Princeton/µµ/99-18Jan. 7, 1999Physics Opportunities at a Muon ColliderKirk T. McDonaldJoseph Henry Laboratories, Princeton University, Princeton, NJ 08544The case for a future high-energy collider based on muon beams is briefly reviewed.1 I Wan t to Believe...• That elementary particle physics will prosper for a 2nd century with laboratory exper-iments based on innovative particle sources.• That a full range of new phenomena will be investigated:– mass ⇒ a2nd3× 3 (or larger?) mixing matrix.– Precision studies of Higgs bosons.– A rich supersymmetric sector.– ... And more ...• That our investment in future accelerators will result in more cost-effective technology,capable of extension to 10’s of TeV of constituent CoM energy.• That a Muon Collider [1, 2] based on ionization cooling is the best option to accom-plish the above.2 Ionization Cooling (An Idea So Simple It Might Just Work)• Ionization: takes momentum away.• RF acceleration: puts momentum back along z axis.•⇒Transverse “cooling”.Particles are slowed along their path (dE/dx)Particles are accelerated longitudinallyOrigin: G.K. O’Neill (1956) [3]1.• This won’t work for electrons or protons.• So use muons: Balbekov [4], Budker [5], Skrinsky [6], late 1960’s.3 The Details are DelicateUse channel of LH2absorbers, rf cavities and alternating solenoids (to avoid buildup ofangular momentum).One cell of the cooling channel:But, the energy spread rises due to “straggling”.⇒ Must exchange longitudinal and transverse emittance frequently to avoid beam lossdue to bunch spreading.Can reduce energy spread by a wedge absorber at a momentum dispersion point:Absorber wedgeNominal energyEnergy too highEnergy too low Equal energies[6-D emittance constant (at best) in this process.]4 What i s a Muon Collider?An accelerator complex in which• Muons (both μ+and μ−) are collected from pion decay following a pN interaction.• Muon phase volume is reduced by 106by ionization cooling.2• The cooled muons are accelerated and then stored in a ring.• μ+μ−collisions are observed over the useful muon life of ≈ 1000 turns at any energy.• Intense neutrino beams and spallation neutron beams areavailable as byproducts.Muons decay: μ → eν ⇒• Must cool muons quickly (stochastic cooling won’t do).• Detector backgrounds at LHC level.• Potential personnel hazard from ν interactions.Table 1: Baseline parameters for high- and low-energy muon colliders. Higgs/year assumesa cross section σ =5× 104fb; a Higgs width Γ = 2.7MeV;1year=107s.CoM energy TeV 3 0.4 0.1p energy GeV 16 16 16p’s/bunch 2.5 × 10132.5 × 10135 × 1013Bunches/fill 4 4 2Rep. rate Hz 15 15 15p power MW 4 4 4μ/bunch 2 × 10122 × 10124 × 1012μ power MW 28 4 1Wall power MW 204 120 81Collider circum. m 6000 1000 350Ave bending field T 5.2 4.7 3Depth m 500 100 10Rms ΔP/P % 0.16 0.14 0.12 0.01 0.0036d 6(πm)31.7 × 10−101.7 × 10−101.7 × 10−101.7 × 10−101.7 × 10−10Rms nπ mm-mrad 50 50 85 195 290β∗cm 0.3 2.6 4.1 9.4 14.1σzcm 0.3 2.6 4.1 9.4 14.1σrspot μm 3.2 26 86 196 294σθIP mrad 1.1 1.0 2.1 2.1 2.1Tune shift 0.044 0.044 0.051 0.022 0.015nturns(effective) 785 700 450 450 450Luminosity cm−2s−17 × 103410331.2 × 10322.2 × 10311031Higgs/year 1.9 × 1034 × 1033.9 × 1033Comparison of footprints of various future colliders:A First Muon Collider to study light-Higgs production:45 The Case for a Muon Collider• More affordable than an e+e−collider at the TeV (LHC) scale.• More affordable than either a hadron or an e+e−collider for (effective) energies beyondthe LHC.• Precision initial state superior even to e+e−.Muon polarization ≈ 25%, ⇒ can determine Ebeamto 10−5via g − 2spinprecession [7].tt threshold:345 355 365E + 2 mt [GeV]0.00.20.40.60.8σ [pb]Effect of Beam SmearingIncludes ISRμμeemt = 180 GeVISR + BeamISR onlyμμ: R = 0.1%ee: R = 1%Nearly degenerate A0and H0:H0• Initial machine could produce light Higgs via s-channel [8]:Higgs coupling to μ is (mμ/me)2≈ 40, 000× that to e.Beam energy resolution at a muon collider < 10−5,⇒ Measure Higgs width.Add rings to 3 TeV later.• Neutrino beams from μ decay about 104hotter than present.Possible initial scenario in a low-energy muon storage ring [9].Study CP violation via CP conjugate initial states:⎧⎪⎨⎪⎩μ+→ e+νμνeμ−→ e−νμνe.6 Future Frontier Facilities(A Personal Assessment)• Hadron collider (LHC, SSC): ≈ $100k/m [magnets].≈ 2kmperTeVofCMenergy.Ex: LHC has 14-TeV CM energy, 27 km ring, ≈ $3B.5• Linear e+e−collider (SLAC, NLC(?)): ≈ $200k/m [rf].≈ 20 km per TeV of CM energy;But a lepton collider needs only ≈ 1/10 the CM energyto have equivalent physics reach to a hadron collider.Ex: NLC, 1.5-TeV CM energy, 30 km long, ≈ $6B (?).• Muon collider: ≈ $1B for source/cooler + $100k/m for ringsWell-defined leptonic initial state.mμ/me≈ 200 ⇒ Little beam radiation.⇒ Can use storage rings.⇒ Smaller footprint.Technology: closer to hadron colliders.≈ 6kmofringperTeVofCMenergy.Ex: 3-TeV muon collider, ≈ $3B (?), would have physics reach well beyondthe LHC.7 Muon Collider R&D Program• Targetry and Capture at a Muon Collider Source [10, 11].Baseline scenario:To achieve useful physics luminosity, a muon collider must produce about 1014μ/sec.– ⇒> 1015proton/sec onto a high-Z target ⇔ 4 MW beam power.– Capture pions of P⊥<∼200 MeV/c in a 20-T solenoid magnet.– Transfer the pions into a 1.25-T-solenoid decay channel.– Compress π/μ bunch energy with rf cavities and deliver to muon cooling channel.6Proposed R&D facility:• Ionization Cooling for a High Luminosity Muon Collider [12, 13].Test basic cooling components:– Alternating solenoid lattice, RF cavities, LH2absorber.– Lithium lens (for final cooling).– Dispersion + wedge absorbers to exchange longitudinal and transverse phasespace.Track individual muons; simulate a bunch in software.Possible site: Meson Lab at Fermilab:Power Supplies (two floors)Cooling ApparatusMuonBeamlineshieldingshielding for primary beamshielding for primary beamtarget anddump(other experiments)(other experiments)7Cooling channel components:8 Upcoming Workshops(See http://www.cap.bnl.gov/mumu/table workshop.html)• Muon Collider Collaboration Meeting, May 20-26, 1999, St. Croix.• Neutrino Factories Based on Muon Accumulators, July 5-9, 1999, Lyon/CERN.• Muon Colliders at the Highest Energies,