Muon Colliders

[phongn]

Instead of very long linear accelerators (e.g. ILC or CLIC), some guys are proposing using muon accelerators as the next step beyond the LHC. Here's a brief overview of a proposed accelerator:

Why Muons at the Energy Frontier?

Muons are the heavy cousins of electrons. They have the same electric charge and interact with matter in a similar way: muons and electrons belong to the same family of particles known as leptons. Unlike protons, which comprise subatomic particles called quarks, muons and electrons come in one piece: they are elementary particles.

Muons could eliminate a big problem that scientists face when accelerating electrons: in a circular accelerator, electrons emit light and lose energy as they go around the ring. This puts a limit on the maximum energy that the electrons can reach in such a machine. The solution could be a straight accelerator, in which electrons don’t emit light. But scientists would need to build a very long (and hence expensive) linear accelerator to propel electrons to record energies.

Because muons are 200 times heavier than electrons, they emit less light and lose less energy when traveling in a circle than electrons do. Hence scientists are developing the concept of a circular muon accelerator. Sending the muons through the same loop and the same accelerating cavities repeatedly reduces the number of cavities needed and the footprint necessary to accommodate a collider. The machine would easily fit on the Fermilab site.

Experiments done using a muon collider would complement experiments at the Large Hadron Collider at the European laboratory CERN. The LHC accelerates protons and makes them collide at record energies. Scientists expect that the collisions will reveal the nature of dark matter, extra dimensions of space and the origin of mass.

Once the LHC experiments reveal new physics phenomena, physicists could use a muon collider to make more precise studies of those discoveries. The discoveries at the LHC will determine the energy needed for a muon collider to explore these new phenomena.

[Simon_Jester]

Hmm. Hadn't thought of using muon beams to cut the synchrotron radiation problem. It would work, but not as well as a proton beam. You'd still have more trouble with radiative loss of energy than you would with protons, not less. So in terms of trying to achieve the greatest possible energy, this is not your friend. Note that the advertised energy level for the Hypothetical Muon Collider is lower than that planned at the LHC (if they ever get the damn beams running at the same time).

Fermilab would get about the same collision energy from this muon collider as they already do from their proton collider. Which is great for a lepton collider; previous lepton collision energies have been limited to the ~100 GeV range. But it's a "works along with" the big proton colliders, not "the next step beyond."

Beam cooling sounds tricky; I don't know the details well enough to evaluate their plans. Cooling is important, because there are a lot of important figures of merit for a collider beyond just the collision energy. Maybe I'll ask around at work and see what people think.

[Wyrm]

There's also a small problem that muons are unstable. For all their faults, electron-positron collisions use particles that will exist until you smash 'em together. Same with proton-antiproton collisions.

[Fingolfin_Noldor]

There's also a small problem that muons are unstable. For all their faults, electron-positron collisions use particles that will exist until you smash 'em together. Same with proton-antiproton collisions.

Maybe if they accelerated them fast enough, they'd overcome the decay problem.

That is of course, if they can accelerate them fast enough.

[phongn]

There's also a small problem that muons are unstable. For all their faults, electron-positron collisions use particles that will exist until you smash 'em together. Same with proton-antiproton collisions.

Maybe if they accelerated them fast enough, they'd overcome the decay problem.

That is of course, if they can accelerate them fast enough.

That's what they're planning on doing.

[Gil Hamilton]

I'm not clear where you get muons TO accelerate. Muons, as I understand it, aren't something you can make on command, since we can really make energetic events with the energy necessary to make them, so all muons that exist on Earth are the result of cosmic radiation bashing the atmosphere. Electrons and protons are easy to "get", I'm just not clear where you "get" muons.

[phongn]

I'm not clear where you get muons TO accelerate. Muons, as I understand it, aren't something you can make on command, since we can really make energetic events with the energy necessary to make them, so all muons that exist on Earth are the result of cosmic radiation bashing the atmosphere. Electrons and protons are easy to "get", I'm just not clear where you "get" muons.

"To create lots of muons, scientists use a high-intensity proton accelerator that steers protons into a target. The collisions create short-lived particles called pions. Within 50 meters the pions decay into muons and neutral particles called neutrinos. The muons have an energy of about 200 MeV." (Fermilab)

[Fingolfin_Noldor]

They'd have to accelerate the muons to speeds comparable to or greater than the speeds of muons that hit earth via cosmic rays. Otherwise, the decay time isn't substantial longer.

[muon]

It's not too difficult to create and capture muons, given that they have the lowest rest-mass of any (EM- or strongly-interacting) particle aside from the electron (and hence are produced in copious quantities once collision energies pass at least a few hundred MeV, easily accomplished with a small linac striking a stationary target), and their mean lifetime permits them to travel hundreds of meters even before hitting the accelerator stages.

One of the advantages a muon facility would have over hadron colliders is that the COM energies are effectively higher than they are for baryons (as protons are bundles of quarks, whereas muons are essentially point particles), thus they should be able to accomplish similar physics research at lower beam energies.

More than just being a useful engineering design, it has long been proposed to construct large muon storage rings: by having the muons decay in the long straight sections, it's possible to create intense beams of neutrinos, which can then be directed (straight through the earth) at neutrino detectors hundreds of kilometers away. In short, a "Neutrino Factory". Neutrino physics is currently a very active field of research, and these types of experiments (with neutrino source to detector baselines of hundreds to thousands of kilometers) could provide lots of additional data on neutrino oscillations and related phenomena. See a short article about the subject here; some cool recent papers about the neutrino research here, here, here, here, and here; and some large reports here and here.

One significant problem with muon colliders and storage rings though: since the muons will continuously decay in the ring, and since the produced neutrinos will be of exceptionally high energy (up to TeV-scale or beyond), there will be a neutrino radiation hazard in the ring plane, particularly along the axis of any straight sections! (Although neutrinos at lower energies are indeed relatively inert - "a light-year of lead" and all that - at higher energies their interaction cross-sections are large enough to become dangerous.) See here.

Indeed, it has even been proposed (very speculatively at this point) to use this neutrino-radiation effect as a possible futuristic weapon that could reach targets shielded by thousands of km of rock! See here - Stuart, are you reading this? (A possibly more practical design for such a weapon has been proposed here.)

[phongn]

One of the hopes for Fermilab, at least, is that they'll be able to build a neutrino factory with the muon collider and point it at the (proposed) Deep Underground Science and Engineering Lab. With ILC pretty much out of the question for US hosting, the lab really needs a new big project for the future else we're in trouble - after all, LHC is going to take over where Tevatron left off.

As for the neutrino beam weapon: "We also note that a 1000 TeV machine requires the accelerator circumference of the order of 1000 km with the magnets of ≃ 10 Tesla which is totally ridiculous."

[Fingolfin_Noldor]

One of the hopes for Fermilab, at least, is that they'll be able to build a neutrino factory with the muon collider and point it at the (proposed) Deep Underground Science and Engineering Lab. With ILC pretty much out of the question for US hosting, the lab really needs a new big project for the future else we're in trouble - after all, LHC is going to take over where Tevatron left off.

As for the neutrino beam weapon: "We also note that a 1000 TeV machine requires the accelerator circumference of the order of 1000 km with the magnets of ≃ 10 Tesla which is totally ridiculous."

Whether the US is willing to push for a collider is a big question mark though.

In theory, they could use the existing Fermilab tunnels and perhaps lengthen them to accommodate the accelerator, but as the defunct SSC shows, political will is low. And now the US is terminating the ISS in remarkably short period of time.

[Simon_Jester]

Muon (and probably phongn) appear to know more about this than I do. However, I would like to give my take on some of the questions that have come up:
________

There's also a small problem that muons are unstable. For all their faults, electron-positron collisions use particles that will exist until you smash 'em together. Same with proton-antiproton collisions.

Yes. It's a drawback, but for electron-positron colliders you're already stuck with the huge drawback of synchrotron radiation that kills your accelerator above the 10-100 GeV range anyway; muons are far more resistant to that problem by virtue of being so much more massive. As I recall from doing the math earlier this summer, you couldn't build a TeV-range electron-positron collider even if you ran the thing around the Earth's equator, for God's sake. It's ridiculous. Proton beams can run in that range, but you run into other difficulties. A muon collider might make a good compromise.

Maybe if they accelerated them fast enough, they'd overcome the decay problem. That is of course, if they can accelerate them fast enough.

Sort of. They're talking about accelerating the muons in the storage rings to 200 GeV, which gives them a relativistic gamma of about 2000... in our frame that extends the half-life to a few milliseconds... running around a pipe length on the order of 20 to 50 kilometers (not sure exactly) at approximately the speed of light... yeah. They can at least keep the beam alive for several circuits around the collider, long enough to get collisions out of it. They're still going to have to keep generating muons from their proton beams, but I see no fundamental reason why the decay problem stops them from getting some results, even if it's a horrible pain in the electric bill to keep the thing going.

I'm not clear where you get muons TO accelerate. Muons, as I understand it, aren't something you can make on command,

Sure you can. Slam protons into a wall hard enough and you will get muons. That's how cosmic ray collisions generate them in the first place. And muons are charged, so you can steer them with the same toolkit you'd use on protons. It's a problem, but it's an engineering problem, not a laws-of-physics one.

[muon]

There's also a small problem that muons are unstable. For all their faults, electron-positron collisions use particles that will exist until you smash 'em together. Same with proton-antiproton collisions.

Yes. It's a drawback, but for electron-positron colliders you're already stuck with the huge drawback of synchrotron radiation that kills your accelerator above the 10-100 GeV range anyway; muons are far more resistant to that problem by virtue of being so much more massive. As I recall from doing the math earlier this summer, you couldn't build a TeV-range electron-positron collider even if you ran the thing around the Earth's equator, for God's sake. It's ridiculous. Proton beams can run in that range, but you run into other difficulties. A muon collider might make a good compromise.

It's not quite that bad, although nearly so. Per the brehmsstrahlung formula (for acceleration perpendicular to velocity as is the case in a collider), the synchrotron radiation loss is proportional to the fourth power of relativistic gamma, and inversely proportional to the square of circular path radius. Thus, as LEP was able to reach ~100 GeV beam energy (an important target milestone, permitting W boson pair production) in a circumference of ~27km - see collider parameters (pdf link) - then in theory, scaling this up to planet-circling size, a ring with circumference of ~40000km should be able to reach energies over 38 times higher, nearly 4 TeV. Using ILC as a baseline reference, even higher energies could be achieved... but yes, that would be a lot of bending magnets! In any case, since muons will have a synchrotron radiation loss (105.7/0.511)^4 = 1.8E9 times less than electrons for a given collider, they're essentially as good as protons in this regard, for any machine we're likely to build in the foreseeable future.

Maybe if they accelerated them fast enough, they'd overcome the decay problem. That is of course, if they can accelerate them fast enough.

Sort of. They're talking about accelerating the muons in the storage rings to 200 GeV, which gives them a relativistic gamma of about 2000... in our frame that extends the half-life to a few milliseconds... running around a pipe length on the order of 20 to 50 kilometers (not sure exactly) at approximately the speed of light... yeah. They can at least keep the beam alive for several circuits around the collider, long enough to get collisions out of it. They're still going to have to keep generating muons from their proton beams, but I see no fundamental reason why the decay problem stops them from getting some results, even if it's a horrible pain in the electric bill to keep the thing going.

If muon colliders and storage rings are in fact developed, we should be able to do considerably better than that even with today's technology. See p.71-72 of the '02-'03 report I'd linked previously - here - 5 Tesla bending magnets would permit muons to complete ~1000 turns before decay (e.g. a beam energy of ~3 TeV, a gamma of ~30000, in a reasonably small ring). Again, note the advantage of muons over hadrons for HEP work - per the illustration on p.72, in lepton colliders the attainable collision energy is simply the CoM energy, but as protons are composite particles, for hadron colliders the CoM energies attainable in hard ("quark-quark") collisions are much less than the total beam energy; thus, it's possible to do equivalent or superior physics work with smaller and lower-energy rings.

As for the neutrino beam weapon: "We also note that a 1000 TeV machine requires the accelerator circumference of the order of 1000 km with the magnets of ≃ 10 Tesla which is totally ridiculous."

(mad_scientist_rant)

I was actually quite surprised at the buzz that paper created all over the web; although it was obviously a purely theoretical study, a lot of folks seemed to be taking it more seriously than isomer bombs! That said, the concept of such a weapon (that, with currently-known science, could not be blocked by any type of remotely practical shelter!) is darkly intriguing; if the speculation in that other paper I linked proves to be at all correct, and there is a near-singularity of the Z0 pole (and we can effectively produce monochromatic neutrino beams, e.g. via inverse beta decay)... well, it would certainly be a lot more practical to produce 90 GeV beams than a PeV (!) beam, and life might start to get "interesting" for various superhardened bunkers. Naturally, even aside from the pure-physics considerations, such a weapon would have to be made at least reasonably portable, if it were not to be immediately targeted upon construction.

(/mad_scientist_rant)

...but as the defunct SSC shows, political will is low. And now the US is terminating the ISS in remarkably short period of time.

Sigh... yeah.

[Simon_Jester]

It's not quite that bad, although nearly so. Per the brehmsstrahlung formula (for acceleration perpendicular to velocity as is the case in a collider), the synchrotron radiation loss is proportional to the fourth power of relativistic gamma, and inversely proportional to the square of circular path radius. Thus, as LEP was able to reach ~100 GeV beam energy (an important target milestone, permitting W boson pair production) in a circumference of ~27km - see collider parameters (pdf link) - then in theory, scaling this up to planet-circling size, a ring with circumference of ~40000km should be able to reach energies over 38 times higher, nearly 4 TeV. Using ILC as a baseline reference, even higher energies could be achieved... but yes, that would be a lot of bending magnets! In any case, since muons will have a synchrotron radiation loss (105.7/0.511)^4 = 1.8E9 times less than electrons for a given collider, they're essentially as good as protons in this regard, for any machine we're likely to build in the foreseeable future.

I'll have to check my math from USPAS. Thank you for the partial correction.

[Fingolfin_Noldor]

Physics Today or something had an article on particle physics in US a while back. The general consensus is that particle physicists in the US want a collider, but convincing the government to part with 10 over billion will be hard.

Also, never mind that if the Higgs isn't found at the LHC, it might be potentially extremely hard to convince people to fund another collider. The question in everyone's heads will be: "How much more energies do we need to find this particle?"

Granted it might spur a lot of High Energy theory research, it might sound the death knell for big Physics for a long time.

[Gil Hamilton]

It's not quite that bad, although nearly so. Per the brehmsstrahlung formula (for acceleration perpendicular to velocity as is the case in a collider), the synchrotron radiation loss is proportional to the fourth power of relativistic gamma, and inversely proportional to the square of circular path radius. Thus, as LEP was able to reach ~100 GeV beam energy (an important target milestone, permitting W boson pair production) in a circumference of ~27km - see collider parameters (pdf link) - then in theory, scaling this up to planet-circling size, a ring with circumference of ~40000km should be able to reach energies over 38 times higher, nearly 4 TeV. Using ILC as a baseline reference, even higher energies could be achieved... but yes, that would be a lot of bending magnets! In any case, since muons will have a synchrotron radiation loss (105.7/0.511)^4 = 1.8E9 times less than electrons for a given collider, they're essentially as good as protons in this regard, for any machine we're likely to build in the foreseeable future.

Clearly we need to go the "Forever Peace" route and send robots to the Jovian system to build a supercollider that entirely circles Jupiter.

[Fingolfin_Noldor]

I remember reading somewhere that maybe in the future, we'd need to build a collider that orbits the sun....

[starslayer]

I remember reading somewhere that maybe in the future, we'd need to build a collider that orbits the sun....

The real, super-duper, ultimate collider of DOOMTM would actually be larger than the Solar System, and it would generate enough energy to probe the Planck length. Good luck building it, though...