Published online 13 June 2007 | Nature | doi:10.1038/news070611-7

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'Stuff' not made of particles could be seen soon, in theory.

What would the track of an 'unparticle' look like? No one knows. LAWRENCE BERKELEY LABORATORY / SCIENCE PHOTO LIBRARY

The universe could be filled with stranger stuff than anything physicists have ever seen: stuff that, unlike all known matter, isn't made of particles. Howard Georgi of Harvard University calls it 'unparticle stuff'.

According to theory, such stuff could barely be felt by everyday matter, making it invisible to us and our instruments so far. "It could be that at the energies we can probe today, we just don't see the unparticle stuff at all," says Georgi. But the high energies about to be probed by machines such as the Large Hadron Collider (LHC) at CERN, the European centre for particle physics near Geneva, Swizerland, might show up the strange stuff, he says.

The story of fundamental physics has largely been a search for particles — atoms and their constituents, such as quarks and electrons. The latest hunt is for the elusive Higgs boson, which is thought to be responsible for giving other particles the property of mass. The Higgs is thought to be very heavy, meaning it will take a lot of energy to make one — such as the energy from crashing other particles together at awesome speeds in the LHC.

This is the way it is with particles: their mass corresponds to a particular energy, as Einstein famously formulated in E=mc2. At the energy levels that prevail in our everyday world, matter consists largely of the familiar particles that make up atoms: protons, neutrons and electrons. If energies were a thousand times greater, a new suite of particles would predominate, as is thought to have been the case in the early instants of the Big Bang. The Universe would then look very different.

Georgi's unparticle stuff doesn't behave like that, he argues in Physical Review Letters1. You can boost energy levels as much as you like, and the unparticles would look just the same. This property is called 'scale invariance'.

Light fantastic

There is already known stuff like this, Georgi points out: light, made of particles called photons. Their scale invariance is possible because photons have no mass and so can have any level of energy.

Particles with non-zero mass can't show scale invariance. But that prohibition doesn't extend to 'stuff' that is not made of particles, says Georgi (he is careful not to call it 'matter', which implies particles). He says that theories of scale-invariant stuff have been understood mathematically for a long time. "But it's hard to describe this stuff because it is so different from what we are used to," he says. In particular, he notes, it can't be particulate.

It would only be possible to see this unparticle stuff via the effect it has on regular matter, and presumably (since we haven't seen it yet) that effect is normally small. "If all the stuff that is scale-invariant couples to all the stuff that isn't, in a way that gets weaker and weaker as the energy gets lower, there could be a scale-invariant world separate from our own that is hidden from us at low energies because its interactions with us are so weak," says Georgi.

This is what the ghostly particles called neutrinos are like. "Neutrinos have some properties in common with unparticle stuff. They are nearly massless and therefore nearly scale-invariant. They couple very weakly to ordinary matter at low energies, but the effect of the coupling increases as the energy increases," he notes.

Half a neutrino

Georgi has considered how unparticles might start to make their presence felt at higher energies, by analogy with the way this happens for neutrinos. The answer, he says, is weird: ordinary particles interacting with unparticles would behave as though they were interacting with a fractional number of massless particles — a bit like interacting with, say, five and a half photons. "This is the first glimmer of an answer to the question of how unparticles begin to show up," Georgi says.

The counterintuitive aspects of unparticles seem likely to leave some physicists scratching their heads. Physicist William Unruh at the University of British Columbia in Canada wonders whether the fingerprint of an unparticle would really be distinct from one a particle would leave. "If, say, the unparticle field were to interact with the atoms in a cloud chamber, would we not get definite tracks just as we do for photons?" he asks. "In other words, would this unparticle field not behave in many situations just like a particle?"

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Georgi thinks not. "What one would 'see' in a detector is way beyond what I understand," he admits. "But it wouldn't be particle tracks."

"I and other researchers are now trying to push these ideas harder, and other weird properties of unparticles have already emerged. It is great fun," he says. "Of course", he adds, "it would be even greater fun if we actually saw stuff like this at the LHC." The LHC is expected to begin its first runs in early 2008.

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