavities –High Temperature Superconductor for up to 50 T magnets Acceleration TechniquesAcceleration Techniques –Dual-use Linac –Pulsed Recirculating Linac

27. Feb 12, 2008 TJRProspects for a Muon Collider27 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 -.

28. Feb 12, 2008 TJRProspects for a Muon Collider28 Fernow-Neuffer Plot HCC 400 MHz HCC 800 MHz HCC 1600 MHz PIC REMEX & Coalescing Start Cooling: After Capture, Decay, Phase Rotation, Pre-Cooling Acceleration To 20 GeV End Cooling: Start Acceleration to 2.5 TeV

29. Feb 12, 2008 TJRProspects for a Muon Collider29 Viewgraph-level Design Target, pion capture, Phase rotation Helical cooling channel Proton driver 2.5 km ILC-like linacs 2.5 + 2.5 TeV muon storage ring with two IRs 1 km radius (= Fermilab Main Ring, but it’s not deep enough) Final cooling, preacceleration μ+μ+ μ–μ– 10 recirculating arcs In one tunnel L ~ 10 35 cm -2 s -1

30. Feb 12, 2008 TJRProspects for a Muon Collider30 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!)

31. Feb 12, 2008 TJRProspects for a Muon Collider31 Neutrino Factory

32. Feb 12, 2008 TJRProspects for a Muon Collider32 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

33. Feb 12, 2008 TJRProspects for a Muon Collider33 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

34. Feb 12, 2008 TJRProspects for a Muon Collider34 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

35. Feb 12, 2008 TJRProspects for a Muon Collider35 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

36. Feb 12, 2008 TJRProspects for a Muon Collider36 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).

37. Feb 12, 2008 TJRProspects for a Muon Collider37 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