Cern Courier Article: December 1997 issue
Muon colliders move nearer

by Steve Geer (Fermilab)
Our present theoretical prejudice is that new physics "beyond the Standard Model" will emerge in quark-antiquark and lepton-antilepton collisions at or approaching the TeV energy scale. To fully explore the TeV scale will require at least one multi-TeV hadron-hadron collider to make a broad search for new physics, and one or more TeV scale lepton-antilepton colliders to make precision measurements of new phenomena. The next big step forward in advancing the hadron-hadron collider energy frontier will be provided by the CERN Large Hadron Collider (LHC), a proton-proton collider with a center-of-mass energy of 14 TeV due to come into operation in 2005. The route towards TeV scale lepton-antilepton colliders is less clear.

The lepton-antilepton colliders built so far have been e+e- colliders like the Large Electron Positron collider (LEP) at CERN and the Linear Collider (SLC) at SLAC. However, electrons are very light and like to radiate away their energy when they are accelerated. In a circular ring like LEP the energy lost per revolution in keV is 88.5 E^4 / rho, where the electron energy E is in GeV, and the radius of the orbit rho is in meters. Hence, the energy loss grows rapidly as E increases. This limits the center-of-mass energy that would be achievable in a LEP-like collider. The problem can be reduced by building a linear machine like the SLC, but technical challenges still appear to make it very difficult in practice to approach and go beyond the TeV energy scale.

For a lepton with mass m, the energy losses per revolution in a circular ring are inversely proportional to m^4. Hence, the energy loss problem can be solved by using heavy leptons. In practice this means using muons, which have a mass 207 times the electron mass. The resulting enormous reduction in radiative losses enables higher energies to be reached, and smaller collider rings to be used. Estimated sizes of the accelerator complexes required for 0.5 TeV and 4 TeV muon colliders are compared with the sizes of other futuristic colliders in Figure 1. At least two generations of muon colliders would fit on existing laboratory sites. Although the cost of building a muon collider is still unknown, since it would be relatively small and fit within an existing laboratory, muon collider proponents expect the cost to be cheaper than alternative futuristic machines. Muon colliders also offer some physics advantages. The small radiative losses permit a very small beam energy spread to be achieved, allowing very precise measurements of the masses and widths of any new resonant states scanned by the collider. In addition, since the cross-section for producing a Higgs-like scalar particle in the s-channel (direct lepton-antilepton annihilation) is proportional to m^2 this extremely important process could be studied at a muon collider but not at an e+e- collider.

The muon collider idea dates back to Tinlot (1960). Unfortunately there are significant challenges in designing an accelerator complex that can make, accelerate, and collide mu+ and mu- bunches before nearly all of the muons have decayed, which they do with a rest lifetime of 2.2 microsecs. However, at the Sausalito workshop in 1995 it was realized that, with modern ideas and technology, it may be feasible to make muon bunches containing a few times 10^12 muons and, before more than about half of the muons have decayed, compress the phase space occupied by the muons by a factor of 10^5 - 10^6 and accelerate the resulting intense muon bunch up to the multi-TeV energy scale. It was also realized that with careful design of the collider ring and shielding it may be possible to reduce to acceptable levels the backgrounds within the detector that arise from the very large flux of electrons produced in muon decays. These realizations led to an intense activity involving almost 100 accelerator and particle physicists, which resulted in the muon collider feasibility study report prepared for the 1996 Snowmass workshop. One year after Snowmass, encouraged by further progress in developing the muon collider concept together with the growing interest and involvement of the high energy physics community, the "Muon Collider Collaboration" became a formal entity in May of this year. The collaboration is receiving strong support and participation from 3 US laboratories (BNL, FNAL, and LBL) and a growing number of university groups. The goal of the collaboration is to complete within a few years the R&D needed to determine whether a Muon Collider is technically feasible, and if it is, to design the First Muon Collider.

Figure 2 shows a schematic of a muon collider accelerator complex. In the example illustrated, proton bunches containing 5 x 10^13 particles are accelerated to energies of 16 GeV. The protons interact in a target to produce approximately 3 x 10^13 charged pions of each sign. These pions are produced with only a very limited component of momentum transverse to the incident proton direction, and can be confined within a beam channel using, for example, a 20 Tesla co-axial solenoid with an inner radius of 7.5 cm. To collect as many particles as possible within a useful energy interval, rf cavities are used to accelerate the lower energy particles and decelerate the higher energy particles. Muons are produced by allowing the pions to decay into a muon plus neutrino, which they do with a rest lifetime of 2.6 x 10^-8 secs. For example, at the end of a 20 m long decay channel consisting of a 7 Tesla solenoid with a radius of 25 cm each incident proton results in about 0.2 muons of each charge. With two proton bunches every accelerator cycle, the first used to make and collect positive muons and the second to make and collect negative muons, there are about 1 x 10^13 muons of each charge available at the end of the decay channel per accelerator cycle. If the proton accelerator is cycling at 15 Hz, in an operational year (10^7 secs) about 1.5 x 10^21 positive and negative muons would have been produced and collected.

The muons exiting the decay channel populate a very diffuse phase space. The next step in the muon collider complex is to "cool" the muon bunch, i.e. to turn the diffuse muon cloud into a very "bright" bunch with small longitudinal and transverse dimensions, suitable for accelerating and injecting into a collider. The cooling must be done within a time that is not long compared to the muon lifetime. Conventional cooling techniques (stochastic cooling and electron cooling) take too long. The new technique proposed for cooling muons is called ionization cooling, and is illustrated in Figure 3. The idea is that the muons traverse some material in which they lose both longitudinal and transverse momentum by ionization losses (dE/dx). The longitudinal momentum is then replaced using an rf accelerating cavity, and the process is repeated many times until there is a large reduction in the transverse phase space occupied by the muons. The energy spread within the muon beam can also be reduced with ionization cooling by using a wedge shaped absorber in a region of dispersion (where the transverse position is momentum dependent). The wedge is arranged so that the higher energy particles pass through more material than lower energy particles. Initial calculations suggest that the six-dimensional phase space occupied by the initial muon bunches can be reduced by a factor of 10^5 - 10^6 before multiple coulomb scattering limits further reduction.

At the end of the ionization cooling channel each muon bunch is expected to contain about 5 x 10^12 muons with a momentum of order 100 MeV/c. Rapid acceleration to the collider beam energy can be achieved in either a recirculating linear accelerator and/or a rapid cycling synchrotron. Positive and negative muon bunches are then injected in opposite directions into a collider storage ring and brought into collision at the interaction point. The bunches circulate and collide for many revolutions before decay has depleted the beam intensities to an uninteresting level. For example, in a collider with 2 TeV muon beams (4 TeV center-of-mass energy) and an orbit circumference of 8 km, the muon bunches would make about 1000 revolutions, and would then be "dumped". New muon bunches would be injected into the collider about 15 times a second.

There are many interesting and challenging problems that need to be resolved before the feasibility of building a muon collider can be demonstrated. For example, (i) the very intense proton bunches will destroy a solid target, necessitating the development of a liquid metal jet target, and (ii) to attain the desired cooling factor in the ionization cooling channel current designs require the development of rf cavities with thin beryllium windows operating at liquid nitrogen temperatures in high solenoidal fields and also the development of long liquid lithium lenses to provide very strong radial focusing for the final cooling stages. Despite these and many other technical challenges, no "show stoppers" have yet been identified, and there is growing enthusiasm amongst the particle and accelerator physics communities to find out whether a muon collider is really feasible. If the answer is yes, then the first muon collider would probably be a "low energy" machine and might be, for example, a Higgs boson "factory" if a Higgs-like boson exists with a mass in the 100 - 500 GeV/c^2 range. Perhaps at last we have an answer to the famous question I.I. Rabi asked after the discovery of the muon: "Who ordered that ?". The answer must be "High-energy lepton-antilepton collider enthusiasts".

Figures and Figure Captions

More Information

More information about muon collider R&D can be obtained at the web sites at Fermilab and Brookhaven , or from the spokesperson for the muon collider collaboration (R. Palmer). Information about the ionization cooling R&D can be obtained from S. Geer (spokesperson), and information about the muon production R&D program can be obtained from K. McDonald and R. Weggel (Spokespeople).
Suggestions ? Contact sgeer@fnal.gov