Physicists propose factory to spew out Higgs particles

No sooner has one mammoth accelerator delivered its first big result, than discussions begin on what should replace it.

At the annual get-together of Nobel prizewinners in Lindau, Germany, this week, all the talk was of whether the Large Hadron Collider is the right instrument to find out what exactly the LHC has found.

The problem is that the LHC collides two beams of protons. Protons are made of a melange of smaller particles, quarks and the gluons that hold them together, so when two of them hit, the result is a confusing array of shrapnel.

Finding something that looks like the Higgs boson has required painstaking reconstructions of what was fleetingly produced in the violence of the collisions. So much different stuff is produced that it might simply be too confusing an environment in which to pin down with any certainly what the putative Higgs's true properties are, and so reach a conclusive identification.

"The question is, will the LHC be able to do it at all? Or do we need something else?" says Carlo Rubbia, an experimentalist who as the head of CERN, Europe's particle physics lab near Geneva, Switzerland, played a major part in getting the LHC project off the ground in the 1990s.

Particle physicists' traditional answer to this problem has been to suggest building a machine to collide electrons and their antiparticles, positrons. Unlike protons, electrons are elementary particles, so don't disintegrate on impact, making it clearer what is produced when they collide.

The problem is that an electron-positron collider of the right energy would be even bigger than the LHC. A proposal to build such a machine, the International Linear Collider, has been on the table for some time, but with an estimated price tag of $20 billion, few countries are willing to commit. "It's not likely to come in the next two decades, maybe not in my lifetime" says David Gross, a Nobel prizewinning theorist at the University of California, Santa Barbara.

Muon-smasher

Rubbia thinks there could be a smarter solution. Muons are particles similar to electrons, but 200 times more massive. That means a muon-antimuon accelerator could reach the necessary energies over a far shorter distance.

With the putative Higgs checking in at about 125 gigaelectronvolts, you would need to bash together a muon and an antimuon with each having just over 60 GeV of energy. "That should be doable with a machine 100 times smaller than the LHC," says Rubbia. The result would be a "Higgs factory" producing the particle and little else.

There are a few technical hurdles to be overcome before such an accelerator becomes reality. Muons are produced in the decay of other particles, pions, as a hot, randomly moving gas, and they need to be cooled before they can be focused into a high-intensity beam.

But Rubbia thinks that we already have the expertise to overcome such problems, because similar techniques are used to cool antiprotons. Work is also being done to surmount these difficulties at CERN and Fermilab in Batavia, Illinois.

If that proves too challenging, and the electron variant too expensive, we are left with making the best of the messy collisions at the LHC to find out more about the Higgs. Gross is confident that something will come out even if we are left with that. "Our colleagues are extremely clever," he says. "They'll find a way to do it."

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