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Physicists Dream of a Muon Shot

Alexander Hellemans

When you plan to accelerate subatomic particles to astronomical energies and collide them to spawn new forms of matter, your choice of a projectile is critical. Hadrons--protons, for instance--shatter on impact into smaller pieces such as quarks and gluons, making their collisions messy and hard to interpret. Electrons are indivisible and yield cleaner collisions, but they emit energy-wasting synchrotron radiation when they are accelerated in circular machines. At a meeting last month in San Francisco, a group of physicists considering the next great collider--a hoped-for successor to the Large Hadron Collider (LHC) now being built at CERN in Geneva--pinned their hopes on the electron's chubby brother, the muon.

Catching muon beams. High-energy protons (green) collide with a target to produce pions (blue), which gradually decay to produce muons (red).


Pointlike, negatively charged particles 207 times more massive than the electron, muons generally have a fleeting existence in the debris resulting from particle collisions. Partly for this reason, they have never before been used as accelerator projectiles. But they and their positively charged antiparticles have some compelling advantages, promising the clean collisions of electrons without their wasteful synchrotron radiation. Although some researchers favor a hadron or electron collider as a successor to the LHC, the group that met at the 4th International Conference on Muon Colliders "is becoming more and more enthusiastic about muon colliders," says Andrew Sessler of Brookhaven National Laboratory (BNL) in Upton, New York.

Indeed, Sessler and his colleagues are now proposing a large-scale test of muon collider technology to see if they can generate and marshal these ephemeral particles into a coherent beam. "This is becoming more and more a real thing," he says. "And we expect that we can do it for less money."

The $5 billion LHC, which will begin colliding protons and antiprotons at an energy of 14 trillion electron volts (14 TeV) in 2005, may well offer a glimpse of the Higgs boson, a hypothetical particle that would help explain the varied masses of other particles. The machine may also reveal supersymmetric particles, heavier partners to known particles, which are predicted by a theory called supersymmetry. But to follow up on these clues, researchers will need a new machine that can produce Higgs and supersymmetric particles en masse and precisely measure their properties.

Proton-antiproton collisions are ill suited to making these precision measurements, says Howie Baer of Florida State University in Tallahassee: "You get lots of extra quarks and gluons, making the events very 'messy.'" And because electrons give off copious synchrotron radiation when a magnet bends their paths, a next-generation electron collider would probably take the form of a linear accelerator, or linac, tens of kilometers long.

Muons might offer the advantages of an electron collider without the expense of a huge linac. Because of their larger mass, they lose much less energy in the form of synchrotron radiation, so they can be accelerated in relatively small circular machines. "You don't have to build full-energy linacs," says Sessler. A 1-TeV muon collider "can fit on an existing laboratory site and use some of the existing infrastructure, and that is a tremendous advantage," adds William Marciano of BNL. What's more, because of muons' mass, their collisions spark physical processes that should generate Higgs particles far more efficiently than electrons can. "We can build a Higgs factory," says Sessler.

But producing and handling muons are still largely terra incognita, says Alvin Tollestrup of the Fermi National Accelerator Laboratory near Chicago, one of the proponents of the muon-collider idea. Muons are scarce in nature because they survive for only a few microseconds before decaying into electrons and neutrinos. In current designs, the first step toward making them is to collide an intense proton beam with a liquid metal target, producing quark-antiquark pairs called pions. The pions then decay into muons. In what Tollestrup calls "the critical part of that sequence," these muons have to be "cooled"--marshaled into a beam in which they all move at the same velocity. Only then can they be accelerated to nearly the speed of light, which extends their lifetime through Einstein's time dilation.

Sessler and his colleagues are now hoping to test muon-cooling schemes, which would rely on arrays of magnets and energy-absorbing materials. "We are proposing a $30 million experiment to be put in Fermilab," says Sessler. They expect to complete the final proposal early this year and hope to win funding in the fiscal year 2000 science budget.

Still, any push for a muon collider will encounter plenty of skepticism. Alvaro de Rújula, a theoretical physicist at CERN, acknowledges that the "clean physics" of a muon collider would be attractive. But he thinks the technical problems with these machines are daunting. "There are two types of machines of the future: the hadronic machine, where the experiments are extremely difficult, and the muon machine, where the machine is difficult."

Alexander Hellemans is a science writer in Naples, Italy.

Volume 279, Number 5348 Issue of 9 January 1998, pp. 169 - 170
©1998 by The American Association for the Advancement of Science.

Copyright © 1998 by the American Association for the Advancement of Science.